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[TowerTalk] re: coax as balanced line (really quad tuning)

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Subject: [TowerTalk] re: coax as balanced line (really quad tuning)
From: (Pete Soper)
Date: Wed, 30 Jul 1997 17:13:22 -0400
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. 


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