I happen to think that the K line and the off-centre-fed dipole make for an
ideal compact station, particularly for those with very restricted land on
which to put just one aerial. As you have concluded, common mode current
must be conquered and I think the GM3SEK chokes are ideal where weight is
not a factor, eg on the ground and in the shack. I built one in a
weather-proof box with the LF version and the HF version all in together for
a very neat solution.
Your problems were mainly with transmit, but these chokes also work on
receive keeping noise off the coax which would otherwise creep right inside
the radio.
As others have said, you need a choke at the feed point after the balun and
others down the line depending on your layout. A feeder that does not drop
vertically from the aerial induces further cmc onto the coax. Another good
and easy thing to do is ground the coax shield before it enters the house,
but not everyone can do that, so a choke at the radio does the job.
DJ0IP/NJ0IP is putting data on his website from a very large number of
measurements on various combinations of ocf dipoles to show which chokes and
baluns work best. His project is not yet finished but it is the most
comprehensive set of measurements I have seen on practical aerials.
David
G3UNA
...the most common response
was common mode current on the feeder caused by OCFD unbalance. I did
research and decided to build the renowned GM3SEK choke balun. I chose the
mid range version which covers broadly 5-14MHz and installed it just south
of my 4:1 balun at the North end of the feeder. No Joy. Further ideas from
reflector pundits suggested that it might still be common mode, but
coupled
from the antenna into the feeder where my GM3SEK choke wouldn't help.
Fortunately I had bought enough Fair Rite oval cores to build all three
(Lo/Mid/Hi) balun designs, so in a final roll of the dice I built the two
core Hi Band balun and installed it in the Shack next to my rig. The hi
band
version has a fairly flat impedance curve and reaches down to 7MHz and up
to
30MHz. IT WORKED! I have run 100W on all the bands from 40m to 10m and I
have seen no run away SWR readings. My 25W tune value stays rock solid. As
a
final push-my luck step I turned the KPA500 to operate and ran 400W SSB.
No
Sweat! All of these things were no-nos yesterday.
Ray Coles, C.Eng. M0XDL
10 Littlemoor Road,
Weymouth DT3 6AA
Tel: +44 (0) 1305 833699
Mob: 07831 516517
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