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/
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