Jerry wrote:
"Roy Lewallen" wrote in message
...
Richard Harrison wrote:
Build a small scale model that can be tested indoors and report its
characteristics. Antennas are scaleable.
That's more easily said than done. One of the critical characteristics of
a small antenna is loss. And to correctly replicate loss in a scaled
antenna requires scaling the conductivity of the conductors as the square
root of the frequency. To scale to a higher frequency requires that the
conductivity be better than the original. Unless the original is made from
lead and the scale factor moderate, this wouldn't be possible.

Roy Lewallen, W7EL

Hi Roy

I'm curious to know why you didnt use stainless steel as an example rather
than lead as your example.

The main reason is that many years ago I was involved in using physical
models scaled up in size in order to optimize microstrip transitions and
other features used for time domain equipment having rise times on the
order of 10 ps. At that time, I looked to see if it was possible to
model the loss accurately, and found I'd need a semiconductor to do it.
I recall that lead was about the least conductive common metal
available. So I tossed that one out in my example.

I don't remember investigating stainless or other steels, but that might
indeed be a way to do it. To make a strictly accurate scale model, the
permeability and permittivity (dielectric constant) stay fixed with
frequency, so a non-magnetic stainless steel would be necessary.
However, since the skin depth is inversely proportional to the square
root of permeability, a magnetic material has the loss of a non-magnetic
material having a conductivity lower by a factor equal to its relative
permeability. So a magnetic material such as steel or magnetic stainless
steel might be used to extend the range of possible loss values
available for making larger physical models. There's a very large number
of steel and stainless steel alloys, and good information on the
required parameters can be hard or impossible to find. So samples would
probably have to be measured.

In the case in question, however, a smaller scale model was proposed,
and we can probably assume that the original is made from copper. So
it's not really possible to create a smaller scale model which
accurately imitates the loss of the original.

Roy Lewallen, W7EL