Κατασκευάστε ένα πολυμπαντικό δίπολο.

Just got my Kenwood TS-120S up, with the help of Extra Class Phil/W4VPI. I have a 40m dipole setup, with a 949E tuner... But, as expected... All Ill be able to work is 40m. Sigh, Id love to work more than one band. So, I do some extensive research. I find many plans for multiband antennas. Everything from fan dipoles, to trap dipoles to the renouned G5RV. I decide Im going to make a G5RV, so I get the materials I need and proceed in building one! I built it to specs for an 80m full size version... only to find out that silly me has not the yard to setup such an antenna. I already finished building it, but the legs are *way* too long... Oh well, I go ahead and position them anyway, its not anywhere near right, but I give it a whirl. Cant tune squat. So much for that. The next 2 days I search more. And in one article I read, a ham posted basicly ""just put up as much wire as you'r yard can handle and feed it with balanced line"". Well, that sounded too simple, and a big claim for less than $30 worth of supplies... when there a lots of antennas claiming to do multi band for $100 and up!

Well, what the heck! I already have the legs and the twinlead, all it would be is a simple matter to modify the current faux G5RV to a balanced fed doublet! The pictures will outline the process... Keep in mind Ive deleted some of the pictures that were not necessary to this write up... as I took them during the G5RV build. You will see the pieces I used for the pigtail, which was no longer used with the resulting antenna. #12 solid copper wire with black plastic shield and PVC couplings from Home Depot, 300ohm twinlead from Radio Shack. Total invested: About $30.





Time to start drilling holes, in the following pictures pay close attention to where I drilled.



I measured 1/2 inch back to drill 1/4 inch holes. The straight couplings are insulators for the legs, the "T" is for the apex.



One of two leg insulators finished.



Both the leg insulators and the "T" drilled... there is only one hole on the base of the "T", two on each end, and one on the top for the pull string.



Top view of the "T".



Back view of the "T".



Here is about how much you want the split end of the twinlead to extend from both ends. Use a pencil and mark about where you want to make a hole to secure it to the "T".



I used the un-tipped end of a princes iron to melt a hole where I marked for the mounting of the twinlead to the "T".



The resulting hole.



With a single tie-wrap, the twinlead is secured to the "T" with the hole we made.



The core of the "T" is built... now for the legs, and a few finishing touches.



A double knot in some nylon rope prevents the pull line from coming out the top of the "T" which is now the apex of the antenna.



This is how the antenna apex will be suspended.



A loop is tied and the end is melted to prevent fraying. The apex will be pulled up with another rope, of your choice.



About 4 inches of insulation is stripped in segments from the copper wire, this will go through the apex and end insulators, both ends stripped the same.



The first leg is attached to the "T".



Twist on the exposed twinlead wire and soldier. Almost done.. This setup will allow for flex to be used either in an inverted-v (my choice) or as a flat top.



The end insulators are twisted and soldiered the same. Were ready to run the twinlead into the house, put up the apex and test the system out!



I did "encase" the open ends of the "T" in Scotch Super-33 electrical tape (anything less is junk). This will offer some protection from the elements, at least for a year in my estimate.



The antenna is up! Lets play the "Find the dipole" game! Seriously, look... you can find it! I ran it up with black polyester flat rope, and also used the same to secure the legs. The rest of any white PVC that was exposed was fully taped up. What I ended up with is what I was hoping for, a dipole that you cannot detect in the yard. A covert antenna.



The apex is only up about 20 feet, though after testing I was able to tune from 80 to 10 meters without a problem. 80 and 10 I can get down to as little as 1.5:1, 40, 20 and 15 I can tune almost flat.



Can you see the leg, and the rope that secures it?



How about now?



The leg end insulator and rope. Covert.



From this angle, about all you notice is some twinlead coming out of the tree, if you were looking. Most would likely not even notice, Im sure.



So for review, about $30 in supplies and about an hour of messing around (about half an hour if your serious about the project and not taking pictures), you will have a 66 foot (33 feet each leg) dipole antenna. Mine is feed by about 75 feet of twinlead, though you could use the entire 100 foot roll, or more if you need to splice for length without problems. The twinlead is connected to the back of a tuner at the "balanced line" post.





