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I tried to make a dual-band (2m/70cm) dipole this morning by following the instructions on this page: http://www.amateurradio.bz/2m-70cm_vertical_dipole_antenna.html, with the rather major significant exception of placing the antenna's elements in a groove I routed out of a 2x4 and I'm using bare #10 copper wire instead of stainless steel.

I plugged a VNA into it and the antenna's lowest SWR was ~1.09 at ~124MHz. I tried it in a couple different environments and orientations and made sure there wasn't anything conductive right near the antenna (other than the wood, which I'm expecting to reduce the effective velocity factor of the wire.

Since I have no shortage of #10 wire, I started trimming the ends in 1/2" increments. I got the lowest SWR to 1.04 at 148MHz... but only while horizontal! Orienting the antenna vertically causes the lowest SWR point to jump to 1.52 at 153MHz.

What's happening here? Why was the tuning so far off? Is it the wood? Is it the copper wire? Why does orientation affect the tuning so much? I know I've got a lot of variables here, but is there one that is obviously the most important difference?

William
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It was the wood!

It may also be the fact that I'm using copper wire instead of stainless steel rod, but I can't test that as I don't have access to any stainless.

Further experimentation this evening show much more predictable results. While I thought the 2x4 would be relatively transparent to RF, simply being near the 2x4 seems to cause the whole system to seem electrically much longer than it is, thus making the antenna very difficult to tune.

At first, I thought that it was conducting RF to the wood because the wire was bare, but I cut new elements just now out of my solid-core #10 THHN, this time leaving the THHN on everywhere except where the bolts contact the wire and had basically the same results. Bending the wire away from the wood such that the tips of the dipole (where voltage is highest?) caused the match to jump from 118MHz to 137MHz.

My best guess here is that the lumber is not fully dried and whatever water left in it is causing it to be a (poor) conductor, causing some capacitative coupling. I'd still love for anybody with more experience to chime in though, as I'm just guessing and am pretty new to making antennas.

William
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I suppose you were standing on the ground when you tested your antenna so rotating the elements in a vertical position made them closer to the ground effectively detuning your antenna.

To avoid that, the antenna elements must be above the ground at least 1/4 of the wavelength - in your case higher than 2m / 4 = 50cm at all times.

Another factor could be your feed line. Try coiling some coax near the feed point (simple RF choke) and see if your tuning changes - http://www.k3dav.com/rfchokecoil.htm

Paul S
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This is an interesting antenna with but with no information included about behavior of the unique part at 70cm.

For RF insight into the design, a horizontal 2 meter dipole for 144 MHz was modeled using #8 copper wire at 2 meters elevation. The 70cm portion was formed by wires from the feedpoint connections going down then to the sides beneath the 2 meter arms. This is an approximation of the "U" shaped elements described in the article.

Tuning for 144 MHz was easy. It worked out to be 19.055 inches for the long arms with a feedpoint separation = 1.05 inches. Total Width=39.16 inches.

Horizontally: SWR=1.5, Z=69 Ohms, Efficiency=77%, Gain=7.5 dBi.

Vertically: SWR=1.4, Z=68 Ohms, Efficiency=51%, Gain=2.6 dBi.

However, tuning for 446 MHz was critical and required radically different shape and dimensions.

Spacing between arms was much wider: 3.89 inches down and the length of the arms much shorter: 2.12 inches... with total width=5.29 inches.

Horizontally, the "L" shape still radiates like a dipole with a very minor vertical component.

Horizontally: SWR=1.05, Z=50 Ohms, Efficiency=73%, Gain=9.4 dBi.

Vertically: SWR=1.02, Z=51, Efficiency=60%, Gain=6.2 dBi

Note: The Z impedance of a vertical is not effected by feedpoint height. Not so horizontally. Z goes up and down around 72 Ohms (1.4 SWR) so tuning changes with height.

Dick Reid
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