Κεραία loop για λήψη

Ragchewing on 75 meters with the locals (several hundred miles) can be a lot of fun. The summer months, however, with their high static and noise levels, can be brutal on the ears. I have found that a small receive-only loop antenna can be used to make the situation much more tolerable. This page describes an easy-to-build loop that I have built several times for myself and friends. We tend to congregate on 3.805 MHz, so I call this version the 3805er.

The perimeter length of the loop is 12 feet, and it is in a diamond shape. It is approximately 4 feet wide and 5 feet tall. It is certainly possible to use the loop in the shack, although I find the performance to be better when the loop is outside.

A 3805er up against the garage

• Introduction
• Design
• Parts List
• Assembly
• Checkout and Adjustment
• On-the-Air

Introduction

In order to improve reception, it is necessary to improve the signal to noise ratio. Common 80 meter antennas such as low dipoles, inverted Vees, or verticals, suffer on receive because they have a nearly omnidirectional response pattern. While the desired signal is arriving from one direction, noise can arrive from all directions. A loop antenna such as the 3805er improves this situation because it has broadside nulls at very low wave angles. Most radio signals within several hundred miles tend to arrive at much higher wave angles (above 45 degrees), and at those angles, the loop response is nearly omnidirectional. Often times, noise is relatively local, and arrives at a low angle. The loop can be oriented to reduce the reception of this noise. Less noise, same signal, improved signal to noise ratio.

Technically, the 3805er is a single-turn shielded loop antenna. This type of loop is described in the ARRL Antenna Book, and the ON4UN book. Some sources state that the shielded nature of the loop provides additional noise immunity by shorting the (noisy) electric field to ground while responding to the magnetic field. Because of that belief, this type of antenna is also called a magnetic loop. I'm not sure if I believe that particular theory, but in any case, the loop can improve reception of short-distance radio signals.

In my experience, this type of antenna is most useful for short-distance work in the summer months. I do not consider it to be a good DX antenna, although it has been used for DX by some. When winter comes, and 80 meter noise tends to drop naturally, the advantage of the loop often disappears. Please note that the 3805er is a receive-only antenna. In order to use it, it is almost a necessity that your radio provides a separate receive antenna input jack. This antenna, like most receiving antennas, will have very low output compared to transmit antennas. In some cases, a preamp can be useful. It is entirely normal that when using this antenna you will have a nearly S0 noise level, and the station you are working will barely move the meter. What will be missing is the deafening noise crashes and static bursts that contribute to ear fatigue.

Design

The antenna design is certainly not original. The loop is constructed from small diameter 50 ohm coax, such as RG8X. Any coax diameter or characteristic impedance can be used, so long as an appropriate capacitor resonates the antenna at the desired frequency.

In order to preserve the broadside nulls, the literature suggests that the length of the loop wire should be less than 0.1 wavelength. That limits our perimeter length to no more than 25 feet. I chose the 12 foot length since it naturally falls out from using standard 4 foot dowels for spreader arms. I also wanted a loop that could be used inside as well as outside, and the 4 foot dowel-based form is light and not difficult to move around.

I use a small trimmer capacitor to resonate the loop at the desired frequency, 3.805 MHz. A loop built following my dimensions and geometry has an inductance of approximately 5.3 uH at 3.805 MHz. This is around 125 ohms of inductive reactance. A 325 pF capacitor will be needed to achieve resonance. I happened to have a bag of 270 pF hamfest trimmers, so I parallel an 80 pF silver mica capacitor across the trimmer to get up to the needed value. I also usually include a small step-up transformer to match the low loop impedance to the 50 ohm feed line. This is an optional part of the design. I do find that the addition of the transformer increases loop output. The transformer is wound on a 1/2 inch ferrite core. Any mix suitable for the frequency will do (#75 (best), #43, etc.).

3805er Schematic

Parts List

In order to build your own 3805er, collect the following parts:

Parts List
Part Description	Quantity
12 feet of 50 ohm mini coax (RG8X)	2
PL-259 coax connector with mini coax insert	1
4 foot dowel rod, 5/8 inches in diameter	2
3/4 inch aluminum tubing, 3 inches long	2
3/4 inch aluminum tubing, 18 inches long	1
2 3/4 inch X 2 1/8 inch X 1 3/4 inch aluminum project box	1
1/4 inch grommet	3
1/4 inch X 20 stainless steel bolt, 2 inches long, and locking nut	1
1/4 inch screw eye	3
#8 X 1/2 inch long stainless steel machine screw	3
terminal strip, 5 lug	1
variable trimmer capacitor, approx. 350 pF (see text)	1
cable ties, small	3
cable ties, large	3
1/2 inch ferrite core toroid, #75 or #43 material	1
solid hookup wire, #22	2 feet
electrical tape	8 inches

3805er Parts

In addition to these parts, and common hand tools, you will need a drill and several different drill bits. One bit makes the hole for the grommets. For a 1/4 inch hole grommet, a typical drill bit size is 3/8 inch.  You will need a 1/4 inch drill bit for the 3 inch aluminum tubes and the 1/4 inch X 20 stainless steel bolt that joins the tubes and dowels. Finally, you will need a drill for the #8 screws that attach the project box to the 18 inch aluminum tube. The bit needs to be slightly smaller than the screw so that you can use the screw in a self-tapping manner. The diameter is approximately 1/8 inch, but size it according to your screws.

If you wish to accurately resonate the antenna, and optimize the output of the antenna, you will need an antenna analyzer that measures SWR as a function of frequency.

Assembly

Here's how I assemble a 3805er.

• Select one 12 foot length of coax to be the antenna. To both ends, remove 1 inch of the outer jacket, and collect the braid into a single stranded wire. Remove 1/4 inch of the inner conductor insulation. Find the middle of the coax (6 feet from either end). Remove a 1 inch section of the outer jacket, and then remove the braid from around this 1 inch region. The braid must be electrically broken at this point. Seal the exposed braid ends, especially if you are going to mount this antenna outside. I paint the braid ends with liquid electrical tape, cover the entire exposed area with electrical tape, then finish off the region with a piece of heat shrink tubing. No doubt this is overkill.