UPDATE: 14 MAR 07

I had the wrong measurments prior to this update, the legs are 33 feet each, giving a 66 foot tip to tip doublet.



UPDATE: 14 MAR 08

I have come to find, although it can tune up 80m with a Z-100, the efficiency is lacking so I do not suggest anything other than 40m - 10m including all WARC bands. I did run 40m, 30m and 20m CW and PSK recently with 100w CW and 25-50w PSK and have had VERY good reports.

Εικόνα ενός πολυμπαντικού διπόλου

        Multiband Dipole

Center insulator


Side view of insulator.

Coil at each end.


A view of each end of the dipole



View of the insulators for 2 wire and 3 wire.



The 68 turns of 20 gauge wire ( door bell wire ) was used on the 80m coils. Start 1" from the end.

Added the 160m coil on the end. Had to add to the 80m leg. It went from 59" to 97".
The 160 leg is 56" long and made of 14 gauge insulated wire. The coil is 2" O.D. and 10" long PVC. There are around 120 winds of 20 gauge wire and start 2" from the end. They are similar to the 80m coil for holes and hardware to attach wires.
I have the center up at 40ft and the legs hanging down to around 15ft.
You may have to tweak the lengths to fit the part of the band that you use the most. I have mine for the top of each band for voice.

Αόρατη πολυμπαντική κάθετη κεραία

I used to live in an apartment complex with 980 units, all of which were outputting some electrical noise. This made copy on the low bands very hard. A small receiving loop helped; it has very deep nulls and made listening on 80 and 160 more pleasant.

An update on the story is warranted, since I wrote this page a long time ago but this page has become fairly popular since. I built this loop because of a particular totally crippling noise that wiped out signals on my doublet on 80m and 160m. After a few months of very successfully using this loop and moving it around the living room slightly, I realized that I was pointing the nulls in slightly different directions depending on where it was in the room!

If you ever have done radio direction finding with a loop like this, you might recognize that moving a few feet to the side and having to adjust the null direction means that you're getting very close to the source. It's straightforward triangulation. If the source is far away, you have to move a huge distance before you really have to point in another direction. The conclusion? As they say in those dumb ham radio horror movies (they have those, right?), the source was coming from inside the house. As it turned out, it was my desktop computer in the workshop/office room. I didn't really use that while I was operating, so I just turned it off. But, I suppose if it hadn't been my computer, the story would have ended differently, and the loop really did a good job nulling out my own computer.

The receiving loop goes on a tripod in the living room.

The loop is 3 feet square. It's two turns of #20 insulated wire on a bamboo frame, resonated with a 0-300pF capacitor for 80m (needs 150pF or something, I think), with a 500pF fixed cap with alligator clips for 160m. (550pF required, total. You can see the 500pF cap dangling from the velcro straps in the picture. The antenna is currently set up for 80m.) I measured the inductance of the loop with my MFJ analyzer, then picked the resonating caps. Coupling to the loop is done by a 16" square loop that connects to coax.

Click the schematic for a larger version.

The other end of the coax has a 1:1 isolation transformer to isolate the coax shield from the rest of the grounds. I think this is not at all necessary, but it's there for an eventual try at a low noise receiving inverted L that is transformer coupled and has a separate ground. This goes into a changeover relay that is currently footswitch operated but will eventually be operated by the TX GND on the FT-857 and/or its cousin on the TS-440S. You can't transmit into this antenna; the relay lets me stomp a switch and go into transmit.

Here's the RX loop / main antenna T/R relay (black) and some coax switches.

I've found that this loop needed to be in the middle of the living room for proper nulling. This put it about 10 feet from the radio. Now, it needed frequent adjustment for resonant frequency and null direction, so I needed to be able to see the radio's S-meter from across the room. I found that the MTR setting for the backlight color on the FT-857 is useful for this; the backlight changes from blue through purple to red as signals get stronger; this plus audio lets me peak the tuning and find the deepest null from too far away to see the meter box.

