"Owen Duffy" wrote in message
...
"Marcus Tait" wrote in
:

Hi,
I have just been tuning a 400MHz 1/4 wave antenna (approx 170mm length
of wire). I connect a receive antenna to a spectum analyser and the
antenna to be tuned to the signal generators 50ohm output. I set the
signal generator to 400MHz and adjusted the length of the antenna to
get a maximum peak on the spectrum analyser. My question is when i
have tuned the antenna for maximum output at 400MHz is the antenna
then tuned to 400MHz or is the antenna's impeadance tuned to 50ohm or
both?

My view is that you have adjusted the antenna to accept the maximum power
from your source which should in this case have an equivalent source
impedance of 50+j0.

Contrary to popular belief, radio transmitters do not necessarily or even
usually have an equivalent source impedance close to 50+j0.

You should not be surprised if you replaced the generator with a
transmitter and performed the same procedure, that the optimum length
might be a little different, and that if changed the feed line length a
little, the optimum length might be a little different again.

You would more usually want to adjust the antenna to be within a given
VSWR tolerance, and to know the final VSWR, you will need some form of
directional coupler on the signal source to determine rho etc.

Owen

The corollary to what you are saying is that a particular transmitter might
put out more power when presented with a load other than 50 Ohms. Assuming
the output is voltage limited, as to 13.8 V at the drain, more than likely a
hot ( high gain ) transistor will put out more power into a lower impedance.
This would also increase the temperature rise in the transistor.

Tam/WB2TT