Dressed coax ends and braid removed from center of antenna coax

• The other piece of coax is the feed line. I have found that a 12 foot length with a PL-256 plug lets me move the antenna to my various sites and simply plug it in and go. If you would prefer a different length or connector, certainly adjust my comments to your situation. Whatever the length and connector, prepare the antenna end of the cable as in the previous step. Remove 1 inch of the outer jacket, collect the braid into a single stranded wire. Remove 1/4 inch of the inner conductor insulation.

• I like to apply some sort of stain/protector to the bare wooden dowels. In some climates, a bare dowel will barely last a year. With staining, mine last for many years.
• Place a 3 inch aluminum tube over each dowel, and center it over the center of the dowel. With normal aluminum tubing wall thickness, the 5/8 inch dowel will fit snugly within the 3/4 inch tube. You could use other combinations of dowels and tubes so long as the dowel fits within the tube. I find that this combination is a good balance between strength and size.
• Drill a 1/4 inch hole in the middle of each 3" aluminum tube. Since the dowel is in the tube, you will also drill a hole in the dowel. Try to keep the hole in the center of the tube, perpendicular to the tube length.
• Put the 2 inch stainless steel bolt through the two tube/dowel assemblies. You should now have a cross or X of dowels. Place the locking nut over the bolt end in order to keep the arms together.

Center of the loop, showing 3 inch tubes around dowels with bolt as center pin

• Install the screw eyes into 3 of the 4 dowel ends. The end without a screw eye is the bottom of the antenna.
• With the cross in front of you, on the floor or a table, lay out the antenna coax around the perimeter of the cross. The ends of the coax come together at the bottom of the antenna. That is the dowel end without a screw eye. The middle of the antenna, where the break in the braid is located, is at the top of the antenna. Take the three larger cable ties and put them around the coax and through the screw eyes. You do not need to pull them tight at this point. Keep a little slack in them at this point so that the antenna coax can shift around as the junction box is installed.
• Drill three, 3/8 inch holes in the aluminum junction box for the antenna ends and the feed line. All of the holes are drilled in the piece that has the ends. Drill a hole in the center of each end for the antenna. Drill the feed line hole in the lower right corner of the same piece when looking into the piece (see the parts photo). Put a grommet in each hole.
• Drill two more holes in the same piece. These two holes, approximately 1/8 inch, are for the two sheet metal screws that hold the box to the 18 inch aluminum tube. The upper hole also holds the terminal strip. See the picture for more information.
• Put the coax ends through the appropriate holes. The goal now is to attach the small cable ties to the coax so that the ends cannot be pulled out of the box. My experience has been that a small cable tie, wrapped twice around the coax, holds much better than a larger and more stiff cable tie wrapped once. I locate the tie about 3/8 of an inch from the end of the outer jacket, on the outer jacket. Wrap the tie twice around the coax, and pull it tight. As you pull the excess coax out of the box, the ties will hit the grommets and prevent the coax from being pulled out of the box. At this point you should have three dressed coax ends in the box, with sufficient length to reach around in the box.
• Measure 7 inches from one end of the 18 inch aluminum tube. Drill   a hole with a diameter that will grip the #8 stainless steel sheet metal screws.
• Put that end (nearest the screw hole) onto the bottom dowel. If the coax is already tied to the dowel ends, it will be necessary to put the tube over the dowel before attaching the box. If you attach the box before putting the tube on the dowel, you will have to remove the antenna from the dowel ends in order to get enough slack to get the tube over the bottom dowel.
• Put a sheet metal screw through the terminal strip, through the box, and screw it into the hole in the 18 inch tube. The tube should be positioned so that the antenna wire is not offering any resistance, but not so far up the bottom tube that the screw enters the dowel.
• With a single screw holding the box to the tube, align the box so that it is sitting squarely on the tube. Drill a hole into the aluminum tube through the remaining box hole. Insert a second screw into that hole, and screw into the tube. The aluminum box is now securely mounted on the tube.
• With the box mounted on the tube, and the antenna running out of the box and around the ends of the dowels, the tube can be pulled away from the center of the antenna, taking up any slack in the antenna coax. If the antenna is unbalanced around the perimeter of the antenna, adjust it at this point. Snug up the cable ties on the dowel ends. Come down about an inch from the end of the tube over the dowel, and drill a hole to hold the last #8 screw. This screw must penetrate into the dowel.
• Mechanical assembly is now complete. All remaining electrical work takes place inside the box. The box is a rather cramped place. The three coax cables are somewhat rigid. I find that a 30 watt pencil-style soldering iron works well for soldering connections in the box
• Connect the three coax braids together, and connect them to the ground lug of the terminal strip. This means that the box and the 18 inch aluminum tube are grounded. I have built loop boxes where the feed line is connected exclusively to the secondary of the impedance matching transformer. Ground is therefore not common between the feed line and the loop shield. I could not tell a difference in loop performance. Either way seems equivalent.
• Connect the two antenna center conductors to two nongrounded terminal strip lugs. Connect the trimmer capacitor to the same terminals. I find that the terminal strip is more than strong enough to support the trimmer.
• Select one side of the trimmer to be the loop output. It can be either side of the trimmer.
• If you are not using a matching transformer, you are done. Electrically, the trimmer is connected across the loop wires and the output signal is taken from either side. If you wish to add the transformer, follow the remaining step.
• Through experimentation, I have found that with #75 ferrite material on a 1/2 inch core, the transformer primary is 5 turns, and the secondary is 15 turns. The primary, the low impedance side, is connected to the loop output side of the trimmer. The other end of the primary goes to ground. The high impedance side (15 turns) connects between the feed line center conductor and ground. I use solid hookup wire to make the windings.

Checkout and Adjustment

Connect the antenna to an antenna analyzer. Set the analyzer to the desired frequency on the 80 meter band. If your analyzer provides antenna reactance (X) data, adjust the trimmer for resonance, that is, reactance equals zero. If not, adjust the trimmer for the lowest SWR. If you are using the matching transformer, you should be able to achieve a 1:1 SWR. I add or subtract turns on the secondary until I get a 1:1 SWR (once I am at resonance). If you are not using the matching transformer, then the point of lowest SWR is probably not the point of resonance, but they should be close.

