Building an indoor multi-band trap dipole

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I decided to build the trap dipole described in An Attic Coaxial-Cable Trap Dipole for 10, 15, 20, 30, 40, and 80 Meters. As a start, I built a 10m (28.15 MHz) trap on a piece of grey PVC pipe. It is the first trap shown in the photo on the right (click on the photo to see a larger version).

Measuring trap resonance with an MFJ-259 HF/VHF SWR Analyzer

I needed to measure the resonant frequency of the trap so as to tune it to the correct resonance by adjusting the coil spacing. I looked at buying a dip meter but the only one I found was expensive. I had a home-built one that I had been given by Art, VE1EP, but it had no coils and I decided that getting it going and wiring coils for it was more work than I wanted.

The manual for my MFJ-259 HF/VHF SWR Analyzer says that it should be able to do the job by connecting the trap to the analyzer’s antenna connector but I was unable to get it to find any resonance at all. Finally, I tried using an RF generator, a digital counter and an oscilloscope but still couldn't find a resonance, no matter what I tried. I wondered if the grey plastic pipe was not good for RF (maybe had carbon in the mix). I eventually put the project aside since, if I couldn't measure the trap resonance, the project was a no-go.

Subsequently, I put up a simple indoor wire dipole using thin white plastic-covered wire that ran the 50 feet length of the attic and then draped along the end walls. It resonated at 5.88 MHz. The K2‘s internal antenna tuner was able to obtain a low SWR on all bands except 80 metres. Later, I trimmed the antenna to be just the length of the attic which was a much neater solution. It resonated at 8.72 MHz.

In 2012 I resurrected the trap dipole project and made a new 28.15 MHz trap on a white CPVC 3/4 inch pipe coupling. If the grey colour of the previous trap was a problem, this white CPVC should fix it. The coupler's outside diameter is 1.09 inches and the BASIC programme I downloaded that calculates these things called for 5 turns of RG58. The new trap is the middle one shown in the above photo. After constructing the trap, though, I found I was no further ahead since I still couldn't find its resonant frequency. However, a Google search for "measure trap frequency" found a YouTube video showing an analyzer (not an MFJ-259) measuring the resonance by using a single loop around the trap. I made a similar loop for my MFJ and it worked! See the photo on the left. Click on the photo to see a larger version but see later about reducing the coupling between the loop and the trap.

I found that the resonance of both the grey and the white traps was too low. By spreading the turns, resonance could be adjusted to between 25.8 and 27.0 MHz. I modified the white trap by decreasing the number of turns from 5 to 4.75 and this brought the resonance to the required value.

The result of measuring the 10-meter trap with the MFJ-259 is shown on the right. Click on the image to see a larger version. The MFJ measures SWR and resistance and both are plotted twice. One plot (light red SWR and dark red Resistance) is with tight coupling as shown in the above photo - the loop is placed around the trap concentric with the windings. The second plot (SWR light green and Resistance dark green) is with loose coupling - the loop is raised above the trap about level with the end. Two things are apparent: the SWR dips before the resistance; and the loose coupling shows resonance to be at a lower frequency than does tight coupling.

I know that when measuring resonance with a dip meter, a loose coupling is more accurate so I decided to go with that measurement. I was undecided about using SWR or resistance but I chose resistance since the measured SWR is likely the SWR of the loop, rather than the trap. Therefore the dark green line shows the result of tuning to 28.15 MHz.

Update: I was subsequently able to check the trap resonance with a working dip meter (see my Web page about dip meters) which gave a reading of 28.05, which agreed more with the MFJ’s SWR measurements than with its resistance measurements. I therefore adjusted the spacing of the trap windings to bring the resonance to 28.15 MHz as shown on the dip meter.

Building my “Ugly Balun”

I also constructed the combination RG58 choke and 1:1 balun that is described in the antenna article. It is designed to match the dipole to the coax and reduce or eliminate any RF interference in the house. The choke is shown on the right in the photo at the top of the page; it has 6 feet of RG58 on a piece of 3" PVC pipe.

However, I subsequently found which says that the balun should use 18 to 21 feet of coax (!) so I built a second balun, the “Ugly Balun”, which is shown in the photo on the right. This time I put a PL259 connector on both ends for convenience. Click on the photo to see a larger version.


Building my indoor trap dipole

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