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antenna tuners

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Subject: antenna tuners
From: (
Date: Mon, 13 Jan 1997 13:21:29 -0500 (EST)
Henry, your question is so good that I am posting to the group also...
Any thoughts on using coiled lengths of RG8 (or even RG8X for low power) for
fixed-value capacitors?  At 26 pf per ft, the most you would need is 10 ft.
 This would be cheaper than a variable capacitor; however, it would require a
lot of experimenting to determine the right lengths per band.

The problem here is distributed inductance... the length of coax is not a
pure capacitance (nothing is of course, as all caps have inductance)... you
can visualize each foot of the coax as a discrete 26pF cap with a one foot
series inductor connecting to the 26pF cap of the next foot... this makes all
the difference in the world...
For instance, on my 160m array I have four 1/2 wave RG8X feedlines fed in
parallel at the shack end... only one of the lines at a time is relay
connected at the antenna ends, the other three coax remain open circuited...
given these lines are 200 foot long I should have over 5000pF of shunting
capacitance per line and over 20,000pF on the four lines.. if this is so
these coax will be a low impedence, RFshort to ground... actually it makes
less than 4 ohms difference in Z when I feed the 4 coax, as compared to
You will find that ten foot of coax is less than 260pF.... certainly, you can
use a bundle of 5 or 10 shorter lengths of coax in parallel, and get fairly

Cheers   ...   Denny

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