Assuming the addition of a transformer to match the loop to the coax and provide a 1:1 SWR at resonance, the 2:1 SWR bandwidth was measured to be 36 KHz.   Once you get approximately 50 KHz away from resonance, the loop signal (and noise) output will drop by several dB. This is not really a problem, as most all receivers have more than enough gain to compensate. Still, if you want maximum output from the loop, you should adjust the resonance point to your desired frequency.

On-the-Air

It is almost a necessity to use a radio that has the ability to accept a separate receive antenna. Fortunately, this feature is becoming standard on most all recent vintage radios.

There are a number of ways to mount the antenna. For inside use, you could simply lean it up against a wall. The antenna should be kept vertical, and potentially rotated to null out local interference. I had a used wooden spool for holding coax that provided a good mount though the center hole. I simply laid the spool on its side, and placed the 3/4 inch aluminum tube base in the spool hole. You could also take a piece of wood, such as a foot long section of a 2X6, and drill a 3/4 inch hole in it to act as a base.

Outside, the simplest mount would be to push the 3/4 inch aluminum tube into the ground. the only down side of this approach is that the tube will get filled with dirt. A variation on the this theme is to drive a short length of 5/8 inch tubing into the ground, then slip the antenna tube over the 5/8 inch tube, since the two sizes telescope. Now the antenna tube will stay clean, and the antenna can be easily rotated. I have also used an elastic cord to strap the antenna to a deck railing. The antenna could also be hung from a low tree branch with a short length of strong string.

Some sources suggest putting a loop on a rotator so that the nulls can be easily moved. I have never had this setup, so I cannot comment on its value. It is interesting to use the loop in the shack, however, where it can be turned by hand. I find this especially interesting right before sunset. So long as there is daylight, it's a good bet that most signals are arriving via ground wave, at a very low angle. As you rotate the loop, the broadside nulls will be quite obvious. There should be very little advantage in raising a loop high off of the ground. Operation at ground level is just fine.

I prefer the loop outside. I have mounted mine on a wooden deck handrail, a few feet off of the ground. This gives me the best results. Inside my second story bedroom shack, there is too much local noise (computers, TVs, etc), and I believe that I lose a few dB of signal (and noise). This is compared to being out in a flat field, almost 100 feet from the nearest building.

Ελικοειδής κεραία μεσαίων

Φορητή κεραία μεσαίων

I wanted a good 160m antenna to work stations while static mobile. After finding out that a commercial one was around £50 I decided to make one. The only expense was a reel of enameled copper wire of .75mm which was less than £10 & had enough wire to make at least 2 coils for 160m with enough spare for an 80m coil.

 Other bits were all found in my garage/workshop, such as some old alloy tent poles & a piece of plastic water pipe. 
                                                
To mount my antenna, I had a friend make me a piece of stainless steel tube , 6 ½” Long by 1 1/8” diameter external & approx ¾” internal diameter. The tube had a 3/8” thread at the bottom to connect to a mag-mount.
  This would allow the antenna with a base tube of less than ¾” diameter, to be mounted to the car.

 The antenna was made using 4 alloy tubes, 1 plastic tube & a 5 foot whip. Once constructed, the antenna would split into two pieces for storing in the car.

  The bottom 2 feet of the antenna was made using 2 pieces of tubing, one nested inside the other. Then I nested a 2 foot length of water pipe over the bottom section. The water pipe was just over ¾” diameter & the tubes were drilled & had a nut & bolt fastened through them so they would remain rigid. Finally I added another one foot of aluminum tubing. This would be one half of the antenna.

   The next half of the antenna was another piece of tubing, just over 2 feet long, with a whip stud fitted so I could use a five foot or six foot whip at varying heights to enable full coverage of 160m.

   I wound about 115 feet of the .75mm enameled copper wire onto the plastic former.
This was close wound & taped on. The ends of the wire were attached to the top & bottom of the aluminum tubes either side of the plastic tube. Then the coil was covered in some blue heat shrink to seal it.

When operating, I found the total height of the antenna was about ten foot tall on my local net frequency of 1.972 MHz   . The antenna was taller than this when used in the DX window, around 1.845 MHz  

  This antenna works well for me & I have worked stations all around the UK & Europe so far. I am currently working on a base loaded antenna for 160m. The antenna was a homebrew one which only stood about 5 feet tall. I’m going to take some wire off the coil so I can gain some height by using a larger whip.

  Check out the pictures of my Homebrew mobile antenna. I also plan to make one for 40m in the future. This will be made so that I can use the top alloy tube & adjustable whip from my 160m antenna. I hope to make the antenna about the same overall size. A smaller coil will be the main difference.

Aerial Support

Base Section

Centre Section Former

Centre Section Coil

Centre section Heatshrinked Coil

Top section

Whole Aerial

Κεραία loop για λήψη

To make this loop take 20 feet of RG59 coax. Half way along the coax , At the point that will become the top of the loop, break the shield for 1 inch. Place a variable trimmer capacitor (400pf) inside a waterproof project box. I mounted mine on some wood, with a hole drilled for the trimmer shaft to sit in. I glued the trimmer cap to the wood.

Next, join the 2 centre conductors to the 2 trimmer tabs (see photos) Join the braids to each other & the braid on a short piece of RG59 to connect to a surface mount SO239 .
The centre of this short piece of coax should join the centre of the S0239 with the variable trimmer.

Finally, adjust the loop for resonance using an analyzer. Start with the trimmer screwed in, then gradually screw out, using an insulated screwdriver.

You can make formers for your loop & mount on a fence like myself, or on a rotator.

Good Luck,

M0VEY, Phil.

