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Dual-Band 2m/70cm PVC Antenna A quick note on the home-brew antenna I mentioned on the net a couple of days ago. The design is based on the "Dual Band Half-Wave Flower Pot Antenna" from VK2ZOI. His article on the web site is well written and has more or less everything you need to know - This is a picture of the completed version of this antenna (not yet weather-proofed). Ingredients are 1" PVC, RG58/U Coax and aluminum foil.

Here is my understanding of the theory of this antenna (which is what caught my attention, in addition to it being easy to build). Basically this is a half-wave "resonant feedline dipole" (RFD) at 2 Meters. The coax choke at the base which is self-resonant just below 140Mhz, isolates the feedline. The actual feed point for the half-wave dipole is the point where the coax shield stops and the center conductor continues. The dual band version is created by adding a sleeve (of aluminum foil) around the feed point of the dipole (the area between the blue tape in the photo). This sleeve and the center wires form quarter-wave matching sections which are end-feeding the two 1/2 wave dipoles in phase. (Actually, 180 degrees out of phase, but they are oriented in opposite directions, so the radiation is in phase). If the velocity factor (vf) of this coax is 2/3, with 440 MHz being near the 3rd harmonic of 144 MHz, the total length (matching sections plus two 70cm dipoles) adds up to (about) a 1/2 wave at 144 MHz. Neat trick. I guess another way of thinking about it is that the sleeve shields the portion of the field that is 180 degrees out of phase with the two collinear half wave elements.

The assumption that the velocity factor is 2/3 is probably not a bad one. Although the coax formed by the PVC pipe contains air, the PVC itself has a fairly high dielectric constant (around 4). Also, you would like the impedance of this coax to be on the high

side of 50 ohms -- the higher the better for the transformer action. This is achieved by the high diameter ratio (higher on the top section than the bottom). There will be some (negative) effect on 2 meters, but probably not a huge one. Effectively, the dipole is shortened a bit, and there is some small transformer action due to the aluminum foil sleeve. The net effect is probably some decrease in radiation resistance (and efficiency), but I didn't do the math. Another potential issue is the dissipation factor of the PVC (which acts both as radome and coaxial dielectric). I put some in the microwave for a couple of minutes and it barely got warm to the touch (compared to a cup of water next to it which boiled) -- so I don't think that is a concern. I measured the VSWR with an MFJ-269 antenna analyzer. I have never used one before, so I am not sure how much I trust it, especially on UHF. But here are the measurements:

Since the feed-line choke is resonant around 130MHz, it will not be totally effective at 440MHz (it will have high reactance, but not as high as the choke at self resonance). I think that the VSWR measurements at 70cm may show some evidence of this. The curve shape may indicate that there is some feed line current present. Because of this, I decided to try a modification. I built a second version with some ferrite cores added to form a sleeve choke. This is shown below, along with measurements. I used 4 toroids with #61 material (because thats how many I had).

Above - modified choke -- ferrite cores added. Below are the measurements with the ferrite toroids added. (Note that this is a second build of the antenna from the same specs -- so some variation may be expected).

There might be a couple of things going on here. The ferrite cores may have introduced some loss, but I don't think that is what is going on. Perhaps the 2 meter design was counting on some of the choke being part of the dipole, and the ferrites are actually working too well in this case (removing all of the air choke from the dipole). Conversely, at 70cm, removing the coil from an arm of the dipole is a good thing, and the match is better. On the other hand, it is likely that at least some of the changes are due to construction differences, since the antenna was built twice, once with and once without ferrite cores. As far as actual use, there is no detectable difference in S-units compared to a Diamond SRH77CA whip on a HT held in the same position -- but S-units are a pretty coarse measure and I would not expect any difference unless the antenna was simply not working. The Diamond claims 2.15 dBi -- about 0dB(dipole) -- for their whip on 70cm. The theoretical gain for the collinear half wave elements at 70cm described here depends on the spacing. In this case, it would be about 3 dB vs. a dipole. Since the antenna I built at least holds its own at 440 (vs the HT + whip) over a 40ft RG-58 cable (3-4 dB attenuation), it is probably actually providing gain on that band. All in all, a simple antenna to build and a fun learning experience. AF6ZE - Guy



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