[Elecraft] Question for Wayne about your OCF Dipole

[Jim Brown]

On 9/18/2017 8:17 PM, Charlie T wrote:
> I wonder about the power handling capability of some of that cheap RG-6, or whatever they use for TV & satellite.

CATV coax is optimized for physical properties and for RF performance at 
UHF (CATV does include "backhaul" information from the customer for 
billing, etc., and that does run at HF).

Nearly all CATV coax has a copper-clad steel center and a shield with 
braid and one or more foils. The shield is usually 100% Al. Loss in coax 
below UHF is all due to I squared R losses. The steel center causes 
increased loss at MF and low HF, while skin effect takes the steel out 
of the picture at VHF and above. And because it's Al, shield resistance 
is higher than for a good copper braid.

Further, an important property of a cable shield is its transfer 
impedance, which is the ratio of differential voltage induced by current 
on the outside of the shield. In other words, its a parameter defining 
the effectiveness of a shield against common mode current. The lower the 
transfer impedance, the better the shielding, and the lower limit on 
transfer impedance is the resistance of the shield at the frequency of 
interest. That's an important reason for using common mode chokes on 
coax used for RX antennas!

W8JI says that "RG6 cables can handle TX power," without qualifying 
which RG6 cables he's talking about (there are hundreds of them). I use 
CATV RG6 extensively for RX antennas, and buy flooded Commscope on 1,000 
ft spools for about $95.

While thinking about OCF dipoles, remember that because they are 
inherently unbalanced, they tend to pick up lots of noise on the 
feedline, and because they're nearly always fed with 2-wire line and 
usually a poor match on most bands, they are impractical to choke 
effectively. [To understand why, study my tutorial k9yc.com/RFI-Ham.pdf] 
No problem if you're in a quiet location in the middle of nowhere, but 
not a great choice if, like most of us, you're surrounded by neighbors, 
each with dozens of RF noise sources.

73, Jim K9YC