Topband: Skin effect on RG6U at top band

[jeremy maris]

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?

73

Jeremy G3XDK