Topband: Modeling verticals (fat, skinny, etc) on EZNEC

[Rick Karlquist]

The recent discussion about fat and skinny verticals, etc
suggests that there is a need for a tutorial on EZNEC modeling
of verticals.  Here's what works for me:

Build the vertical to be modeled (VTBM) at the origin (X=Y=Z=0)
over perfect ground.  Build a receive antenna (RA) in the far
field, say 5 wavelengths away, consisting of a plain vertical,
dimensions not critical, eg 1/4 wl high, .001 wl diameter.  Insert
a load of 36 ohms (also not critical) at the bottom of the RA.  Insert
another load, at the bottom of the VTBM.  Set the value of this load to
what you think your equivalent ground resistance is.  Insert a current
source at the bottom of the VTBM and set it to 100A.  Click on "Source
Data" and "Load Data".  Calculate a figure of merit (FOM)
consisting of the power in the RA load divided by the power generated by
the VTBM source.  This FOM is the "money spec".

It would be nice if EZNEC had a type of source called "power", then you
could set it at 1 watt and not have to do the division.  But EZNEC
doesn't.

You can leave the antenna alone and experiment with ground resistance
to see how much loss you have relative to zero ground resistance.
You could easily be losing .1 dB per ohm or even more.

You will see that taller verticals are less impacted by ground loss
than shorter ones and that wide spaced arrays are less impacted by ground
loss than close spaced arrays.  I developed the 7-hex array with 3/8 wl
spacing to minimize ground losses.   Some other arrays with close spacing
get killed by ground loss unless they are really well outfitted with
radials.

You can also compare various types of antennas.  You will see that an
inverted L has a lower (FOM) than a T antenna, despite higher "radiation
resistance".  This is because the FOM only counts useful vertical low
angle radiation, not useless cloud warming radiation.

Hope that all makes sense.

Rick N6RK