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
one...
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
close...
Cheers ... Denny
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: K7LXC@contesting.com
Sponsored by Akorn Access, Inc & N4VJ / K4AAA
|