W0NCL wrote:
>Im putting up a 20 meter monoband beam and was planning to feed it
with LMR400.
Recently a friend said he had some 75ohm hardline that I could use as well.
Which would I be better using as far as lower loss is concerned? Its about a
250-300ft run btw.
You didn't say which size hardline or
exact length, but I'll assume 7/8" and 250',
and use ARRL's TLW:
Hardline loss: 0.403 dB
LMR400 loss: 1.155 dB,
Difference: >0.75 dB
Cut the hardline to an exact half wave multiple on
14.175 MHz using an MFJ-259B and you won't need
a matching network. 50 ohms in equals 50 ohms out...
plus hardline doesn't deteriorate due to UV like LMR.
You will need to measure the velocity factor
of your hardline and then carefully prune to an exact
half wave multiple. My Commscope's Vf was in the
ball park of 0.87. Assuming that, you will need to
cut an exact multiple of 30.197' nearest to the length
you need to reach the antenna. Depending on how
accurately you measure Vf, you should cut a little
extra and use the MFJ to tune exact resonance using
instructions in the manual. Use 50 ohm coax on
either end of the hardline to antenna and transmitter.
73, Bill W4ZV
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