[Antennaware] Shunt fed towers, modeling gotcha's

Guy Olinger K2AV olinger at bellsouth.net
Sat Aug 22 09:49:44 PDT 2009

Posted an earlier version on topband (I think, still getting used to
Gmail interface)

>> When I model it as a shunt
>> fed tower, ... the gain drops to
>> about 0 (zero) dBi. Is this a fault of EZNEC4....
> I have been having similar problems trying to model my shunt fed antenna and others with EZNEC5...
> I would appreciate advice from Group which earth is recommended for best accuracy when modeling an inverted L

Part of the problem is trying to get an absolute value to mean
something when the local variations in ground defeat such attempts.

When I do any vertically polarized antenna modeling, I use real ground
and COMPARE the result with a Broadcast Grade (BG) vertical using the
same conditions.  I do have the NEC4 version of EZNEC which allows me
to specifically model a quarter wave with 120 buried quarter-wave
radials in average ground, using real wire diameters, tower diameters
and copper resistance.

I can then model anything explicitly in the same environment and
COMPARE it to the gain of the BG vertical.  So I can say an antenna is
so many db below or above the BG vertical in the same environment.
This has proven a much more predictive reference.  That BG vertical
comes up with a gain of 1.18 dbi.  This seems stingy, and doesn't have
the satisfying feel of the outlandish kind of gains put forth by some
antenna companies, but it is a much more reliable reference.

An antenna that predicts within a couple db of a BG vertical, using
the same set of modeling rules and ground specification, will do very
well in ham usage

These are the rules I follow that seem to avoid a lot of trouble.

Do not intersect wires of differing diameters except at straight-on
one-to-one junctions.

Do not intersect wires with different segment lengths at a junction.
This also means do not go around a corner with different segment

Close parallel wires should have the same segment length, and the
segments should be aligned across from one another.  Lines drawn
between segment centers should look like a ladder.

Use identical short segment lengths everywhere in the model to start.
Only adjust those later if adjusting them does not change results. On
160 this is one foot, maybe ten inch segments. Some versions of EZNEC
may get upset about the number of segments. (If you are at that point,
UPGRADE.  It's no more than taking the family out to a nice
restaurant. Keep the programmers working on their stuff.)

Do not place a source or load in a segment adjacent to a complex
intersection (three or more wires at the point.

Do not use ground (any kind) as a conductor in a gamma/omega matched antenna.

Do model ALL conductors on the tower that are not at RF ground at the
base and bypassed or connected to the main conductors at the top. You
may be surprised at the large amounts of current on such conductors.

Do model ALL conductors in a half-wave radius. You may spend a lot of
time modeling the best antenna gain in an ideal environment, only to
destroy it with surrounding conductors.

Literally modeling the tubing size of an antenna at the top of a tower
(e.g. triband beam) being used as a vertical will not change things
much as there is little current there. Using a single 1 inch diameter
for boom and elements will give the same results. (The actual
resonance of the tower cannot be predicted that well anyway, without
precise accuracy in the ground/radial modeling, which is never
available, see below.)

Do not bother to try and choose between elevated and buried radials
based on models. There are poorly understood issues in any such
comparison, with much variation in results vs. estimated, that are
simply not taken to account in anybody's model program.

While I long for something serious and irrefutable to come forth on
elevated vs. buried radials, and do not doubt the veracity of
anecdotes, I remain unconvinced by either side however much I would
like to be convinced.

The definitive work and data still in use for current MF antenna
design and measurement was done pre-WWII, well documented by Bell
Laboratories, BUT using the methods and equipment available at the
time, and using what amounted to unlimited funds from the
then-monopoly AT&T.  There has been essentially zero economic
incentive to repeat what would be a very expensive study with 21st
century tools.

Measurements had to be done AT GROUND LEVEL.  This WAS appropriate to
their goals. With few exceptions, broadcasters at MF are interested in
ground wave. Hams are interested in sky wave.  We still have some
undiscovered country in MF skywave.

73, Guy

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