I have just been given a 1000' roll of shielded CAT 5 cable of seemingly
high grade. I would like to feed a far away start point to a 900 foot
Beverage but not exactly sure on how this will work. Do I tie all cross
pairs together? I understand there will be a need to have a small toroid
to match the Beverage itself and see the advisability of floating the
shield there. But do I ground the shield at the station end to try and
create a Faraday shield to reduce common mode pick up or does this only
work at AC and not at RF?
Just curious
Herb Schoenbohm, KV4FZ
On 8/13/2013 3:16 AM, Jim Brown wrote:
On 8/12/2013 2:10 PM, JC N4IS wrote:
50/75 BALUN
Thanks for the detailed post, Carlos. BUT -- please let's use the
right words to describe things so that people understand what you're
describing and how it works. I strongly suspect that at least some of
those things you are calling a "balun" are really a simple transformer
-- that is, a primary and a secondary with magnetic coupling between
them, and probably on a ferrite or powdered iron core. If it's a
transformer, let's call it a transformer. Likewise, if we have a
common mode choke formed by winding a coil of the transmission line,
it is a common mode choke, not a "balun." Using the word "balun"
confuses things, because that word is used to describe at least a
dozen very different things that I know of.
When we use the word "balun," it's a magic box that few hams really
understand. When we use the right word, most hams have a chance of
understanding what it does in a circuit. :)
Yes, there are arrays of common mode chokes that can be used to
transform impedance, and there are transmission line transformers of
various sorts that can do that as well.
BTW -- your discussion of phasing between elements of an RX array
causes me to add an important post script to my advice that a perfect
match is not required. When ANY passive network is used to produce
phase shift, the source and termination impedances DO matter. The
tricky part, though, is knowing what the input Z of the RX is, and if
you're doing something like a phased array using phasing lines that
end at the RX input, it might be a good idea to actually measure input
Z and the antenna Zs with a VNA.
73, Jim K9YC
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