[TenTec] OT 75 Ohm twin feeder

[Steve Hunt]

Jerry,

Interesting information.

I just took my short length of "twin RG-400" and tried spraying it with 
water. I put a 47 Ohm resistor across one end and found the frequency 
where it was acting as a quarter-wave transformer (31MHz). I measured 
the resistance at the other end with the feeder dry - it was 104 Ohms - 
consistent with my previously measured Zo of 69 Ohms. Then I sprayed it 
liberally. The measured resistance fell almost immediately by 30 Ohms to 
74 Ohms. As it has dried out, the resistance has slowly climbed back up 
- after 5 hours it has reached 98 Ohms. I also noticed that when it was 
wet the reactance had climbed to about 7 Ohms, although it had been 
pretty much zero when dry.

As I said in an earlier posting, the HexBeam application would allow the 
feeder to be protected from the weather within the Centre Post, so it's 
not really a major problem.

Steve G3TXQ


> A lossy feed line won't look bad checking SWR at the transmitter. It
> looks good even when the antenna has fallen down. It should be checked
> with the antenna end open or shorted, though a short can radiate so an
> open can make a higher SWR. Forward power - reflected power is twice the
> transmission loss with the antenna end open circuited. At VHF I found I
> had to terminate a balanced line with a copper sheet as well as a wired
> minimum length short to see a high SWR at the transmitter end.
>
> Water can have a significant effect on antennas. Water has a fairly high
> dielectric constant and a molecular resonance about 650 MHz. That's why
> water in 9913  causes lots of attenuation at 432 MHz but doesn't show a
> high SWR.
>
> Then real water even rain water isn't pure and is the worlds best
> solvent and so gathers stuff like SO2 and NO2 from the atmosphere to
> make sulfurous, sulfuric, nitrous, and nitric acids that all give it
> considerable conductivity. And dirt is dissolved even better with these
> acids. Reduced producing of these produces from fuel combustion has
> reduced the acids, but they are not totally gone and it doesn't take
> much to make water conductive. The Collins 821A-1 applied 60 KV peak to
> a column of water and that water had to be distilled and deionized to
> keep the leakage below what would boil the water in the 1/2" PA boiler
> feed pipe. Hot water left as steam in a much larger pipe so the
> transmitter itself distilled the water.
>
> So dust on the insulator will become conductive in rain or heavy dew (or
> a British fog).
>
> The best cure of rain (ice effects are stronger) is prevention by
> keeping water effect points dry. That might involve enclosing home made
> twinlead in heat shrink tubing (but watch for RF losses in the tubing),
> possibly with bits of low RF loss rope in the cleavages of the two
> conductors on both faces to hold the water further from that high field
> strength area. I know teflon heat shrink tubing exists, but I don't know
> of a similar Teflon rope. Though I suppose Teflon tubing would serve
> that spacing purpose well. The whole purpose of the spacer is to keep
> water away from the strongest electric fields. Its important to seal the
> upper end of such a feed line to keep water out and the lower end would
> be best for water by being open to drain, but insects that nest in
> cavities will nest there if left open.
>
> Low impedance twin leads tended to be oval for that purpose. There used
> to be a 200 ohm transmitting twin lead that was also oval in cross
> section. The flat twin leads don't quite concentrate the majority of the
> electric field in the plastic so are more sensitive to water (and ice).
>
> 73, Jerry, K0CQ
>
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