[Elecraft] K3/KPA500/KAT500 SWR problem Update2

[Vic Rosenthal K2VCO]

Depending on the location of the feedpoint, the currents may be balanced 
even if the antenna is asymmetrical. Consider the "end-fed Zepp" antenna 
in which there is a tiny or non-existent horizontal section on one side 
and a multiple of half-waves on the other.

Of course, if one side of the antenna is lower than the other or closer 
to conducting objects, the theoretical balance will not be obtained.

An additional problem, even if the antenna is center-fed, is that the 
feedline must come away from the antenna at a 90 degree angle from the 
wires for some distance. Otherwise, the feedline will pick up RF that 
was radiated from the antenna, in common mode. I suspect that pick up of 
radiated RF is more likely with an OCF antenna.

A choke/balun at the feedpoint will force the feedline currents to be 
balanced even if the antenna currents aren't. But if RF that is radiated 
by the antenna is being picked up on the feedline (e.g., on the outside 
of a coax line or on a balanced line in common mode) then you will also 
need a choke at the entry to the shack.


On 1/10/2014 3:04 AM, Ray Coles wrote:
> Buck K4IA casts aspersions on the OCF Dipole by saying that it "Is
> inherently unbalanced"; but although the antenna is geometrically
> asymmetric, it is not inherently unbalanced as far as the feeding is
> concerned. See W8JI paper on the subject which shows that the standing wave
> pattern across the dipole OCF antenna is exactly the same as a center-fed
> version. The main thing that changes is the feed impedance.
>
> Having said that, I have responded to all who suggest that common-mode
> currents on my feeder could be my problem by making myself what is often
> referred to as the finest and most versatile common mode choke balun, as
> described by GM3SEK. Yesterday I completed it and installed it just South of
> my 4:1 Balun under the antenna feed point (I made the mid band version, and
> used all the right Ferrites and winding arrangements). I really believed
> this would fix my problem (which mainly affected 40-14m), but it didn't
> unfortunately.  If anything, my problem seemed worse. My next step will be
> to move it from the feedpoint to the shack entry, because I am worried that
> since replacing and re-routing my coax feeder I may have exposed it to
> closer coupling to the antenna (it's now more parallel and unburied). I hope
> Reflector readers are not getting too bored with my problems and attempts to
> overcome them. I am still driven by the 10 months I had using my OCF and
> Elecraft rig on all bands from 80-6meters and working lots of DX!
>
>
>
> Ray Coles, C.Eng. M0XDL
>
> 10 Littlemoor Road,
>
> Weymouth DT3 6AA
>
> Tel: +44 (0) 1305 833699
>
> Mob: 07831 516517

-- 
73,

Vic, K2VCO

Fresno CA

http://www.qsl.net/k2vco/