Loop Trimmer Capacitor

The Trimmer capacitor & way of joining the coax to make the receive loop operational

Οικονομική κεραία 160m - 6m

Preamble
It was developed simply because of my own personal circumstances which meant I often had to play radio from restricted QTH's. My employment in discrete antennas sure helped me out, as not only did I have demanding customers for discrete antennas but I find myself missing amateur radio when operating from hotel rooms abroad or "digs" in the UK!
Of course I've done all the large antennas from home but it's too easy and I wanted to have a bit of a challenge from building COMPLETE indoor stations totally indoors and so the following has been in use for the past few years by myself almost daily.
The antenna is nothing more than a simple 2.4 metre square loop "drawing pinned" to the internal brick wall of the spare bedroom. Yep, thats right, the inside wall of the spare bedroom - ideal for flat dwellers, hotel rooms or whinging neighbours!
It is currently used with it's base height at 3m AGL.
The loop has a simple switched inductance at the top of the square loop and uses a simple coaxial stub to tune the antenna. An additional variable capacitor placed across the feedpoint can be used to fine tune the resonance of the antenna. The basic configuration is shown below.

At J1 or J2 the coaxial stub can be replaced by a good quality ATU, preferably one which doesn't use toroids!

A balun is not required and at the most a simple choke style balun made from about 6 turns of coax and about 6 inches diameter can be used to attenuate radiation from the feeder cable. Good quality ATU's are expensive and often cost around 300 pounds sterling so to cut costs the simple coaxial stub can be use to utilise the self inductance and capacitance to from a tuned matching circuit to provide a 50ohm match to the amateur tranceiver. It is also possible to connect a variable capacitor at either the beginning or the end of the coaxial stub in order to provide across band fine tuning. The antenna is fed by simple 50 ohm coaxial line.


Here's the description of the antenna at various bands.
160m
Use 100uH inductance at the top.
A coaxial stub which is open ended RG58 style coax is 48cm long.
Open ended means just dangle a piece of coax at point J2 with braid to one side and centre to the other. The end is left "open" or with a simple variable capacitor for fine tuning. Results have been reasonable for local working but not amazing. Generally I qso 50 miles or so.
80m
Using ZERO inductance and a 139cm stub plus 5 watts I have made plenty of UK/EU qso's with this antenna. Obviously larger antennas are better but expect your signal to be about 2 s-points down from stations using full size antennas and assuming the same power output.
40m
Using 10 watts I have had some QSO's around the UK but I am not keen on this band so my experience is limited. Zero inductance and a 39cm stub is required.
30m
THIS BAND AND QRP IS BRILLIANT! 5 watts rarely fails to get a QSO. I have only operated in the daytime when I have had numerous QSO's around europe with just 5 watts with no hassle. Just use a 30cm stub and zero inductance.
20m
Numerous european and north american qso's made in the direction of radiation of a normal quad loop. Use 15uH of inductance for best results.
17m
Use a 3cm stub and zero inductance.
Numerous north americans, the middle east on 1 watt and Russian ragchews are easy.
Due to the radiation pattern of the antenna in my QTH I am best placed for north American and middle eastern QSO's.
15m
All over north america, the carribean and some south Americans with 10 watts.
1 watt to the Lebanon and recently my first call was answered by the 3B9C expedition using 25 watts of ssb. Countless europeans of 5w cw. North Americans worked by the truckload. Brazil also worked. Use zero inductance and 6cm of coaxial stub.
12m
Plenty of north americans and europeans. Use 9cm of RG58 stub and zero inductance for best results.
10m
Plenty of north American, the caribbean and the middle east as well as Europeans on 29MHz FM and I only used a max of 25 watts. Brazil also worked. Use 15cm of coaxial stub and zero inductance.
Remember the quad loop up on all these freqs is like a horizontal dipole and as such is directional.

HOWEVER we can perform a useful trick here - use 60uH of inductance and you can rotate the radiation in the opposite direction and with vertical polarisation! A rotatable indoor dipole without rotating it! This time you use 60uH of inductance at the top of the loop and approx 20cm of open ended coaxial stub.
I've worked gawd knows amounts of north americans, south americans, central americans, the middle east, europeans and Russia and south african stations with only 10-20 watts ssb.
Remember all of the above has taken place outside the sunspot maxima.
6m
In the summer sporadic E season it is typical to be able to work distance about 1500 miles. With only 5 watts cw/ssb I have had plenty of qso's at this distance with ease in the direction of the loop, which is in the same direction of a normal quad loop. I don't frequent 50Mhz all that often but it's pleasing to work exactly the same typical distances as every one else with this silly antenna!
NOTES
This antenna is like any other antenna - it's just a resonant circuit that radiates. The loop is the inductance and all you need to do is add the required capacitance to it to for a tuned and radiating circuit.
Rules of thumb are that the smaller the loop the more capacitance you need to resonate it. When tuning a coaxial stub you simply snip off 5mm at a time to provide the required match. Circumstances vary! So be prepared for slight differences to stub lengths etc. It is perfectly in order to have a reasonable variation in loop length and adjust sub length/variable capacitance in order to suit.
All polarisation is horizontal except for 160m where it is vertical OR on 28Mhz using 60uH inductance it also becomes vertical polarisation.
This antenna as described is in use by G0FTD every day and provides many happy hours of operating - so I know it works.
With the restrictions placed upon so many of us these days it really is pleasing to report a real antenna that allow just about anyone to play amateur radio with some sense of normality. Youngsters with parents can put up this antenna in their bedrooms, pensioners can (and have) used this antenna in retirement homes and restricted accommodation, students and flat dwellers can use it to at least continue their chosen hobby.
Noise is a big problem on receive and is often very restrictive. Sorry folks but it's a problem we ALL suffer from. Apart from operating at night when hopefully the local TV's have been turned off there's not a lot we can do.

CONCLUSION
Amateur radio IS possible under the most extreme circumstances so long as you don't expect all freqs to be 100% qrm free on recieve and that you'll have to expect your signal to be a few s-points lower. In practice it's not really a problem and then it pays to think that if your using a bit of "damp string" and a few watts your acheivement is damn good compared to all the sillywotsits who spent 1000's of euros/pounds/dollars and massive
effort for towers/kilowatt amps/expensive aluminium tubes to make yagi's blah blah.
It's all relative folks - so enjoy amateur radio under your own circumstances and stop feeling left out!
A BETTER LF ANTENNA.
I have also tried a 4.2m x 2.4m loop with excellent results on the LF bands. It seems to outperform the previous antenna on the LF bands by quite a large margin. The antenna is a corner fed loop as shown here.

