I have gone through the trouble to model a tower with beams, etc, and monkey
around with lengths, attachment points, different tower heights, etc.
The complex model behaves essentially like a more manageable simple one, a
(physically impossible) tower consisting of a vertical #12 wire, with wires
for mast, boom and outer yagi elements. If anyone wants it I will email the
.EZ file as an attachment for those with EZNEC 3. Please request this off
the repeater.
The model validates a lot of apparently contradictory anecdotal material,
and explains a good deal. The model predicts the following ....
1) The only way to get a feedpoint/wire length that repeats is to have an
IDENTICAL tower, ground, attached antennas, metallic guys, AND feedlines.
2) Otherwise, the connection point and length of wire to obtain Z of 50+0x
is wildly variable, and not practically reachable in some situations.
3) Having established such a point and length, ANY change to the tower has
the possibility of detuning it.
4) The antenna will have some high angle radiation, as with a 5/16 wave
inverted L with ground radials (useful in contests), will work within a
couple db of the peak gain of a good regular vertical, even with a poor
ground, and has a good 360 degree low angle pattern, with a mild weakness
away from the sloper.
There are some NON-INTUITIVE aspects to the antenna, some quite curious,
some useful. In no particular order....
The antenna puts MORE power/current to the top of the tower system than a
gamma/omega matched tower, and running 1.5 kw can have all of the burn,
arcing at the tower top problem that the matched tower setup does. The tower
above the wire connection point is the MAJOR radiator of this antenna. The
antenna's real name ought to be "tower-top-plus-half-sloper".
The wire does not have to be straight. This may make possible a VERY DECENT
160m DX antenna on a lot for which a good tower-base radial system (60x128')
is not possible, particularly tower-close-to-house situations.
100+0x (using a 1/4 wave 75 ohm matching section to feed) could be more
attainable in a given situation, and can move the connection point up,
improving the efficiency. In general, going up with the connection point
raises the R component of Z, going down lowers it.
It will be a lot harder to locate the match point just using SWR and not
knowing the R component independent of the X component, because ....
Moving the connection point up and down the tower changes R more than
changing the wire length does. Changing the wire length changes the X
component more than changing the connection point does. Which suggests a
259B or the like can help find a good match point using the procedure: A)
Move the connection to get 50 (or 100) R. B) change the wire to get 0 X
or lowest SWR. C) Repeat A) & B) until cooked.
The highest current is in the wire. The next highest current is above the
connection, in the tower, about 95% of the wire. The current below the
connection at the tower base is greatly diminished, about 15 % of the wire
current, which makes this antenna far less affected by sub-optimum grounds
than a gamma/omega matched tower. This aspect seems independent of what is
on the tower.
The miscellaneous stuff up and down the tower, beams, booms, metallic guys
directly attached to the tower, etc, in general do not radiate, since these
are more or less balanced. BUT they all change the match point whether they
are above or below the connection point. The tower/mast do the radiating
above the connection and the wire radiates below it. The current on the
tower below the match point tends to be flat, like the single wire feed to
an old-fashioned Windom.
The presence of maximum level current ABOVE the connection point can give
extreme low angle radiation superior to a regular vertical. In particular
the vertical high current point of this antenna will help low angle
radiation where the vicinity is cluttered with buildings. It is almost sure
to beat an inverted V for DX.
Very interesting antenna, indeed.
73 & HNY,
Guy, K2AV
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