Jeff,
Your 70 ft of radiator wire is not a multiple of a halfwave on any
amateur bands, so it is not resonant within any ham band.
The web article does state that the coax length should be 18 feet, which
strongly implies that they are using the coax shield as part of the antenna.
Yes, a counterpoise is needed, and if you want to use the antenna as you
have it configured, I would suggest that you create a low impedance
either at your shack end or at the SO-239 jack at your UNUN. That low
impedance can be created by a quarter wavelength of wire for each band
you intend to operate. Tie the near ends of the wires together, and the
far ends should be fanned out away from the other wires. Treat the far
end of each wire as though it were the end of an antenna wire -
insulated and away from where people or pets might come in contact with
it - it will have a high RF voltage on it.
If you connect the near ends of the wires to the SO-239 jack shell, use
a current choke (often called a current balun) on the coax at that point.
If you bring the end of those counterpoise wires into the shack rather
than attaching to the SO-239, you may get by without a current choke.
The ends of those wires is the RF Ground for your shack - note that it
is not a safety ground which should be implemented as a heavy wire (#6
or larger) connected to driven ground rods, or at a minimum to the
utility entrance ground rod. If you drive extra ground rods, they must
be connected to the Utility entrance ground rod by a #6 or larger copper
wire.
The use of those counterpoise wires will likely change the tuning of
your antenna. Right now, it is using the coax and probably most of all
the equipment in your shack as a 'counterpoise' even though it may be
ineffective. You are effectively bringing part of the radiating antenna
into your shack.
73,
Don W3FPR
On 5/31/2013 3:35 PM, Jeff Ellis wrote:
I got the kit John talks about and made the antenna. Its was quick and fun to
make. I have a question about it though. I had a 70' wire that I had been
using just for RX to test a IC-718 and I hooked it up to the UNUN box instead
of the 30' wire that came with the kit. Its below 2:1 on 6m, 12m, 17m, 40m and
less then 4:1 on 10, 20m, 80m and about 6:1 on 30m, 160m. The only really high
one is 60m. So really all but 60m are tunable by the KX3 tuner board. I am
using what I remember to be about 15' of RG-58/U. My Shack area is on the
second floor of the house and the wire goes out my window and down to the back
fence.
My problem is I am getting RF coming back into the shack (on just 5W) where it
will reset my USB hub a few feet away from the radio. Not fun when trying to
work digital modes. This only happens on some bands but I know happens on 20m
where I try and work the most. I expect its just the ones less resonate that
its doing it on but I need to check again.
Will adding a number of ferrite on the feed line help choke the RF? Will
winding the feed line around a tube to make an air choke work? I guess that one
is cheep enough I should just try it. Buying 10 or more ferrite to clamp to the
line costs more ;)
What about using a longer feed line? I really don't want to add a counterpoise
and the instructions said it does not need one. I could use the 30' antenna
wire it came with if that is going to help. I would not think so since its
resonate on so many bands and tunable on the others.
Thanks for the help!
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