I have NOT tried coaxial stubs with this antenna. I have only use an atu for matching at the point of J1. Never use an atu at the rig end!
However, I have used computer modelling to assist me attain a high degree of accuracy with the previous antenna, so here are the recommended starting points for using open ended coaxial RG58 stubs.

Band		Stub Length
160m		70cm stub
80m		5cm
40m		19cm
30m		38cm
20m		48cm
17m		6cm
15m		7cm
12m		12cm
10m		5.5cm
6m		8.5cm

Δέκτης COLLINS 75A-2

The Collins Model 75A-2 Receiver is designed for the amateur bands in the frequency range of 1500 kc to 30 mc. The receiver provides facilities for the reception of CW, MCW, and AM PHONE reception. Two octal sockets have been provided for internal plug-in attachment of a Narrow Band Frequency Modulation Detector unit and a Crystal Calibrator unit which provides reference frequencies every 100 kc. Controls for these accessories are provided on the front panel and are wired ready to use. The receiver uses the double-conversion superheterodyne principal to obtain high image rejection. Stability is obtained by the use of quartz crystals in the high frequency oscillator stage and a Collins Type 70E-12 sealed VFO in the low frequency oscillator circuit. Additional features of the receiver are separate noise limiters for PHONE and CW, amplified AVC, crystal filter, direct reading dial with frequency readings accurate to within 1 kc up to 21.8 mc and 2 kc from 26 to 30 mc. Provision has been made to connect a blocking bias to the receiver to mute the receiver audio when the key of an associated transmitter is closed.

DESCRIPTION & SPECIFICATIONS

FREQUENCY COVERAGE - The amateur bands are covered as follows:

160 meters - 1.5 - 2.5 mc        15 meters - 20.8 - 21.8 mc
   80 meters - 3.2 - 4.2 mc        11 meters - 26.0 - 28.0 mc
   40 meters - 6.8 - 7.8 mc        10 meters - 28.0 - 30.0 mc
   20 meters -14.0 -15.0 mc

The above table shows the tuning ranges within which the amateur bands fall. The exact frequencies of the amateur bands are given in the latest amateur radio handbooks.
BANDSPREAD - The permeability tuning system employed in the 75A has been engineered to give linear tuning on each band. Ten turns of the vernier tuning dial cover each of the individual ranges shown above. Each division of the vernier tuning dial (which has 100 divisions) represents 1 kc on the 160, 80, 40, 20, and 15 meter bands, and 2 kc on the 11 and 10 meter bands.
ACCURACY AND STABILITY - Visual tuning accuracy to within 1 kc from 1.5 mc to 21.8 mc and 2 kc from 26 mc to 30 mc provided the vernier dial corrector (zero set control) is exactly calibrated at the centers of each tuning range. Extreme variation in plate supply voltage causes a change of only a few cycles in the CW note. Furthermore, the CW note is absolutely independent of all except the tuning controls Physical shock will not disturb the frequency unless the shock is severe enough to change the dial settings. The stability is available after a very short warm up.
IMAGE AND I-F REJECTION - The circuit design of the 75A receiver has inherently high rejection to spurious frequencies. Image rejection is a minimum of 50 db. I-F rejection is 50 db minimum.
SENSITIVITY AND SIGNAL TO NOISE RATIO - A 10 db signal to noise ratio and one watt of audio output is obtained with signal inputs of 2 microvolts or less.
SELECTIVITY - The crystal filter controls provide a bandwidth that is variable ln 5 steps from approximately 4 kc to 200 cycles at 2 times down (6 db down from the peak of the resonant frequency). There is only slight loss in gain caused by use of the crystal filter with the exception of the extremely sharp position which gives about 6 db loss. The fixed I-F selectivity provides a bandwidth of approximately 13 kc at 1000 times down (60 db down from the peak of the resonant frequency).
PHASING - The crystal filter includes a phasing control which provides a rejection notch for suppressing heterodynes. The range of rejection of this control has been extended downward to 250 cps or lower.
AUTOMATIC NOISE LIMITER - The 75A receiver contains a series type noise limiter which automatically adjusts its limiting threshold to all carrier levels.
CW NOISE LIMITER - A shunt type noise limiter with front panel control of limiting level is provided for CW operation.
AUTOMATIC VOLUME CONTROL - Delayed, amplified AVC gives constant output within 6 db for a change in r-f input from 5 microvolts to 0.5 volt. AVC is applied to the r-f stages and three i-f stages. The proper amount of AVC delay is employed for maximum sensitivity on weak signals.
SIGNAL STRENGTH METER - The S meter is calibrated from 1 to 9 in steps of approximately 6 db each, and for 20, 40, and 60 db above S9. Zero adjustment is provided. A reading of S9 is obtained with an input of approximately 100 microvolts. The AVC amplifier tube works into an unusually low value of load impedance which permits quick recovery from noise pulses or strong signals from the associated transmitter, thus allowing. fast break-in when the receiver is used to monitor operation of the transmitter.
AUDIO OUTPUT - 2.5 watts of audio power are available.
TERMINAL IMPEDANCES
INPUT - The antenna input circuit is designed for a nominal 50 to 150 ohms impedance but will accommodate a wide variety of antenna impedances, both balanced and unbalanced, without serious loss. Mounting holes for an Army type SO 239 coaxial connector are provided to allow convenient connection to coaxial transmission lines, such as RG-8/U (52 ohms) and RG-11/U (73 ohms).
OUTPUT - A 500 ohm output and two 4 ohm outputs (one of which is interlocked with the panel headphone jack) is available on a rear terminal board. The panel headphone jack is a four ohm termination so that any value of headphone impedance will function satisfactorily.
CONTROLS - The following controls are on the front panel of the receiver:
Tuning Control - RF Gain Control - Band Switch - Audio Gain Control - CW Pitch Control - Crystal Phasing Control - Antenna Trim Control - CW-AM-FM Switch - Off-Standby-On Switch - Noise Limiter-Calibrate Switch - Crystal Selectivity Switch - Zero Set for Tuning Control - Headphone Jack - CW Limiter Control
CIRCUIT - Dual Conversion superheterodyne. One r-f amplifier stage, 1st mixer stage, crystal controlled h-f oscillator, variable i-f filter, 2nd mixer, three fixed i-f amplifier stages, detector/AVC Rectifier stage, two audio amplifier stages, AVC amplifier/noise limiter stage, CW noise limiter, variable frequency oscillator, beat frequency oscillator, and power supply. All circuits concerned with the tuning process are permeability tuned and ganged to one control.
POWER SOURCE - Power supply self-contained. Requires 115 volt 50/60 cps source. Power consumption about 85 watts.
DIMENSIONS - CABINET - 21-1/8" wide, 12-1/2" high, 13-1/6" deep. The receiver chassis is mounted on a standard 10-1/2" x 19" panel and can be removed from the cabinet and mounted in a standard relay rack. Depth behind the panel is 13-5/16".
WEIGHT - 50 lbs.
FINISH - St. James Gray wrinkle.

