Wednesday, October 23, 2013

Tuning the TH1vn + 17 Meters Hybrid Trap Dipole

I have gone as far as I will go for the present with multi-band tuning of the TH1vn trap dipole with its 17 meters addition. First I will finish up the discussion of its mechanical design since I have made changes.

The feed point was a week point since the original TH6DXX driven element has nothing more than clamps to attach wires to each half-element. This is suitable for the Hy-Gain BN86 voltage-balun plus beta match on the yagi, but not so much when used standalone as a trap dipole. I replaced my rat's nest of wires with a more structured and robust rat's nest, as shown in the adjacent picture.

As you can see I continue to use pieces of the PVC pipe in additional projects since it is cheap and strong, and sitting in my basement ready for use.

The wires to each half-element clamp run through the pipe to the SO-239 UHF connector. The connector nestles inside the cut-out I made in the pipe centre. The centre pin of the connector fits through a drilled hole in the back of the pipe (not visible in the picture). The connector is screwed onto the pipe, with one screw doing double duty as a post to connect a wire and round lug. It sits diagonally across the pipe since there is no practical way to mount it to the pipe otherwise since the pipe is about the same width as the connector.

The pipe is slightly bent due to the screw tension. While there is no danger of breakage I plan to add a spacer to eliminate any risk of a wire touching the large steel clamp that holds the element halves.

Wires run from the same feed point to hose clamps on the 17 meters element. The stainless steel hose clamps are rated down to ¼-inch pipe (or aluminum rod in this case) but would not work properly without the aluminum shims I added between the clamps and wires. After the picture was taken and initial tuning the solid 12 AWG wires to the 17 meters element were replaced by 14 AWG stranded wire. There was some wire motion in high winds and I want to avoid a mid-winter failure due to metal fatigue.

Here is a somewhat-blurry picture of one half-element of the completed antenna on the tower, sitting 9 meters above ground. You can see how the PVC spacers and plastic ties hold the 17 meters element a constant distance from the trap dipole.

The spacers are secure enough with the antenna mounted and the wind blowing but are prone to rotation when the antenna is moved around during lifting, mounting and tuning procedures. Keeping the aluminum rod directly below the trap dipole is more for aesthetic appearance rather than functional performance. I may eventually snip the ends of the ties.

Tuning the antenna is tricky. The TH6DXX was not designed to accommodate additional parallel dipole. In particular the narrow spacing between the traps for 10 and 15 meters allows little tuning room on 15 meters. To make gross tuning adjustment meant lowering the antenna to the ground. This is easy enough since the tower is short and the antenna lightweight, but would push the spacers out of position as it dragged on the ground or bumped into the tower and guys.

Smaller adjustment I made on the tower by tying the antenna to the side of the tower and accessing the parts of the antenna to be adjusted. This consisted of either the end tip of the trap dipole element or the lateral position of the 17 meters rod.

During this procedure I left the 10 meters tuning alone. It still resonates up in the SSB segment the SWR is a bit higher at the bottom of the band where I primarily operate. Now at 1.6 to 1.7 SWR at 28.0 MHz it work just fine. The other reason I left it alone is that lengthening the inner section of the antenna would lower the resonance on 15 and 20 meters. As I said above, there is little room between the traps to shorten the antenna for operation on 15 meters. This matters since as is now stands the hybrid antenna resonates a little below the band bottom, at about 20.950 MHz. This is fine for CW but the SWR is between 2 and 3 across the SSB segment.

Tuning on 20 meters should be straight-forward since the ends of the element are easily telescoped and have almost no effect on 10 or 15 meters. It did not turn out to be quite that easy.

As I mentioned in the previous article, after adding the 17 meters element the minimum SWR point dropped from 14.0 MHz to 13.7 MHz. Due to the intervening traps this results in a narrow-band SWR response and and SWR above 2 even at 14.0 MHz. The reason for the major impact is circled below.

The end of a dipole is particularly sensitive to the presence of metal, in this case the 17 meters aluminum rod. The same occurs with tuning on 17 but this can be independently adjusted with the wires between the coax and rod. This is why, in general, antennas are shortened with capacity hats and linear loading section connected towards the ends of the dipole elements.

My "toy model" of the hybrid antenna in EZNEC captured the mutual sensitivity at the tips but not the magnitude of the interaction. The model told me to reduce the length of the 20 meters element by 20 cm at each end. I did this and ended up with a resonance at 14.6 MHz! With some quick mental math I decided to try again with 8 cm shortening and the result was right where I wanted it, near 14.1 MHz.

On 17 meters the SWR dipped up around 18.5 MHz but was still broadband enough to work well at 18.068 MHz. Even so, when I spliced in the stranded wire (as described above) I took the opportunity to tune the antenna. It now resonates around 18.2 MHz with an SWR that is low across 17 meters.

In summary, the SWR minimums on 20, 15 and 10 are not far different from what they were before. The main difference is that the SWR bandwidths are somewhat narrower, especially on 20 meters. Plus 17 meters, of course.

Even a dummy load has a 1.0 SWR so this is not an indication of performance. However I can report that it works quite well. Not better than a dipole, but it loads, hears and transmits just fine. It was usable during the tuning -- which was done intermittently over the past week -- and was available for use during the fantastic recent band conditions. I have now rocketed past 150 DXCC countries this year with my QRP plus wires station. The most recent was VU2 over-the-pole on 10 meters, using this antenna.

Since I already have antennas for 10, 15, 17 and 20 meters was this exercise worthwhile? Yes, it was. Apart from the challenge of doing it I do have additional reasons. More on these in a future article.

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