[Elecraft] Four Band K1 and antenna

[Ron D'Eau Claire]

> Just wondering what kind of portable antenna anyone uses when they are out
> portable in the field.
>
> I have the bands of 40, 30, 20 & 15 meters. Looking at a doublet type
> antenna, no verticals.
> Ron Polityka
> WB3AAL

Doublets are great antennas, for either fixed or portable use.

If you do NOT have an ATU (either the KAT1 or an external ATU), a half wave
on each band is the easiest to feed with 50 ohm coax. The 40 meter doublet
works okay on 15 in most cases where it is 3/2 waves long.

If you have an ATU, by all means put up whatever you can and feed it with
open wire line - ladder line is a LOT easier for portable use and darn near
as good. 300 ohm "twinlead" will do if that's all the volume you can take
into the field and it will still be superior to any coax when the SWR is
high.

Your radiator can be as short as 33 feet and still work almost as well as a
full sized 66 foot antenna on 40. The difference is less than 1/2 of one dB.

The big thing with a doublet is getting it up high enough if you want to
work DX and you are on level ground. Something between 3/8 and 1/2
wavelength is needed for good low angle radiation.

IF you are "mountain topping" or near a steep cliff, you can mount the
doublet so it is off to the side of the cliff. Some ops use a single pole
supported by a back stay holding it out near horizontal sticking out
perpendicular to the slope with the antenna hung as an "inverted V" from the
pole. The earth behind the antenna makes a great reflector that will produce
very good low angle radiation in the direction away from the cliff.

The optimum height above the ground for using the earth as a reflector is
about 0.2 wavelengths, although the height is not critical as long as it is
less than about 3/8 wavelength. If the antenna is really close to the ground
(< 0.1 wavelength) you will start to lose some signal due to ground
absorption, but you will still keep the low angle of radiation off of the
side of the cliff.

When calculating your height when it is hung as an "inverted V", use about
80% of the center height.

If you want to work relatively short skip 600 - 800 km (out to about 500
mi.) on 40, you will do best to place your antenna about 0.2 wavelengths
above the ground on level terrain. That produces a powerful lobe straight up
that gives you great 'short skip' contacts. Some ops working from
mountaintops have found it very hard to work stations less than 1000 km (600
mi.) away until the put their doublet on the very top of  the ridge and kept
it low to the ground. BTW, this sort of installation has the current
nickname NVIS antenna for "Near Vertical Incidence System". You may have
seen articles about them recently. They've been the mainstay (and bane) of
hams working short skip or stuck with low supports since the wavelengths
below 200 meters were first explored, but they seem to have been
'rediscovered' with great fanfare by many hams recently.

Of course, these doublets require balanced feed, although they will work
quite well without it in many cases. Without balanced feed, the feeders will
radiate as much as the "flat top" and you will have a lot of vertical
polarization, assuming your feeders go straight up. Also, you want to be
sure your feeders are in the clear and go up as directly as possible, to
avoid losses coupling lots of r-f into other objects. For balanced feed so
your feeders do not radiate, you can get away with using a balun  between
the balanced feeders and the single-ended ATU in most cases, although it is
not uncommon for some bands to present such a high or low impedance at the
feedpoint that the balun acts more like a dummy load than a balun. Still, a
decent balun works more often than not.

Finally, you mentioned that you were NOT interested in a vertical, but you
can also feed a horizontal antenna at one end just fine! That may be easier
with the KAT2 or most ATU's designed for unbalanced feed. Simply throw out a
1/4 wave long 'counterpoise' wire attached to the case of the rig to provide
an 'r-f ground'.. Some multi-conductor wire with sections cut to each band
work well. The trick here is to try to get your radiator close to (but not
exactly) 1/2 wavelength long. That will raise the feedpoint impedance which
vastly reduces losses in the "ground" connection or counterpoise. If you
made the radiator exactly 1/2 wave long, electrically, on our lowest band -
40 meters - the system will be the most efficient but the KAT1 and most
ATU's cannot handle the high impedance the antenna will present. So you want
to find a length that gets you close, but not exactly on the 1/2 wavelength.
Of course, 30 meters will be way off the mark, but it should work well
anyway. As you go up in frequency, the impedance at even the current loops
will raise, making the system more efficient in any case.

The "end fed" wire requires that you get it right up and in the clear, but
it eliminates the need for a separate feedline.

Good luck.

Ron AC7AC
K2 # 1289