ACCESSORIES

SPEAKER TYPE 270G-2 - An external 10 inch speaker, not furnished, is available, mounted in a matching cabinet. The speaker cabinet measures 15" wide, 11-1/8" high, and 9-1/8" deep overall. Weight 15 lbs.
HEADPHONES - Any good headphones may be used. The 4 ohm receiver output impedance provides sufficient signal level for low or high impedance headphones.
ANTENNA -Any good antenna may be used; however, the receiver input circuit is designed For antenna impedances in the order of 50 to 150 ohms. In most cases, the transmitting antenna will also be the best choice for receiving. Connections on the rear permit the use of both balanced and unbalanced lines. Mounting holes have been provided for installing a Coaxial connector. This allows advantage to be taken of the low noise pickup of coaxial transmission lines.
CRYSTAL CALIBRATOR - The type 8R-1 Crystal Calibrator is available on order. The 100 kc crystal oscillator in this unit provides reference frequencies every 100 kc. This unit plugs into a socket within the receiver. Operating voltages and controls are provided in the receiver.
NBFM ADAPTOR, TYPE 148C-1 - This unit is also available on order. With it, narrow band FM signals can be detected and fed through the receiver audio circuits. This unit also plugs into a socket within the receiver. Operating voltages and controls are provided in the receiver.

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Under the hood...

75A-2 CIRCUITRY

MECHANICAL

GENERAL
The 75A2 receiver is constructed in two major units. the receiver unit and the speaker unit. The receiver is constructed on an aluminum chassis. Both the receiver and speaker cabinets are constructed of heavy gauge steel. The receiver cabinet has a hinged cover utilizing inside hinges. Ventilation openings are punched in the sides and rear of the cabinet. The front panel is flush and trimmed for neat appearance. Both the receiver and the speaker cabinets are finished in a hard St. James gray wrinkle finish.
TUNING
The vernier tuning dial is directly coupled to the lead screw of the variable frequency oscillator thus eliminating any possibility of back lash. The iron cores that tune the RF, first mixer, first IF and second mixer stages are all mounted on a movable platform. This platform is geared and belted to the VFO shaft by means of split gears and metal belts thus giving ganged tuning. The slide rule guide pointer is cable driven. The BFO coil is placed for most efficient operation and a long shaft is used to connect the tuning capacitor with the panel knob. All other stages are fixed-tuned with iron cores.
BAND SWITCHING
Band switching of RF stages is accomplished by means of a multiple section switch gang. In addition to RF circuits, the band switch selects high frequency oscillator crystals.

ELECTRICAL THEORY

CIRCUIT - As shown in the block diagram, figure 1-1, the receiver has one stage of pre-selection. A high gain 6CB6 tube is used here because of its excellent electrical characteristics and desirable physical features. Following the RF stage is the first mixer of the double detection system. The signal grid of the tube, a 6BA7, is tuned to the received frequency, the injection grid receives voltage from the fixed high frequency oscillator circuits at a frequency within a band of either 2.5 to 1.5 megacycle or 5.455 to 3.455 megacycles removed from the received frequency. This oscillator voltage is supplied by a 12AT7 crystal oscillator tube. Since the high frequency oscillator frequency is fixed (by the quartz crystals) the output frequency of the first mixer tube varies. This necessitates a variable IF channel for the first intermediate frequency. Two tuned circuits are used in the variable frequency IF stage. The second mixer is a type 6BA7 tube, the injection grid of which is tuned to the frequency of the variable IF. To produce the second IF of 455 kc (fixed), the output of a precision variable frequency oscillator is fed into the signal grid of the second mixer tube. This oscillator employs a 6BA6 tube in a highly stabilized temperature compensated circuit followed by a 6BA6 isolation stage. The output of the second mixer tube is amplified by a 455 kc IF channel composed of three 6BA6 tubes. A 6AL5 tube as a detector and AVC rectifier follows the IF channel. The audio produced by the detector is amplified by 1/2 of a 12AX7 voltage amplifier and a 6AQ5 power amplifier. AVC bias is produced by 1/2 of 12AX7 tube in an AVC amplifier circuit. A type 6BA6 tube is used in a BFO circuit coupled to the detector input for CW reception. Single conversion is employed for the 160 meter band wherein the signal is amplified by V-1 and fed directly to the grid of the second mixer through the variable IF filter.
TUNING - Tuning of the RF stage, the first mixer, the variable IF stage, the second mixer and the VFO is accomplished by changing the inductance of the tuned circuits by means of powdered iron cores varied within the magnetic field of the coils involved. The tuning cores of all of the above stages are ganged together and are varied as one unit. The inductance of each coil is trimmed with a similar iron core whereas the capacitance trimming of each coil is done with a variable ceramic capacitor.
An unusual method of band change is employed in the 75A receiver for all bands other than the 160 meter band. In the RF and first mixer stages, the inductance of only one set of coils, the 80 meter set, is directly varied by the tuning cores. To change bands, the 80 meter coils are paralleled with tuned circuits having characteristics which will combine with the 80 meter coils to produce tuned circuits suitable for the new frequency range. Five sets of tuned circuits are used, one set for each band. In each case, however, the 80 meter coil is the only coil in which the inductance is directly varied by the tuning apparatus. Refer to the complete schematic, figure 5-5. The 160 meter band has its own separate antenna coil. The first mixer and crystal oscillator are not used in 160 meter operation. The high frequency range of the variable IF channel is produced by paralleling the tuned i-f coils with additional fixed tuned circuits.
The tuning ranges of the coils in both the RF portions and the variable IF portions are 1000 kc in the 160, 80, 40, 20 and 15 meter bands and 2000 kc in the 11 and 10 meter bands. The frequency coverages of the RF stages are:

