80/40m Trap Dipole

Computer model Measurements of the dipole are: 9.92m from feedpoint to trap, 7.15 MHz traps 0.2m long, 3m from trap to end, and the capacity loads have a radius of 0.5m at the ends. Both sides have the same dimensions. All charts are based on 8m height – increasing height produces lower angled nodes of high gain. For best DX results, height should be about 25m on 40m or 80m. Scroll down for the 4NEC2 file.
80/40m Trap Dipole (VE3NCQ, you’re S9+20)

Everybody who has heard me on 80m knows that I’ve been stuck using a compromised antenna for 80m. My small city lot didn’t have enough room for a dipole, so I went up instead. The large maple tree in the backyard served as a good support for a vertical wire, which I could only throw high enough to be resonant at about 4.2 MHz. Ground radials were out of the question, so a counterpoise was installed along the property line and fence.

The wire was sensitive to wind, often becoming wrapped in leafy branches and driving the SWR crazy. It worked better than the 40m dipole on 80m and 20m, but was never wonderful, particularly on receive. The vertical was very sensitive to noise, but not much else. It was clear that there had to be a better way, and then I modelled the vertical on the computer and saw the problem.

Using 4NEC2 software, the model indicated a gain of -11dB, excluding SWR and coax losses. The pattern that was produced sent the power everywhere except in the direction needed, and probably warmed up plenty of earthworms as well. It served me for a log full of DX contacts and contests despite the poor performance, but it was time to try modeling something better.

Computer modeling provides some advantages. No materials have to be cut too short and wasted, antennas can be tested in the middle of the night when nighthawk hams would otherwise risk having to explain to the police why they are climbing trees in the dark, and it’s easy to adjust parameters to tweak a specific characteristic. After trying out various concepts, I settled on a trap dipole, since it would probably fit between the trees, even if didn’t sound very easy.

SWR Plot Using 300 ohm twin-lead smoothed out the SWR and widened the bandwidth. Values are highest on 160m, all easily handled if feedline length doesn’t cause excessive voltage at the tuner. SWR losses with twin-lead are minimal on all bands except for 160m, but topband reports have been encouraging (until the tuner arcs). On 20m, RG-213 coax would have 5.7dB of combined loss over a 50′ run, but twin-lead losses amount to just 0.9dB, barely perceptible. Antenna impedance at the feedpoint is about 39 ohms on 80m and 60 ohms on 40m.

The design was based on trap coils of high-Q with inductance chosen to produce 80m sections that would fit in the available space. The middle section was simply a 40m dipole, which was expected to provide little improvement over my old dipole. Insulators were made from sections of PVC pipe, with holes drilled in the ends to secure the wires.

An important consideration in the design was multi-band use. Even with a tuner, SWR losses from using coax would prohibit the use of 160m, and reduce performance on the higher frequencies. Using 300 ohm twin-lead would raise the resonant SWR to about 4:1, but provided much wider bandwidth, and the SWR on all bands could be handled by the tuner. The penalty was completely offset by the advantages, particularly when loss calculations showed dramatic losses with even the best coax when the antenna was operated off-resonance with high SWR at the feed point. Only ladder line and open line would provide lower loss, but the twin-lead was attractive at 9¢s; a foot, and I had some already. Using balanced feedline meant that each side of the dipole had to be kept symmetrical to avoid feedline radiation.

Space constrictions prevented the 80m extensions from being longer than 3m. By adding capacity hats on the ends, less inductance was required in the trap coils, with better current distribution in the shortened wires. There is more than one combination of inductance and capacitance that will resonate, but only one inductance value will work with a specific wire length – in this case, 3 meters end-loaded. With satisfactory gain patterns and SWR graphs plotted on the computer, it was time to start cutting wire and building traps.

RF Trap The traps generated some concern and curiosity by my kids, while my wife wondered why I had a roll of duct tape hanging from the sky.

The trap coils needed to be 15uH, which were constructed by winding 13 turns of insulated #12 household copper wire around an 85mm cylinder, secured with duct tape. The trap capacitors had to be homebrew as well. The value of 33pF would have to withstand some high voltages at some frequencies, so I decided to try using lengths of scrap 75 ohm coax as capacitors. The impedance didn’t matter – we’re only interested in the capacitance, which was estimated to be 21pF per foot. Once connected to the coils, this turned out to be a good choice that easily facilitated tuning the traps. Simply connect the shield to the outside of the trap, and the inner conductor to the 40m side of the trap, and make sure it’s a little long. It might even be possible to use the same piece of coax for the coil and capacitor, with greater difficulty tuning it to resonance, and less flexibility balancing the coil inductance with the wire length. However, variations in coax capacitance between different kinds might yield a combination that works, and could be used where the luxury of changing the outer dipole wire length can assist tuning such a system to resonance.

