Topband: Skin effect on RG6U at top band
jeremy at maris.plus.com
Mon Jan 31 02:19:15 PST 2011
We've used 1 km of coax to feed a remote Beverage (3 wavelength, 60
degrees, for VK and far east). 750m of the coax is RG6U, with copper
braid. The results were poor, despite a 10dB head amplifier at the end
of the Beverage, so we measured the coax attenuation on part of the
run by putting 25W into the coax and measuring power into a 50 ohm
power meter. I appreciate this isn't 75 ohm like the coax but extra
losses from 1.5:1 SWR should be low.
The measured loss in the first 250m segment was ~ 6dB and about ~9dB
in the second segment, 15dB total over 500m. This is way higher than
published figures (eg at http://www.timesmicrowave.com/cgi-bin/calculate.pl)
which suggest about 3dB per 250m.
I'm thinking that the copper plating on the RG6/U inner conductor is
too thin. N6LF gives the skin depth at 2MHz as 40uM (Conductors for HF
Antennas, Rudy Severns, QEX November/December 2000). Does anyone have
any typical figures for the plating thickness of RG6/U or have
evidence that attenuation figures at 2MHz are much higher than expected?
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