I've since moved out of the apartment and even before I did, I pretty much stopped using the loop. Cleaning up my own devices was sufficient to make many of my noise problems go away. But if the noisy computer was my neighbor's computer instead of mine, the loop probably would have been a permanent requirement in my apartment station. I do still use it from time to time for direction finding. I tracked some noise at my new house to the elevators in an apartment building near here. I can just imagine if I'd been living IN that building. I would have to put up with this sound at 59+40dB. Sometimes you need all the help you can get.

Πολυμπαντική κάθετη κεραία

I have to thank Con, DF4SA, of Spiderbeam for giving me the opportunity to inexpensively try their new 18m telescoping pole. I turned it into a great 160m through 30m antenna; as a bonus it makes a good tree surrogate to hang a 15m dipole from.

Up from the base of the sixty footer. I needed two sets of guys on this one.

The antenna is just the bigger brother of the 40 foot pole vertical and I took essentially the same approach to this antenna, but had a chance to make some improvements. This antenna ended up being a little more mechanically complicated, because guying the bottom section only just seemed much too floppy. I elected to put a second set of guys at the twenty-five foot level, which seems to be sufficient for 25MPH or so winds. It hasn't been through a major windstorm yet, and I'd probably prefer not to find out what happens. I may add a third set of very light monofilament guys very close to the top to cut down the sway, but we'll see. The guys on this antenna are attached using hardware you can find at your local home improvement store. I used flexible drain pipe couplings as nice guy attachment points.

A strap is wrapped several times around the shackles and held with a small bolt and nut.

The original bottom guy attachment ring had the three stainless steel shackles threaded onto the top hose clamp before assembly. It can be seen on other pictures on this page. Lee, W9OY, has pointed out that this very well might be asking for trouble, because the screw might slip on the clamp and the bottom guys would be lost. I've since significantly reinforced the attachment of the shackles with a bolted stainless steel strap.

Snap links are used at the ends of the black 3/16" Dacron bottom guys for easy detachment if I want to take the whole thing inside (or to Field Day!) The rubber protects the pole surface from damage. The upper guy attachment ring (another flex drain coupling) is very similar, except that there are three snap hooks attached to it to clip onto bowline knots at the end of each of the top ropes. Up top, I used 3/16" black Dacron because it was what I had on hand. Smaller rope could have been used, but the only small twine I had was white nylon, and I felt that the black guys were much more attractive. All guy anchor points are 16" tent stakes driven into the lawn. The bottom guys come down 7' from the antenna base and the top guys come down 14' from the base.

With the guy ropes attached, there's a significant downward load on all of the sections below the upper guy ring, so I made use of the Spiderbeam supplied stainless steel clamps on the sections below the upper guy attachment. I originally left them off above it because it takes significantly longer to put this antenna up because of all the extra ropes and clamps to deal with, but I had an incident in 25MPH wind where the top half of the antenna fell down into the bottom half. A thirty foot fall is no joke, and I was merely lucky that there wasn't any damage, so now I'm going to take the extra few minutes to put the clamps on all the way up.

A painted and hacked up garbage can houses the matching networks.

For the *current* matching solution, see the stepper driven matching switch project.

The antenna is naturally resonant around 3.6MHz, but I wanted to use it on 160m, 80m/75m, 40m, and 30m. I didn't add 60m matching because I found I don't really use the channels there. This antenna is slightly more than 5/8ths wavelength long on 30m, and so the antenna is not particularly good on bands above 30, but that's no hindrance here. The matching networks all fit into the top three-quarters of a kitchen-size garbage can which I mounted to a wooden board. I painted the whole thing with textured green spray paint for plastics so I didn't have a stupid shiny white garbage can in the backyard. If I had been in less of a hurry to put the antenna together, I might have built another house-like matching box, just because it's cuter, but the painted can isn't unacceptably ugly.

