Now spouting way too much theory to support my suggestions...
Antenna effectiveness is always affected grossly by physical location,
height above ground and nearby obstructions. If you don't know where you
need to setup in advance I assume you want to take some measurements and
thus coverage predict when you do.
I'll stick to VHF/UHF systems only..
Antenna systems are relatively easy to predict performance of. Assuming
you follow known gain figures, the only big problem (IMO) you will
strike is excessive skying of radiation due to feedline etc radiation.
This is commonly cured by "decoupling the line" through some means. -
but I digress.
(Its strange you are looking at emergency comms, that is what the pager
service was I worked on!)
I disagree that max ERP is undesirable. I assume of course you are
talking omnidirectional antennas that tend to compress the vertical
beamwidth. About the only time this isn't desirable is if you are in
high mountainous country and need to either radiate into valleys or gain
reflections from high angles.
I can see the problem you are trying to resolve now. I think however you
need to take a multi tier approach and not just rely on a one time
measurement at a test site. There are just too many variables to allow
for when you move to a "real" location.
Do a number of basic field strength tests in variable topography at
"normal" operating distances and maybe 3-4 locations with a calibrated
measuring system. It doesn't have to be calibrated to an absolute figure
but you need to be able to replicate the process from on test to the
next. At some stage you will be able to create a table of -dBm vs
whatever device you are using for measurement.
If you want to be pedantic play around with likely base antenna mounting
height and method.
Make sure you do a distribution or at least minimally an average
measurement over several wavelengths. When you come back and do the
other antennas use the same measuring location. (A distribution will
also give you an idea how "choppy" the signal will become for a mobile
station)
Weather conditions may also influence results so try and do them at
close to the same time/day
By variable topography I am talking a hill top, flat area and then a valley.
You'll now have some operating distance parameters that you can plug
into a RF coverage program (like RadioMobile). You should be able to
work backwards from the figures you got in the field to establish the
actual antenna gain and radiation angle/lobe etc characteristics. You
will even see the slight bump in the horiz plane pattern of a jpole.
The next step now in setting up for real world is to take the known
antenna parameters and model actual locations that you need to cover for
the emergency. IMO this will give you a much better idea of what your
coverage will be without needing to do actual site measurements.
In other words you have now characterized your antennas and used a PC to
establish what the coverage will be.
When you want to compare another antenna you'll need to go back to your
test site for the greatest accuracy.
I assume you have done coverage modeling. The link below is not a good
representation but will give you an idea of what the output looks like.
In this case it is a 25W base to mobile 2m setup with a 5/8 on the car
and 6dB collinear at the base. The base is off to the upper right of the
image, the map is about 25 miles square and dBm is the scale on the top
left. It is Tyler TX.
http://pages.suddenlink.net/vk2yqa/f...in%20Tyler.jpg
I hope you find this useful. I believe it far more accurate and useful
for your application than comparing antenna ERP by itself.
Cheer Bob W5/VK2YQA
Tom Horne wrote:
As you can see from some of the replies I gave to others 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. Hence the desire to set up some sort of antenna
experiment that will allow us to compare the antennas against each other.
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?
--
Tom Horne, W3TDH