160 meters =  2.5 to  1.5 mc     15 meters = 20.8 to 21.8 mc
   80 meters =  3.2 to  4.2 mc     11 meters = 26.0 to 28.0 mc
   40 meters =  6.8 to  7.8 mc     10 meters = 28.0 to 30.0 mc
   20 meters = 14.0 to 15.0 mc

The frequency coverage of the variable i-f stage is: 160, 80, 40, 20, 15 meter bands = 2.5 to 1.5 mc; 11 and 10 meter bands = 5.455 to 3.455 mc. In order to produce heterodynes suitable for amplification by the variable frequency i-f stage i.e., 2.5 to 1.5 megacycles or 5.455 to 3.455 megacycles, six high frequency oscillator outputs are necessary. These are obtained by the use of a crystal oscillator and six crystals (one for each band except 160 meters).
In each case, the high frequency oscillator output is higher in frequency than the received signal by 2.5 to 1.5 megacycles or 5.455 to 3.455 megacycles depending upon which band is being used.
Refer to figure 4-2. In order to get a 455 kc heterodyne for the second, or fixed, IF amplifier stages, it is necessary to introduce another signal to beat against the variable IF. Since the output of the variable IF changes from 2.5 to 1.5 megacycles or 5.455 to 3.455 megacycles, the output frequency of this new signal must also be variable and in the ranges 2.955 to 1.955 megacycle and 5.910 to 3.910 megacycles . These requirements are met by the use of a Collins 70E-12 precision oscillator which has a fundamental output frequency range of 2.955 to 1.955 megacycles . The second harmonic of the oscillator is 5.910 to 3.910 megacycles; the second harmonic output is used when the variable IF is 5.455 to 3.455 megacycles (when tuning in the 11 and 10 meter bands). The output of the variable i-f and the VFO are mixed in V-4 and the resultant 455 kc output is fed to the first 455 kc amplifier V-5.
The 455 kc intermediate frequency is amplified by a three stage amplifier, the output of which is rectified and sent through the noise limiter and audio amplifiers.
The beat frequency oscillator employs a 6BA6 in a highly stabilized circuit. The dial used in varying the VFO frequency is calibrated +1 and -1 kc; a feature useful in CW work for reading frequency. With the receiver tuned to zero beat, if the dial is set at +1 kc, add 1 kc to the vernier dial reading at zero beat for the exact frequency of the received station or if the dial is set at -1 kc, subtract 1 kc. The BF0 PITCH control allows approximately +/-2000 cps change from zero beat.
Summarizing the above description of the tuning scheme of the 75A receiver; the received signal beats against the output of a crystal oscillator and produces an intermediate frequency which varies across the band. This variable intermediate frequency is mixed with a variabLe oscillator output to produce a fixed 455 kc i-f signal. The 455 kc signal is rectified and the resulting audio is fed through an automatic noise limiter to the audio stages. Linear tuning is accomplished by the use of a cam wound coil, in the VFO, which has the coil turns spaced non-linearly in such a manner that linear movement of the tuning plug within the coil produces a linear frequency output of the oscillator. In addition, a mechanical frequency correcting mechanism is attached to the oscillator tuning slug. All coils which are tuned by movement of the tuning dial are wound similar to the oscillator coil.
CRYSTAL FILTER - Refer to figure 5-5. The crystal filter in the 75A receiver functions as follows: The 455 kc IF channel input transformer T-3 has a tuned primary which is tuned to the intermediate frequency. The secondary on the transformer is a low impedance coil, the center tap of which is grounded. One stator of phasing capacitor C-58, is attached to one end of this secondary winding while one side of the filter crystal is attached to the other end. A bridge circuit is formed by attaching the rotor of the phasing control to the opposite side of the crystal. This point of attachment must return to ground (or center tap of the secondary of T-3) to complete the bridge of the circuit. This is done through the SELECTIVITY control resistors R-18, R-19 and R-20 or through IF coil L-24. The bridge circuit is necessary to balance out the capacity of the filter crystal holder plates to prevent the signal from bypassing the crystal. If the point of attachment of the rotor of C-58 and the output plate of the crystal was returned directly to ground, the Q of the crystal would be too high, therefore, resistors R-18, R-19, and R-20 are placed in series with the crystal circuit t vary the Q. When the SELECTIVITY switch S- 2 is in the zero position, the crystal is short circuited and the selectivity is determined by the receiver circuits only. When the SELECTIVITY control is in position 1, the crystal Q is at its lowest point because of the return circuit through L-24 (a parallel tuned circuit having high impedance). When the SELECTIVITY control is in position 2, the Q of the crystal circuit is improved because of the lower value of series resistance and so on through positions 3 nd 4 until at position 4 the series resistance is at the lowest useful value and the crystal Q is highest with a resultant high degree of selectivity.
Because the phasing capacity is across L-24, detuning of L- 24 would normally occur when changing the setting of the phasing condenser. To neutralize this effect, an additional set of stator plates has been placed on the phasing capacitor to compensate for this detuning.
NOISE LIMITER - A series type noise limiter is used in the 75A receiver for phone reception. This limiter employs 1/2 (pins 1 and 7) of the type 6AL5 dual diode tube V-10. Refer to figure 4-3. Due to AC loading of the second detector, heavy noise impulses are automatically clipped from the positive audio peaks in the detector. The noise appearing on the negative side of the audio cycle is clipped by the noise limiter. In operation, a negative voltage produced by rectification of the carrier, is developed across capacitor C-84. This voltage cannot change rapidly due to the size of C-84 and R-42 through which C-84 is charged. This negative potential is placed upon the cathode of the noise limiter tube through R-41. The cathode is then negative in respect to the plate of the noise limiter tube and plate current flows. This plate current is modulated by the receiver audio. The modulated plate current produces audio on the noise limiter cathode (to which the grid of the audio amplifier section of V-9 is connected). The noise limiter diode will conduct as long as the cathode is negative in respect to the plate, however, when a heavy noise impulse is received, the plate is being driven negative faster than the cathode can follow (due to the time constant of R-42 and C-84). If the plate is driven more negative than the cathode, the tube will cease to conduct and no audio will reach the grid of the following audio tube. The audio cannot reach the cathode of the limiter tube directly from the bottom of the detector transformer because of the filtering action of R-42 and C-84. The percentage of modulation, at which the limiter clips, can be adjusted by changing the values of R-39 and R-40. Increasing R-39 and decreasing R-40 while keeping the sum of their resistances at approximately 100,000 ohms will raise the percentage of modulation at which limiting starts. In this receiver, limiting starts at approximately 35% modulation with sine wave input. Distortion will be evident on heavily amplitude modulated signals, particularly if clipping is used at the transmitter. Switch S-4 bypasses the audio signal around the noise limiter when receiving conditions do not require its use.
