[Elecraft] Antenna query

[Ron D'Eau Claire]

Ralph, VE7OM asked:

I have a KX1 which I want to take with me on a trip to Phoenix, AZ in early
March.  I wonder if there is anyone out there that has tried the portable
wire antenna as suggested in the KX1 material but used a feedline of some
sort with it.  I'd like to get the antenna a bit away from the operating
position and wonder if coaxial cable, ladder line, or TV feedline might
work.

Needless to say I'd still need/use the ATU in the KX1. 

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I've used a variety of combinations with the KX1, including the end fed wire
suggested. 

The SWR, hence the losses, with any feedline will be higher than direct feed
to the antenna. Higher impedance feedlines are inherently lower loss, so 300
ohms is preferable to 50 ohms and open wire is preferable to twinlead. The
problem with twinlead or any 'balanced' feedline is that it counts on
balanced, out of phase currents flowing in the conductors to suppress
radiation (and pickup) so simply using 300 ohm twin lead or "ladder line"
won't do much to suppress radiation compared to just hooking a wire to the
KX1 unless you are feeding a wire at the center. 

The best antenna will depend upon how "portable" it must be and which bands
you operate: 40, 20, 30 or all three? 

One approach is to string up a dipole for the band(s) you are operating and
feed them at the center with 50 ohm coax. It works fairly well to simply
hook multiple dipoles to one coax feeder. The SWR should be low enough to
avoid excessive losses in the feedline and the coax will provide you with
shielded feedline. You can even make two dipoles using zip cord or twin
lead, cutting it at 66 feet for 40 meters then unzipping or separating the
second conductor and cutting it to 33 feet for 20 meters. You can add a
third dipole  for 30 meters as well. If you use zip or twin lead for the
radiator, let the ends of the shorter dipole hang down several feet, if
possible, away from the longer radiator to reduce the capacitive coupling. 

Another approach is the old "standby" - a doublet feed with open wire line
or twinlead. Open wire line is lower loss than twinlead. Again, higher
impedance = lower loss in general since the maximum SWR the feeder will
experience is lower. You can use one radiator that way -- whatever you have
room for. You'll find best efficiency (and easier matching) if your antenna
plus twice the feedline is at least 66 feet for 40 meters. You want the
radiator to be at least 1/4 wave long on the lowest frequency, e.g. 33 feet
long for 40 meters, which then requires a 16 foot balanced feeder to make up
the difference. To minimize radiation from the feeder you might try a balun
at the KX1. Indeed, the switchable Elecraft balun might give you the most
flexibility by allowing you to quickly change the impedance transformation
in case you have trouble loading on one band.

The thing to avoid is a single wire with coax connected at the center that
is not quite close to  a half wavelength long - 66 feet, 47 feet or 33 feet
for 40, 30 and 20 meters respectively. Trying to use other lengths of wire
fed with coax will produce quite high feedline losses. The only exception is
when the antenna is 3/2 waves long. A 40 meter dipole works fairly well as a
3/2 wave antenna on 15 meters, for example. But the K2 doesn't cover the
spread of bands needed to consider that option. 

Ron AC7AC