[Elecraft] Zip Cord, Balanced Feeders, and Coax

[James R. Duffey]

Zip cord is lossy due to the plasticiser that remains in the poly vinyl
chloride insulator to make it flexible. The dyes and other fillers have
second order effects on the loss. One would need to use an entirely
diffferent material as insulator to significantly reduce the loss. So until
we have zip cord made out of teflon, polyethylene, or other low loss
dielectric, the loss will remain high. Compared to a rig, feed line is
relatively low loss and hardly a good place to scrimp when setting up a
station. 450 Ohm ladder line can be obtained for $0.15/ft or so and good
coax at $0.35 per foot or so. While zip cord is considerably less than this,
the savings are not much when considered as a percentage of the amount spent
on a station. 

Balanced feeders are better than coax when used under high SWR for two
reasons, high impedance and low cost. The loss in a line, coax or balanced
feeder, depends on the impedance of the line, among other things. Under the
same SWR conditions, 300 Ohm coax or 450 Ohm coax with a center conductor
the same size center conductor as the equivalnet balanced feeder conductors,
would have the same loss (actually less loss) than balanced feeder of the
same imnpedance. Why don't we see 300 Ohm or 450 Ohm coax? Calculate the
outer diameter of 450 Ohm coax made with an 18 gauge center conductor.

One can find coax that nominally has the same loss as balanced feeders
though. The Antenna Book lists Times LMR 1200, 7/8 inch cable TV line and
3/4 inch Heliax all with losses comparable to 450 Ohm window line at HF.
Nonresonant antennas at HF could be fed with these feedlines as effectively
as with balanced feeders, albeit at a cost of 10x or more over the window
line. But the large diameter cable TV coax is widely available, sometimes
for the price of hauling it away, so it is worth considering.

It is difficult to maintain the low loss of balanced feeders after erecting
them. Moisture and dust settle on the feeders to increase loss. It can be
difficult to maintain a large enough spacing from the feeders to prevent
unequal feeder currents. Getting it in and out of the house requires a bit
of creativity. 

I don't recall who called the G5RV an extended double Zepp on 20 M, but it
is not. It is a three half wave (1.5 wavelengths long) "dipole" on 20 M,
having a low resistive feed point. The extended double Zepp is traditionally
1.25 wavelengths long or so, and has a reactive feedpoint.

I hope that I have not added to the confusion. - Dr. Megacycle KK6MC/5
______________________
James R. Duffey KK6MC/5
Cedar Crest NM 87008 DM65