[Elecraft] How Long Is "Long Enough"?

[Ron D'Eau Claire]

A "dipole" (normally 1/2 wavelength long) for 40 meters should be 66.8
feet long for operation at 7 MHz. The normal formula is 468/f where
f=frequency in MHz. A half wavelength is actually given by 492/f. A
value of 468 is normally used to allow for 'end loading' effects caused
by the insulators at the high-impedance ends of the radiator.

Theory and practice show that a doublet (center fed antenna) can be a
LOT shorter than that and work very well as you have discovered. If
you'd like to see just how short you can go, I'd suggest getting a copy
of the computer program "dipole3" which is available free from many
internet sites. One that I know of is
http://www.btinternet.com/~g4fgq.regp/page3.html.  It will give you
expected performance characteristics including efficiency as a radiator
and the L-network values your KAT2 will have to come up with to load it
properly.

In general, the closer to 1/2 wave you can make it the better, when you
are forced to use a dipole less than 1/2 wavelength long. However, the
efficiency doesn't begin to really "nosedive" until the radiator gets
down to about 1/4 wavelength overall. 

How "hard" your KAT2 will have to work to get a match depends a lot on
the feeder. If you use a feedline 1/4 wavelength long with a radiator
1/4 wavelength long, then the KAT2 should not have much trouble - you
are actually loading a full-sized 1/2 wave antenna with half of the
antenna in the feedline. As you get shorter the impedance drops "like a
stone". At higher frequencies, where the antenna is longer than 1/2
wavelength, you may find frequencies where the KAT2 can't handle the
high impedance it may see at the feedpoint. With all the bands we have
available now, it's not unusual to have one band that is trouble to tune
up on, particularly with a 'random' sort of antenna installation. 

The usual big loss item in an antenna like that is the height above
ground. For decent DX performance when it is mostly horizontal, you need
to be near 1/2 wavelength above ground - at the very least about 1/3
wavelength up. I'm betting that your DX contacts were on the higher
bands where the effective height (in fractions of a wavelength) were
pretty good.  Where it is a lot less than 1/2 wavelength high (down to
about 0.15 wavelengths above the ground) it makes a great "short skip"
antenna, putting out a whopping signal straight up that gets scattered
back from an active ionosphere to give you great coverage out to several
hundred miles. These antennas have been "rediscovered" by the newer crop
of Hams and christened "NVIS" for "Near Vertical Incidence Scattering"
(or "Skywave") antennas. Quite a mystique has grown up around them, but
they are really just basic antenna stuff: a two-element beam using the
earth as a reflector. The optimum spacing for such a "beam" is 0.2
wavelengths, so that's the optimum height for such a radiator, although
the range is VERY broad. Lower heights just reduce the upward "gain" as
ground losses increase.

So how short can you make it and still have the KAT2 load? I'm betting
that you can throw out 50 feet of coax with NO antenna and it'll "load"
on some bands.  Radiating a signal is a little different. 

How much signal do you need? Some days you can work the world on a few
milliwatts, so if you are losing 99% of the r-f in your antenna system
and only radiating 1% on good days you will still work DX. On other days
you might want to be able to radiate a little more r-f to get through.
And that's why most of us think about getting as much efficiency as we
can out of an antenna, given the limitations of size, installation, etc.
that we must live with. 

I suggest that you throw away the balun at the center of the antenna.
They are useful if you have a dipole way up in the clear with the feeder
coming away at exactly right angles for at least 1/4 wavelength and you
want to be sure that the antenna has exactly the expected "dipole"
pattern. Otherwise a balun is not going to help and it could hurt.

Ron AC7AC
K2 # 1289 

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The subject is: the "shortest" dipole I can get by with when operating
portable with the K2 ...

I know that kludged-up antenna wasn't very "efficient", but frankly if I
can work people with a shortened dipole that can be thrown up/taken down
in 2 minutes, I really don't care ... week-ending with my K2 doesn't
often lend itself to "pure theory and practice" operations ...

A program I found that says the legs of a 40m dipole should each be
approximately 22+/- feet long, but given how the KAT2 performed with
considerably shorter legs, I'd like to have the shortest dipole that I
can put up quickly & easily, take down and roll-up until the next
week-end trip ... and still see a decent SWR on tune ... and at minimum
cost in case I lose the whole thing in a storm <g> ...

SO ... let's hear from some of you portable ops veterans!

HOW SHORT can I make those dipole legs and still expect the KAT2 to load
to a respectable SWR on 40m thru 10M?

73, Weymouth Walker, K8EAB in Georgia ...