On Sun, 10 Feb 2008 13:20:21 -0000, "Marcus Tait"
wrote:

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?

Well, it could be, both, most times for practical use it's ok.

But antennas also have directional senitivity, and you may just
have optimized the direction diagrams for your experimental setup,
and not for DX wide distance connections. There is impedance match,
resonance and directional characteristics which we have to care for,
all three parameters!
The antenna may be in resonance, but Z not necessarily at 50 Ohms,
or it may have 50 ohms but be out of resonance and thus loose
efficiency and/or it may be radiating down to the earthworms.

Everything in the vicinity influences the impedance and resonance
parameters. Therefore normally we mount the antenna at it's proper
place, on the roof, on the mast or where we want it to use.
Then we measure either at the antenna footpoint or the bottom cable
end. I have seen people mounting an SWR meter at the antenna
in 10 meter height and then take readings with a telescope
from down under.


To optimize transmitter to antenna coupling we need a directional
coupler for 50 Ohm
http://en.wikipedia.org/wiki/Power_d...ional_couplers
and connect the antenna to the output port, the signal generator to
the input port, and the analyzer to the coupled port.
The isolated port must be terminated with a 50 Ohm resistor.
Then the antenna is optimally tuned for transmission when the
reflected signal gets to a minimum, that is, nothing is reflected back
to the analyzer.
The easier way is to use a SWR meter
http://en.wikipedia.org/wiki/SWR_meter where the signal generator is
connected to the input, the antenna to the output, and the reflected
signal is show on the meter instead of an analyzer.
This way we make sure the transmitter feeds all the power to the
transmitting antenna.

But where does the energy go?
Worst case, straight up to heaven.
We need a remote antenna to make sure we did everything right.

w.