On Sat, 10 Nov 2007 04:13:56 GMT, Tom Horne
wrote:

I'm trying to
devise a way of practically comparing antennas available because in
emergency service communications support we have no way of knowing were
we will need to set up.

Hi Tom,

There are one, two, three or four certainties about antenna
performance. There are dozens of factors that are way beyond your
control that degrade system performance. Emergencies rarely conform
to optimization; instead you need to think of flexibility.

Hence the desire to set up some sort of antenna
experiment that will allow us to compare the antennas against each other.

As has been offered by a multitude here, that is both very simple in
description and complex in accomplishment.

Just for the sake of my education is it likely to be true that the
antenna that puts out the most effective radiated power will be a bad
choice in a large percentage of possible sites?

This question alone reveals a most curious idea. First, it presumes a
fact that has never, or should have never grown in your mind from all
the contributions here (or from external study):
Effective = Bad
is a non-starter.

Communications performance is measured by link budgets, not antennas
alone. The link budget is an accumulation of factors such as:
1. Applied Power;
2, Transmission line loss;
3. Antenna Gain (Effective Radiated Power);
4. Path Loss;
5. Receiver Sensitivity;
6. Multipath Sensitivity;
7. Noise in receive path.

This list could be made longer, but as long as it is, in an emergency
you really have no control over 4, 5, 6, and 7 (and you may be at some
risk even with 2 and 3). Your task as an emergency operator would be
to recognize and compensate for them as best as possible where it does
not jeopardize mission. Often, mission will negate any opportunity to
do anything about these last factors. This requires you to plan ahead
so that you recognize where these factors could occur and avoid them
first, rather than being distracted with them after their discovery.

The difference between a J-Pole's performance measured on a range, and
that of the standard ground plane is really negligible in comparison
to putting either antenna into a Fresnel Zone where the multipath
completely nulls the signal. They are BOTH dummy loads in that
situation. So you carry a yagi to compensate and switch out the
J-Pole or ground plane.

Unfortunately, you may not know where your contact is and you point
the "optimal" antenna in the wrong direction. The best antenna does
not supply the best result - but that is not a function of the
antenna, but rather the operator (pilot error).

In a nutshell, the questions you are asking imply you are seeking
assurance for managing risk, risk that is so variable that no
assurance is possible. Links fail in the face of best efforts, that
is why it is a hobby at our level of cash flow. $500 is not much of a
premium payment for some emergencies.

The emergency repeater systems I've worked on have represented many
10s of thousands of (1970s) dollars as built up from surplused (MASTR
II and similar) equipment. We spent more like thousands of (2007)
dollars to get there.

73's
Richard Clark, KB7QHC