"Jerry Martes" wrote in message
news:coTri.7566$yg1.763@trnddc04...
"Steve Reinhardt" wrote in message
...
Gentlemen,
If a man was of a mind to try to get some approximate antenna gain
comparisons, how many wavelengths distant might you like to separate the
antennas?
The proposed scenario is this: make a pair of 2M dipoles, one for
reference, one for receive. I was planning on using the local high school
football field, which is on the order of 50 wl, give or take. Transmit a
few mW at the design frequency, measure the signal strength, then repeat
with an alternate antenna, say a j-pole, collinear, or something else.
Now, this leaves out a whole bunch of useful information, that would be
tough for me to measure, like spherical gain distribution, etc. I'm
hoping for a figure of merit for the actual implementation of the tested
antenna. (Which, as you can imagine, I could model and save myself the
aggravation.)
I was pondering all this, when it occurred to me that I could not easily
determine when I get to the point where the square law behavior
dominates. I've seen a couple of equations relating the antenna dimension
to wavelength, but I must be really stupid today, because it's just not
sinking in.
Anyone care to comment?
73,
Steve
W1KF
Hi Steve
It is fairly easy to record exact radiation patterns of "2 meter"
antennas at 137 MHz, using NOAA satellites at the Illuminator. If you
have any interest in the details, you can contact me anytime.
Patrik Tast developed a (free) program for me that produces elevation
plane patterns of the antenna as the NOAA satellite passes over. Since
each satellite passes over 6 or 8 times per day, decent hemispheric
patterns can be made.
I'd guess that the plot of the antenna pattern, when using Patrik's
program is more accurate than any other method when evaluating ground
based "2 meter" antennas. I am open to learning where I'm wrong about
the accuracy.
Jerry
Examples of the radiation pattern data that can be acquired with Patrik
Tast's SignalPlotter program can be seen on one of his sites
http://213.250.83.83/~patrik/apt/log...22-2007/daily/
I have used this SignalPlotter program to make radiation pattern plots by
recording rssi voltage with a simple voltmeter with a RS232 connection and
with LabJack data recorder.
Jerry