The traps were suspended from the workshop ceiling and excited with a grid-dip meter that I found on eBay for $5, that sadly turned out to be missing the coils. That was easy to fix, at the expense of scale accuracy, but coupling an oscilloscope and frequency counter handily displayed the resonant frequency of the traps. It was amazing how the signal started jumping out of the flat line on the scope as the GDO approached trap resonance. Trimming the coax bit by bit reduced the capacitance and allowed resonance to be set perfectly at 7.150 MHz on both traps. The excess coax was coiled and taped, and the trap was attached to the outside of a 20cm piece of PVC conduit.

All the required wire was unrolled and measured using chalk marks on the driveway. Copper would be fairly expensive, but using a roll of stainless steel picture frame wire that I had left over from my days in the photo business would provide better support without much penalty from the higher resistance. Each part was assembled and secured. The feedline was attached to the steel wires at the centre insulator with cable clamps, and coated with silicon. The hardest part was managing the fragile end-caps. They were made of #10 copper X pieces, with an outer web of #14 copper soldered on. It didn’t take much of a collision on the way up to distort them, but that seemed not to affect performance – at least until the wind started blowing branches into them. The end-caps should probably be made of stronger materials if branches may come into contact with them. Wet leaves don’t seem to affect the performance, but having the end-cap whacked over in the wind onto the grounded support wire by a flapping branch did compromise the performance.

While assembling the antenna, my daughter asked what the coils were. I told her that they were traps. As her eyes grew wider, she asked what I going to trap. I replied, “RF.” With her voice rising in concern, she asked, “Daddy, why does the RF have to be trapped?” She thought I was going to harm some poor creature, so I set her mind at ease by explaining that certain signals are blocked by the traps, so they can’t get into the end wires, making the antenna more useful.

Feedpoint The trees were just a little too close together, so the slack was taken up by pulling the feedpoint to one side, anchored to the 2m antenna a few feet away. No distortion to the pattern was noted as a result of this offset.

Eventually, after several attempts to get a string over the right branches, the antenna finally got off the ground. The trees were still about 2m too close together, so I had to climb up one tree and carefully route the supporting wires over the branches. Nearly every time I tackle a project, other projects become necessary first, such as cleaning the garage to reach the ladder, then fixing it before it can be used, using some tool that wasn’t where it belonged. I once went to change a light switch that was interconnected to something else that needed replacement, and by the time the project was done 3 months later, the entire house had been completely rewired up to modern code. This time, while I was up the tree anyway, I spent a day pruning out many dead branches in the sick tree, and clearing a path for the wire.

Once erected, the antenna had just a little too much slack with no support in the middle, so it was attached with string to the same support used by the 2m Echolink antenna. This did the trick, and the feedline was routed down the roof and into the shack through a small hole. When connected to the tuner, the radio popped to life.

Signal levels on 80m were incredible! Stations that had to be pulled from the noise before were now 20dB over S9. My first signal report was excellent, and the ONTARS net control station thought I was using an amplifier. The best part was that all signals sounded more powerful as well, and the noise level was actually much lower. I had feared that a better antenna would just pick up more noise on 80m, but I hadn’t counted on the receive being improved so much. This antenna has been used on all bands from 160m to 10m, and has worked better than expected. Participating in a couple of contests proved that this antenna had entered me into a whole new realm of amateur radio.

Further refinements could include using copper wire instead of stainless steel, and maybe feeding it with ladder line or open line. This could be used without a tuner if a 1:1 balun is placed at the antenna using coax for feedline, but this would have additional SWR losses if used with a tuner on other bands. Ideally a balanced tuner with a 1:1 balun on the input, and balanced feedline should give best results. It’s always nice when there’s still room for improvement.

Computer aided design based on a good idea, getting more wire into the air with end-loading, and using low-loss feedline all contributed to an end result that far exceeded expectations. It’s even performed well in the rain, and the twin-lead hasn’t been as sensitive as anticipated, with no RF in the shack. It was even discovered laying on the metal eavestrough one day with no apparent harm done. The trickiest part was explaining to my wife that I hadn’t hung a roll of duct tape from a wire over the house. She thought it was a squirrel trap.

4NEC2 Plot of Gain and Pattern The 4NEC2 plot in 3D shows antenna gain and pattern for 3.75 MHz. Software allows the pattern to be viewed from any direction, calculated for any frequency. A 19dB improvement over the vertical was noted, which would be the same as running about 8000 watts into the old antenna, with the same boost for received signals.


NEC antenna file

4NEC2 Software Download

Orchard City ARC online Coax Calculator

RAC online RF Coil Designer

If you build one, please send me some feedback!