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Old October 2nd 03, 06:22 PM
Tom Sedlack
 
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Default 70 ohm dipole to 50 ohm feed line question

I asked this before and was told that for relatively short runs (under 50
ft), using 50 ohm coax to feed dipoles is fine.

I keep coming back to this in my head because I model a dipole and I get a
VSWR of 1.5 using 50 ohm feed and I get 1:1 if I model with 75 ohm feed.

My plan for my little antenna farm is to have a run of 50 ohm coax to a
remote antenna switch. Create a few dipoles and feed them with 75 ohm coax.
I saw an equation someplace that, I believe, gave the length of 75 ohm feed
required to transform the impedance to 50 ohms at a given frequency. I would
cut the cable to this length then connect to the 50 ohm antenna switch.

246 x Velocity Factor
L = -----------------------------
Freq


Does all this make sense? Am I going overboard (a curse of trying to be a
perfectionist)? I don't want to use a tuner because; 1 - I don't own one
and 2 - they waste power (I want to try QRPish operation).

Also, if the above is correct, would I still need to put a current choke
(coil of coax) or would the "fact" that the feed and dipole are matched,
there wouldn't be RF on the jacket?

My head is starting to hurt........


Tom