[TenTec] OT 75 Ohm twin feeder

[Dave Kelley]

All good information, but one minor correction is needed.  The lowest 
molecular resonance of liquid water (or at least distilled water at room 
temperature and standard atmospheric pressure) is around 17 GHz, not 650 
MHz.  At a few hundred MHz and below the dielectric constant of water is 
close to 81.  As the frequency rises through the molecular resonance 
point, the dielectric constant drops steadily, reaching a value of 
around 2 at the upper end of the commonly accepted RF range (300 GHz and 
beyond).

Off-topic info:  By the way, water *vapor* behaves very differently from 
liquid water.  It has several resonant frequencies throughout the upper 
microwave and millimeter ranges.  Along with the resonances of oxygen 
and to a lesser extent other atmospheric constituents, it is responsible 
for the high propagation loss experienced in those frequency ranges.

73,
Dave NB4J

> 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
>
>