[Elecraft] Question for Wayne about your OCF Dipole

[Vic Rosenthal]

With any OCF or similar antenna there will be some bands on which the SWR on the coax is very high. And if you are trying to operate it at frequencies below its halfwave resonance, it can be astronomical.
I'm running a 10m long rotatable dipole, center fed, on all bands from 40 through 6m. There is no way this could work with any kind of coax feed; the SWR on 40m is around 100:1.
The feedline is 34 feet (about 10m long). I discovered that the losses in a balun at the rig end are too great -- I nearly destroyed a 5kw rated DXE balun trying to do this (power was 1.2 kW). You need either a balanced tuner, or some balanced arrangement to cancel the reactance before the balun. I chose the latter.
I recently upgraded my balanced feedline to no. 12 spaced 10 cm. I put a twist in it about every 1m to improve balance. Calculated loss on 40m (by far the worst band) is less than 2 dB.

Vic 4X6GP 

> On 19 Sep 2017, at 6:57, Jim Brown <jim at audiosystemsgroup.com> wrote:
> 
>> 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
> 
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