I added some extra radials as I was putting this antenna together. The radial system consists now of the original 27 radials out to the edges of the 40' x 40' backyard, as well as about ten new ones that extend to the front yard around the house, each about 85' - 90' long. I wanted to collect a little more ground current on 160m. I haven't done an A/B test with the new vertical and the new radials to see if they make a real difference, but the new additions probably result in almost twice the area covered with radials with less than 0.02λ between tips. Consider the fact that I'm using a very small ground system if you try to duplicate the matching networks below. My tap points, especially on 160m, probably will be significantly altered if you have a good ground system. I've used all the space I've got.

The matching mess... a thing of beauty.

I used a hodgepodge of matching networks on this antenna, just as I did on the forty foot one. I hand wound all inductors, some of them on lathed forms. Matching network electrical details can be found here:

matching_networks_18m.pdf

The networks are switched by a motorized wafer switch just like the one on the other antenna, except that I wanted a mechanically more rugged switch arrangement, because the contacts on the 40-foot matching switch were stressed a little too much by the wires attached to the switch. The resulting switch is pictured below

The ruggedized bandswitch managed to get itself dubbed "the moonlander".

The wafer switch is mounted in the center of a polystyrene drain pipe cap. Twelve pairs of #6 stainless machine screws and brass nuts stud the outer perimeter of the pipe cap, and each screw bolts a solder lug to the inside of the cap. Each switch contact is connected by a small piece of AWG 18 bare wire to its corresponding bolt. That way, all connections are made simply by clamping another solder lug to the outside and the bolts take all the stress. The wafer switch has two wafers and 12 positions. One wafer switches the antenna wire to all the "ant" ports on the matching networks. The other wafer switches the "tx" ports to the coax. All the matching shunt elements just go to the common ground point.

One switch position is hardwired inside the housing to be a bypass for general RX. There are four switch positions (1-4 in the photo) for 160m; three are currently hooked up. There are three positions to cover all of 80m, one position for 40m, one position for 30m, and a couple more unused ones that I intend on saving for matching a more efficent 160m antenna option, maybe an inverted L or T antenna with some top loading. As usual, I used a servo controlled gearmotor to drive the switch. The feedback pot is hidden from view, coupled to the bottom of the switch. A drawing of the bandswitch arrangement is available here:

moonlander_drawings.pdf

The aluminum piece below the moonlander provides a common ground point. I attached a copper buss wire around the perimeter (underneath the aluminum flange) with stainless hardware so I could solder things to ground.

The 160m base loading and matching coil is about 25 turns of #10 wire 6TPI

The base loaded antenna on 160m only has about a 20kHz 2:1 SWR bandwidth per coil tap. I can cover the bottom 60kHz of the band at present and will probably make the fourth switch tap for 160m be up high in the phone band for keeping in contact with regional friends. The 160m coil is wound on a lathe-grooved piece of grey PVC electrical conduit and is MUCH larger than necessary. A drawback to this is that one turn is too much inductance to get slightly overlapping tap points, so I added some small 2" diameter inductors to fine-tune the resonances so I could cover all frequencies between 1.800 and 1.860. I may eventually go to a smaller, less tightly wound inductor. I was using about twice this inductance to base load the 40-footer as an experiment, and just had this coil, so I used it. This antenna is much better than the 40-footer on 160m. I measured this antenna as having a 6dB field strength improvement over the 40 foot one, more than expected.

Three tap points and modest reactances cover all of 75/80m

Moving on to the 3.5-4.0MHz range, I found that I only needed three networks to cover the whole band. The first two taps both share a shunt inductor (about 10 turns, see the .pdf). The lowest tap has a small series inductor as well. The middle tap is simply a hairpin matching arrangement; that is, the antenna is slightly capacitive by itself and the shunt inductor steps the impedance up to 50 ohms. The highest 75m tap is a "capacitive hairpin" arrangement. The radiator is slightly inductive by itself, and a shunt 500pF doorknob capacitor steps the impedance up to 50 ohms. With these three switch positions, I can cover the whole band with quite low SWR.

The forty meter network needs a big coil, relatively.