CW NOISE LIMITER - A separate noise limiter is used during CW reception. This limiter, a shunt type is bridged across the audio line to the 6AQ5 grid. This limiter short circuits the audio line on noise impulses above the level chosen by the operator. The value of limiting is adjustable by R-62, the CW LIMITER control. Refer to figure 4-3. A dual diode tube is used in this limiter. The adjusting bias applied to pin #1 is obtained from the main power supply. Capacitors C-86 and C-87 accumulate a charge so that clipping will occur equally on both the positive and negative portions of the audio cycle. This limiter is turned on automatically when placing switch S-3 in the CW position. If limiting is not wanted, the CW LIMITER control should be rotated to the counterclockwise position.
AUTOMATIC VOLUME CONTROL - The problem of blocking due to strong signals or heavy static is reduced by the use of an amplified AVC system and a low impedance AVC line. Refer to figure 4-40 The second triode section of V-8 is used as an AVC rectifier to produce the control voltage for the AVC section of amplifier tube V-9. The AVC voltage applied to the grids of the controlled tubes is produced by the voltage drop across resistor R-55 when plate current flows through the AVC amplifier tube V-90 Plate voltage for V-9 is obtained from the voltage drop across resistors R-36, R 37 and R-38 which are in series with the center tap of the power transformer to ground. V-9 will not draw plate current, however, with no signal input to the receiver because of approximately 11 volts of bias placed upon its grid by the voltage drop through R-36. This bias voltage for V-9 is taken from the end of R-32 through which the rectified carrier flows in opposition to the bias voltage. Thus, when the rectified carrier becomes strong enough to overcome the bias voltage on V-9, V-9 will draw plate current and produce a voltage drop across R-55 thereby producing AVC voltage in proportion to the strength of the received signal. The bias on the grid of V-9 is high enough to produce adequate delay in the generation of AVC voltage to allow the receiver to function with full sensitivity on weak signals. Resistor R-33 and capacitor C-81 form the time constant in the AVC circuit. R-34 and C-82 and R-35 are used in a degenerative circuit to prevent the AVC amplifier tube from responding to low audio frequency. The AVC is turned off by opening the plate circuit of the AVC amplifier tube V-9. Tubes controlled by the AVC bias included V-1, the RF amplifier, V-5, V-6 and V 7, the 455 kc IF amplifier tubes.
AUDIO AMPLIFIER - Two stages of audio amplification are employed in the 75A receiver. The first stage utilizes the second triode section of V-9 in a resistance coupled amplifier arrangement. A type 6AQ5 miniature pentode power amplifier tube is used in the audio output stage. This stage is biased with fixed bias obtained from the voltage drop produced across R-38 in the center tap lead of the high voltage transformer secondary. The 500 ohm secondary of the audio output transformer is tapped at 4 ohms to excite the voice coil winding of a speaker directly. Both the 500 ohm and the 4 ohm outputs are terminated on the rear of the chassis on terminal strip E-3. Headphone connections are also made to the 4 ohm tap. When the headphones are plugged into the headphone jack J-l, the speaker is disconnected and a 10 ohm loading resistor is connected across the 4 ohm winding in parallel with the headphones to load the 6AQ5.
148C-1 NARROW BAND FREQUENCY MODULATION ADAPTOR - The Model 148C-1 NBFM adaptor employs a type 6AU6 tube as a limiter and a type 6AL5 tube as a frequency discriminator. The limiter tube provides constant input to the discriminator tube due to the high value of grid load resistance (R201). The discriminator circuit used in this adaptor relies on the phase difference be tween primary and secondary in coupled circuits. A 90 degree phase difference exists between the primary and secondary potentials of a double tuned, loosely coupled transformer when the resonant frequency is applied, and this phase angle varies as the applied frequency varies. The potentials at either end of the secondary winding with respect to a center tap on that winding are 180 degrees out of phase. When the center tap of the secondary is connected to one end of the primary, the potentials between the other end of the primary and each end of the secondary will reach maxima, one above and the other below the center frequency. At the center frequency, the resultant difference of potential between the two is zero. These potential differences vary at audio frequency rate when a frequency modulated signal is applied to the discriminator input. The audio frequency voltage is taken from the diode load resistors and sent through a de-emphasis network, R208 and C208, to pin number 2 of the power plug P203. The unit is ready to operate at all times by merely throwing the CW-AM-FM control on the 75A-2 Receiver to the FM position which disconnects the AM detector and substitutes the FM adaptor. The regular receiver audio circuits are used for FM reproduction. Operating voltages are provided by the receiver.
8R-1 CALIBRATOR UNIT - The 8R-1 Calibrator Unit uses a type 6BA6 tube in a Pierce circuit. A 100 kc crystal is used to give check harmonics at every 100 kc spot on the receiver dial. Capacitor C-301 is provided for zero beating the calibrator output with a known frequency standard such as a broadcast station in the tuning range of the 160 meter band or WWV at 2.5, 15 and 30 mc. The calibrator receives its operating voltages from the 75A-2 Receiver power supply and is turned on when the LIMITER control on the 75A-2 Receiver is placed in the CAL position. The output of the calibrator unit is coupled to the grid of the r-f amplifier tube V-1 through the capacity between pins 3 and 4 of crystal calibrator socket E-5.