Roy,

Thank you. My first objective is hopefully relatively simple. I would like to
determine resonance and measure Z. In order to do that I will use an HP 8405A
and directional couplers. Your suggestion is to practice that on known antennas.
That makes sense. Given that I have relatively good generators and can measure
forward and reflected signals at the antenna, is that sufficient to measure
model impedance?

Dan

Roy Lewallen wrote:
To make an exact scale model of an antenna, you not only have to scale
all physical dimensions, but also the conductivity - the model's
conductivity has to be freal/fmodel times the real conductivity.
(Dielectric constant and permeability remain the same in the model as in
the real antenna.) So in order to test the efficiency of various ground
systems, you'll have to bury radial wires in model "dirt" which is
several times more conductive than real dirt, while having the same
dielectric constant as real dirt. (For example, if the real antenna is
for 7 MHz, the model "dirt" will have to be about 20 times as conductive
as real dirt but with the same dielectric constant.) Technically, you
also have to do the same with the antenna conductor, but hopefully the
conductor loss will be small enough that you won't have to worry about
it. If the loading coil loss is significant, you'll have to construct a
model coil with a scaled inductance but the same resistance (that is,
the same Q) as the real one.

The pattern of a vertical antenna is heavily influenced by the
conductivity of the ground extending several wavelengths from the
antenna. So if you want to realistically evaluate the pattern, you'll
need to extend your highly conductive model "dirt" to at least that
distance.

Then there's the problem of measurement. Nearly everyone seriously
overestimates his ability to accurately measure RF quantities such as
impedance and field strength at HF. It's even more difficult at 2
meters. If you even want to attempt this, you should begin with models
of several antennas of known characteristics, such as a quarter
wavelength vertical and a resonant near-quarter wave vertical. See how
closely your measurements agree with theoretical results. If and when
you can get close agreement, then you have some chance on trusting
measurements of other antennas. But even then it's not a certainty.

Those are some of the pitfalls.

Roy Lewallen, W7EL

dansawyeror wrote:

All,

I have been trying to determine the operation of a loaded vertical on
75 meters and over a less then perfect ground. It is inconvenient to
measure and change do its location.

After many attempts to solve this I have struck upon a solution to
'get close'. I have decided to build a model that works on 2 meters.
This will be small enough to construct in the shack and to experiment
with various 'grounds'.

What are the pit falls in doing this?

Thanks Dan kb0qil