Topband: Fwd: EZNEC Model earth for Inv. L Re: EZNEC Shunt Feed Model Gain

[Guy Olinger K2AV]

<johnr1g3pqa at onetel.com> wrote:

> George K8GG wrote:-
>> 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 modelling an inverted L. Real-mininec-earth or real
> high-accuracy-earth?


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

When I do any vertically polarized antenna modeling, I use real ground and
COMPARE the result with a broadcast grade 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 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 accurate 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.

As to the models themselves, here are some 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 lengths.

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.

Literally modeling the tubing size of an antenna at the top of a tower 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.

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.
You may be surprised at the large amounts of current on those 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.

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 for current MF antenna design and
measurement was done in the 1930's, well documented by Bell Laboratories,
BUT using the methods and equipment available at the time, and using what
amounted to unlimited funds from a monopoly telephone company.  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. Broadcasters at MF are interested in ground wave. Hams are interested
in sky wave.

73, Guy