The high base impedance of the 40m antenna required a big coil for its L network. (I think it's about 8μH as installed). The step down L-network uses a large shunt coil on a PVC form and a nice Hammarlund variable cap that I've had for years and never found particularly useful because it has a maximum capacitance of 120pF. I needed about 60pF for this L-network, though, and it was perfect. This network easily covers the whole band. The antenna used on 40m could potentially use a parallel-resonant circuit as well, but the L-network seems to work just fine. The impedance on 40m didn't agree entirely with the EZNEC model and I ended up having to take a couple of turns off of the coil to get midband resonance.

The thirty meter network with a homebrew capacitor.

On 30m, I found that I'd run out of capacitors and really only needed a very small one (50pF) and also knew that even if I eventually got an amp, I wouldn't need high voltage capabilities on 30m with the 200W power limitation, so I built my own. The capacitor uses copper foil for plates and some overhead transparency material (I think that stuff is polyester) for the dielectric. It seems to work fine. I found on 30m, again, that my EZNEC modeled impedance was significantly off from the real one. I had to reduce the shunt coil by several turns in order to find a match midband on 30m.

Rounding out the electrical details, there's a common mode choke at the feedpoint.

I added a choke at the feedpoint mainly because I've got a lot of noise around here. I'm going to try some more aggressive choking in the future, but it's a start. I never had any problems with RF in the shack with this antenna... not surprising with a ground mounted vertical and only 100W, maybe, but I worry about RF out of the shack here. This is a modest choke, maybe up to 1kΩ across 160-30m, but I can't really measure it. It's a decent guess based on winding a few turns on the thing and measuring with my MFJ-259B, but I didn't check a few turns over the whole range... I just swept and made sure the finished choke was |Z|>650Ω across a large swath of HF, and it is. I also choked the control lines for the moonlander, though they're mostly electrically isolated at the matching box end anyway. The motor frame is plastic and so is the pot frame, so the motor drive system is really only lightly coupled to the RF-energized parts.

All of the HF antennas at N3OX.

I'm plagued with a treeless backyard here, despite the fact that all my neighbors have nice tall trees. This is what got me started on all this fiberglass pole stuff anyway. I figured I'd make the best of being forced to guy this pole at thirty feet from the base, and I added a small plastic clothesline pulley at the top guy point and a dacron rope loop to hoist a 15m dipole between the pole and the house. A similar rope loop holds the house end of the dipole. I have a snap hook on either end so that I can easily drop and detach the dipole when I'm going to take down the 18m pole. The two fiberglass poles in a vee in the background are two sides of a fine-gauge 20m delta loop. The box at the bottom corner of the triangle houses L-networks to match the antenna on 17m and 20m. So, now I have capabilities on 160m, 80m, 40m, 30m, 20m, 17m, and 15m from my small lot here in DC suburbia.

The antennas as seen from the street on the side of my house. As usual, the big pole has a sun-heated curve to it.

I've got to say I'm very happy with this antenna. The 160m improvement was shocking. I've at least got "if I can hear them I can work them" capabilities with this antenna, which makes it easy to have nice QSO's with the larger EU stations. On 80m and 40m, I've busted big pileups with 100W. I'm sure once I figure out some lowband RX antennas, this antenna won't be quite as impressive (as I won't be able to work everything I hear with only 100W) but right now I'm pretty excited to be putting out a pretty big signal on the lower bands.

I'd recommend this pole to anyone in my situation at this point in the sunspot cycle. The pole is inexpensive compared to other 60 foot vertical supports. It's temporary so it's good for renters or those in restricted situations, or those with difficult zoning and building code situations. It's still generally wieldy enough for one person to install and telescope up, though it's more difficult than the forty-footer because the sections are nearly as tall as I am.

I usually leave the pole standing when collapsed.

My best time to telescope it up, put on all the clamps and adjust the guys is about fifteen minutes in the dark. That's from the state in the picture above, add a few minutes if you really must take the thing inside when you're not using it. It's not invisible, but it's certainly fairly low profile visually and is completely invisible from afar at night. It's a little imposing when you're right under it during the day but it's not out of the question to only put it up in the dark. If you need some low-band help, it might just be the ticket.