[TenTec] OT 75 Ohm twin feeder

[Dr. Gerald N. Johnson]

On Wed, 2008-06-18 at 08:45 +0100, Steve Hunt wrote:
> Hi Fraser,
> 
> I have some of Westlake's "400W 75 Ohm twin" where both conductors are 
> totally embedded within the dielectric material. Assuming the field 
> between the conductors is mostly constrained to the dielectric, I 
> wouldn't expect much effect from rain. I've never had any of his Heavy 
> Duty stuff so I don't know whether the same would apply.
> 
> But you raise an interesting point, and I'm wondering whether the same 
> problem might occur with my twin RG400 configuration. Time to go and 
> spray the test assembly with water from the rain butt! If there is a 
> problem I can always put the feeder within the HexBeam centre post - it 
> just takes a bit more effort!
> 
> 73,
> Steve G3TXQ
> 
> Fraser Robertson wrote:
> > Hi Steve et al.  Some years ago I put up horizontal parallel dipoles
> for 20/15/10m and wanted to feed them with heavy duty 75R twin feeder 
> to avoid the weight of a centre balun and coax.  Westlake no longer 
> stocked HD 75R twin but said their black/red DC power cable worked as 
> a substitute.  I found this did indeed work ok, with a balun at the bottom 
> end, except whenever it rained the swr shot up.  Question: does rain 
> have a similar effect on 'proper' 75R twin.73 Fraser G4BJM
> >   
> >

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