[Elecraft] Elecraft: RFI Problems

[Ron D'Eau Claire]

Bob, G3VVT wrote:
 
Not sure that the statement that it would be rather lossy is correct as I
have been led to believe that twin feeder is less lossy than coax and
particularly the 50 ohm version. Even the 75 ohm hard-line coax as used by
cable TV companies was always better than the 50 ohm coax in the loss
figures for a given diameter cable.
 
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Quite true about the relative losses, Bob. We've kicked this subject around
here in the past. It's been pointed out that, while it is true that  there
are dielectric losses in feedlines, feedlines operating at high SWR's are
more prone to resistive losses in the conductors than dielectric losses. 
 
Several people have pointed to the fact that the losses are directly
proportional to SWR as shown in all the transmission line loss charts,
suggesting that what the loss charts are showing are the increased ohmic
losses occurring at the current maxima points (current loops) along the
transmission line as the SWR increases.
 
Voltages and currents do depend upon power, but low power does not guarantee
low currents or voltages on a feedline operating with a high SWR. I melted a
nylon banana jack running 15 watts! The jack was in a grounded panel
connecting my ATU to a voltage-fed antenna. On the other hand, very high
currents can occur at the current loop in a feedline operating at high SWR
even at very low power levels. 
 
That's why "flat" feedlines having little or no standing waves are less
lossy. The current and voltage are directly proportional to the power
applied and do not swing through large extremes along the length of the
feedline. 
 
Feedlines with larger conductors fare better in general.  As you pointed
out, "hard line" and other heavy duty transmission lines have excellent
insulation. They also tend to have larger conductors with more surface area
to reduce the resistance to the RF flowing along the surface. 
 
I count myself lucky to be able to use open wire line with #12 conductors.
We all have to deal with compromises. But that's half the challenge of the
hobby - finding the compromise that produces the best results!  In my case
it's no high supports nor length enough! (Great feedline, marginal antenna!)

 
A lot of modern telephones are incredibly RF-sensitive, having amplifiers
and other circuits that have absolutely no RF protection. Your residual
problem may be simply direct pickup by the phone wires of the RF signal
acting as an antenna. 
 
Ron AC7AC