Gerald Williamson writes:
>I wonder what your application is to need a line like this as it is unusual
> to see a need for a 100 or 150 ohm line?
I'm exploring alternative feeding schemes for a two element Gem Quad. After a
ton of modeling I'm convinced the factory setup results in tradeoffs I can
avoid. Actually, I'll be using the Gem Quad spider and spreaders and a few other
plastic bits but replacing everything else. The tradeoff that really convinced
me to go to a lot of extra trouble was seeing that tying all three feedpoints
together reduces 10m gain to a miserable 5db in freespace with a horrible
gain pattern. (Don't get me wrong, I'm very pleased with the Gem Quad as it
is the one decent antenna that meets my mechanical and dimensional constraints)
So instead of tuning for a common 50 ohm feed as the factory specifies I'm
looking at separate relay-switched connections to 100 ohm feeds on 20 and 15
and to either 67 or 50 ohms on 10 meters. (When tuned for a resonant 100 ohm
feepoint impedance on 10 meters the freespace gain is almost a full db lower
than when lower resonant Z values are arranged, according to NEC2). This led
to thinking about going with the traditional 1/4wl transformer setup or with
2:1 unun/balun combinations (maybe 1.3:1 unun for 10m). For 20 and 15 a 100
ohm balanced line seemed a natural and using short pairs of 50 ohm coax
sprang to mind. I figured I didn't care about velocity factor changes as the
termination would be nominally the same as the line characteristic impedance,
but I didn't have a feel for how much moisture would increase the loss or
change the impedance of the line. In fact I have to wonder about the
mechanism for impedance change as the water can not get much closer to the
center conductors and it's their spacing from the shield that's determining Z,
isn't it? In any case, the notion of having one leg with a big difference in
velocity factor compared to the other hadn't occurred to me. Ouch!
I may reconsider the 72ohm balanced line I have on hand and stick with the
1/4wl route for these two bands. Apart from the need for an exact length that
may or may not be a convenient run to the switchbox, the potential problem
with this stuff (the type with two #12 stranded conductors very closely
spaced) is the breakdown voltage of the insulation between the conductors:
it seems doubtful for QRO power levels.
Regards,
Pete
KS4XG
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