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[TowerTalk] RF choke question

Subject: [TowerTalk] RF choke question
From: Martin Sole <>
Date: Fri, 14 Nov 2008 15:45:07 +0700
List-post: <">>
I plan to use RF chokes at the feed point of my quad elements and would 
like some ideas on a few parameters. Here are some details if anyone can 
help with answers or ideas. Modelling of the multi band 2 element quad 
has shown that most loops in the multi loop configuration show an 
impedance closer to 50 ohms at resonance than the 100 ohms of the single 
quad loop.

Lowest frequency of interest is likely to be 7MHz, highest frequency of 
interest 30MHz but only one band per choke.
Preference for something simple, compact and light. Are any of those 
mutaully exclusive?
Ideally coax around a core if this is workable in all cases but I do 
have plenty of 14 and 16 enamelled copper wire.

How can I determine if it works or not?
I have in mind that if I have a generator and a detector then with the 
coax outers inserted as a simple coil between the generator and detector 
and the ground side of the test equipment connected together I can see 
what the shield attenuation looks like across a range of frequency. I 
guess that's all I'm really interested in. If this works I presume I can 
dimension each choke for best attenuation in the band of interest.

How can I get an idea about power handling capability.
If I use RG142, which I think I have enough of, that should easily 
handle enough power in itself, but what about the core? Sevick says in 
TLT 2nd edition pp9-9 that '10 turns of  RG58C/U on a 2.4" type 61 
toroid has a power capability of probably greater than 2kW of continuous 
power. The voltage breakdown is on the order of 2kV'. I'm not sure I 
feel comfortable with RG58C/U at high power in any situation but I think 
maybe the core is the real limiting factor other than that.

I have a bag of assorted cores and if my test above is good enough to 
characterise them with making a few measurements then I am only left 
trying to understand the power handling. Perhaps the best way is put a 
big  load on one end and feed it lots of power to see if it gets warm?

I cannot just go and buy T200-2 or whatever cores. I can buy cores in 
all sizes but unknown material. Again if my test above is of any value I 
can experiment.

What do you guys think?

MArtin, HS0ZED


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