EXCELLENT info Guy. Thanks for your post
and confirming my *intuitive* analysis of the
sloper from which I developed the N4KG
Reverse Fed Elevated GP Antenna,
tieing several 'slopers' together, feeding
the sloping radials from the center of a single
coax and connecting the braid to the tower.
Your observation that moving the tap point
primarily changes the Resistive component
and that changing the length of the wire
primarily changes the Reactive component
was especially informative.
I *intuitively* estimated the equivalent top loading
of a 24 ft boom TH6 at around 40 ft so I attached
my feedpoint ~20 (25?) ft below the beam and
originally ran 3 separate 1/4 WL slopers. This
resulted in a good match to 50 ohm coax. I assumed
the upper portion of the tower, between the feedpoint
and the beam was doing most of the radiating as
your model confirms.
This placed the feedpoint at the 60 ft level on that
tower. I again *intuitively* concluded that this would
be a high impedance point, being roughly 1/4 WL
above ground on a grounded tower, and therefore
little current would flow in the lower section which
your model also confirms.
I have a sad anecdotal tale that tends to support your
prediction of Low Angle Radiation from an Elevated GP
with Sloping Radials. I had asked VU4GDG about 80M
during a 40M QSO at my sunrise. He said NO but
15 minutes later he disappeared and I heard him CQ
on 80M CW. I called on frequency and K4CEF called
him up 2 on his inverted vee. K4CEF was his first QSO.
My 80M inverted vee had developed a high SWR after
side mounting a 3L20 at 60 ft only a few days before,
seting up an 80M resonance on the supporting 130 ft
tower so I switched over to the Elevated GP.
The pileup was HUGE and lots of inverted vee / dipole guys
were getting through. I could never break through on the GP
with sloping radials. I assume his SSW skew path signal
was fairly high angle at our sunrise.
The next week, I installed an Elevated GP with nearly
horizontal radials at 15 ft on my 40 ft tower top loaded
by a TH7. This antenna has proven to be a more effective
radiator at high and low angles. This is the antenna
described in my JUNE 1994 QST article and in the
ARRL Handbooks and Antenna Book (18 th Edition).
I have worked 230 countries on 80M with this antenna
since I started keeping track of it's performance.
I still need VU4 on 80M, but did work VU4GDG on 160M SSB
when they called me after I had called a station in the Pacific
on schedule at my sunrise. His signal rose from the noise to
56 for 5 minutes then faded out just as fast. I was transmitting
and listening on the shunt fed 130 ft tower.
On Sun, 6 Jan 2002 "Guy Olinger, K2AV" <email@example.com> writes:
> 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
> 2) Otherwise, the connection point and length of wire to obtain Z of
> 50+0x is wildly variable, and not practically reachable in some
> 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
> 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.
> particular the vertical high current point of this antenna will help
> radiation where the vicinity is cluttered with buildings. It is almost
> to beat an inverted V for DX.
> Very interesting antenna, indeed.
> 73 & HNY,
> Guy, K2AV
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