KX1 antenna suggestions

[Paul Huff]

Hi everyone,

I just finished building my 3-band KX1 with the internal ATU.  It sure is a
great little rig!  Today I did my first experiment with a quick-deploy field
antenna.  I had a 40-ft radiator thrown up into a tree, with 3 radials as a
counterpoise.  Most of the radiator was at about a 45-degree angle.  The radials
were cut at 1/4 wavelength - one for each band.  The antenna system was
connected to the rig through a BNC/binding posts connector.  The internal ATU
gave me an SWR on 30 and 40 meters of 1.7:1 and on 20 meters it was 1.1:1.  I
guess this is OK but the 1.7:1 matches showed a reduced output power and I would
like to do better.  I generally like the idea of this kind of antenna, but any
and all suggestions that you might have would be very welcomed.

Thanks and 73,
Paul – N8XMS


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[Ron D'Eau Claire-2]

Y'know a simple way to get a better match without messing with the antenna
is to introduce some capacitive or inductive reactance to the antenna system
at the rig end. Get a 35 mm film can or plastic pill bottle and wind it with
insulated wire. Use some enameled stuff for the easiest working. Every few
turns, put a loop in the wire and twist it, then hit the loop with a
soldering iron to solder the twist and loop to make a tap point. Stagger the
loops around the form so they aren't so close you get shorts.

A couple of holes at each end of the form made with a knife point will allow
you to thread the ends of the wire through them to hold the coil secure.

Hook the 'hot' terminal from the rig to one end of the coil and clip the hot
end of the antenna feeder to one of the taps. IF the tuner can't give you a
good match, try a different tap until it does. It doesn't take much fiddling
around. The idea is to bring the complex impedance presented by the antenna
into a range the tuner can handle. All the ATU is doing while it's
"auto-tuning" is plugging in various values of capacitance and inductance
while monitoring the SWR on the link to the transmitter output for a low
SWR. When it can't find a good match that's because your antenna has a high
enough "Q" that the ATU steps right past the right values or it's because
the tuner doesn't have enough inductors and capacitors and runs out of
"range". That's especially true of the KXAT1 because of the very small
amount of room available inside the KX1 case. Simply changing the reactance
presented by the antenna system can allow it to find a match.

A capacitor can also be used. Put it in series with the antenna hot lead
instead of the coil. If you have an old junker broadcast band receiver
around, the tuning cap from it would do FB, but put it where you won't
ground or touch the frame of the variable. It's all "hot" and grounding it
would short out the antenna or touching it would detune the system. Variable
caps are usually easier to damage than a coil. You could use fixed caps as
well, but use physically *big* caps for good efficiency. Maybe some in the
range of 20 to 100 pf would cover the range needed for HF.

Other ways of accomplishing the task is to use a BL1 balun. Not that a
balanced to unbalanced transition is needed, especially, but because it'll
give you a 4:1 impedance transformation that may bring the impedance to a
value the ATU can handle.

I'll admit that I often "cheat": I use my Elecraft T1 ATU with my KX1.

Ron AC7AC

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