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?


Hi Marcus,

You have tuned a system with a variable stub. To replicate its use in
a practical application, you would replace the generator with an equal
length line terminated in 50 Ohms shielded in a box the size of the
generator (with a dummy line going to the AC wiring to complete the
ground path). You would then have to replace the spectrum analyzer
with your transmitter/receiver in a similar sized box at the end of a
similar length of transmission line. Also during operation, you would
have to inhabit the same vicinity/volume of space as you did when you
clipped the wire. However, precision and resolution may not be that
demanding as you imply by asking for two places: 50 ±j0 Ohms.

In other words, your work would suffer compounding issues of proximity
and parasitic relations - lack of isolation. As a quarterwave antenna
does not present 50 Ohms without extenuating factors, achieving that
figure away from the bench would be more luck than engineering.

73's
Richard Clark, KB7QHC