Topband: Skin effect on RG6U at top band
billw at waveform.net
Mon Jan 31 11:52:03 PST 2011
It is typical for the loss on the CCS (Copper Clad Steel) center
conductor RG6 cables to be a bit higher than you'd expect at the low
frequencies, but it's usually not super significant. For example,
Commscope lists the following for their #5740 (a CCS center conductor,
quad-shield RG6 cable):
1 MHz: 1.21dB/100M
10 MHz: 2.16dB/100M
50 MHz: 4.62dB/100M
100 MHz: 6.3dB/100M
You can see that the 1MHz loss is a bit higher than you'd expect if you
extrapolate out using the stated loss values for some other frequencies
(the actual datasheet gives loss values for up to 3GHz).
The datasheet also lists the center conductor as 18 AWG CCS, and states
the resistance as 28.6 ohms/kft (9.38 ohms/100M). If your cable gives
significantly higher DC resistance than that on the center conductor
then you can probably assume you either have a poor-quality RG6 cable
(which is commonly found at home improvement stores, unfortunately), or
you have some significant damage to the cable somewhere.
Personally, I'd suspect bad connectors if your cable is known to be of
good quality. Assuming you're using F connectors, I've found that if
they get any water in them the center conductor rots away pretty quickly
-- especially if you're running DC on the coax to feed things like
remote preamps and the like.
---- snip 8< ----
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
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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