Rob,
actually, if you go to Gap at a ham convention booth, they likely will give
you the copy of the same paper they handed out at Ham Com; which showed the
theory of the Gap antennas. They are "asymmetrical vertical dipoles", as
the paper in IEEE transactions called it. Originally, they were designed
for one band use as AM broadcast shortened antennas. They have been used
in South America for this purpose.
They also get multibands by linear loading and linear decouplers on the
basic dipole. To get 80m, and a match there, they use the old standard
stub matching section. To make it short enough to fit inside the dipole
tubing, they simply load the end opposite the end that bridges the dipole
feedpoint. It is loaded with one of several fixed capacitors you can select
to tune a certain 100 kHz segment of 80m or 75m of your choosing. The "gap"
is the dipole off center feedpoint on these antennas. (One analogy is a
Windom style antenna turned vertical.)
That is almost all there is to it, but one other neat trick is in the Titan
model. To get good efficiency on 40m they make half of the dipole actually
have a quarter wave of radiator on 40m. They do this by extending the lower
half of the dipole half element with a radial rod connected at its end to a
wire that goes around to other insulated ends of radial rods and finally
joins an insulated line that joins to a similar conductor originating from
the antenna end rod and going the opposite way around the base. This forms
a big square, 30 feet of which is on 40m, and a shorter piece that completes
resonance for 10m.
When I built mine, I used an MFJ 259 to see which pieces caused resonance on
which bands. The GAP models are a clever use of basic principles of
antennas.
Since one end of the vertical is going to couple more to the ground, by
capacitance;thus to get a 50 ohm feed point, you do not have to feed the
dipole exactly in the center. The lower half dipole can be shorter than the
upper half. But to get the feedpoint to look like 50 ohms on all bands, you
have to electrically shorten the half elements on higher bands. The linear
decoupler is a stub made up of half of the dipole and a rod spaced apart.
Several of these take care of various bands, 20m, 15m 17m, 30m, etc.
Another neat antenna is the Ten Tec terminated Vee beams that come in two
models.
They are on the web site along with a good instruction manual showing how
they work with antenna patterns for various installations. One model has
two 100 foot legs and the other has 50 foot legs. The beam feature comes
into play on bands where the leg is at least one wavelength long. Otherwise
they are an inverted Vee with the legs spaced closer than normal.
Terminating a Vee beam makes it unidirectional. Leaving off terminations
will give you a bi directional pattern thru the feedpoint V, although they
do not mention that. (But ARRL Handbook does starting in at least 1942
edition and up to 1957 and probably later).
-Stuart
K5KVH
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