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January 31st 06, 06:36 PM posted to rec.radio.cb

On Mon, 30 Jan 2006 15:19:26 -0500, wrote:

+
++The db calculation were computed after taking the same CB and exciting
++it with a variable power transmitter to see how the noted S-meter
++readings related to power output of the variable transmitter.
+*******
+
+Sorry I put little faith in S-meter readings. I would prefer a
+spectrum analyzer, a preamp and the test antenna to determine gain of
+the antenna.
+
+I'd prefer a Maserati
******

Your tests really do not measure gain perse of the antenna but more
the efficiency of the antennae tested. Now that can be calculated into
a dB ratio and presented as gain. In reality it is not actually the
gain of the antenna. That would reconcile the large variations of
readings between the extremes.

Radiated power(Pr) from a 1/4 antenna is

18.27* Im^2 = Pr

where Im is the magnitude of the antenna current. Gain calulations of
an antenna factor in power loss to heat and other factors(Pl). Thus
the total power into an antenna is the sum of power radiatied as a TEM
wave and those losses which are incurred. While at very small power
levels the heat loss in an antenna does apporach zero. There are other
losses besides heat. The largest of all is ground loss.

Mobile installations can be as poor as 5% efficient. That results in
only 5% of the total power input actually being radiated. A very good
mobile installation can be as good as 65% efficient. In dBs that would
be about 11dB.

Lastly,I beleive that you mentioned that the distance separating the
two antennea were about 200 feet. A better distance would be 350 feet
minimum. This will get you beyond 10 wavelengths separation. In that
distance, near field power is a nonfactor and the TEM wave off the
antenna is fully formed. The near fields from an antenna are
measurable out to about 3 wavelengths from the antenna. It is there
that the TEM wave is nearly formed and by ten wavelengths from the
antenna is its strongest.

Trust me antenna efficiency is far more important than antenna gain in
a mobile installation. HIgh ground losses, can make the best mobile
antenna look horrible.

james