You've gotten several answers, but let me add one more, from perhaps a
little different point of view.

The question of whether the antenna is "tuned to 400 MHz" isn't an easy
one to answer, because it doesn't have any real single meaning. As you
adjust an antenna's length in the vicinity of a quarter wavelength,
there's no magic length where the antenna is "tuned" to work better than
for another length. You can change the length a fair amount in this
region without having any appreciable effect on the pattern or gain. So
your adjusting isn't impacting the pattern or gain of the antenna to any
appreciable degree.

What you are doing is changing the impedance at the base of the antenna.
That impedance is transformed through whatever length of transmission
line you have connected to it, and that transformed impedance is what
the signal generator sees. The signal generator, having a fixed output
impedance of 50 ohms, will deliver the most power when it sees 50 ohms
resistive, causing the spectrum analyzer to show the maximum received
signal. So what you've done is to adjust the antenna feedpoint impedance
so that, when transformed through the line, it provides the best match
to the signal generator output impedance of 50 + j0 ohms. You could say
that the antenna itself is "tuned to 400 MHz" when its feedpoint is
purely resistive (that is, the antenna is resonant). If you have a
perfect 50 ohm match at the signal generator, then you know that the
antenna impedance is 50 + j0 ohms (assuming you're using 50 ohm
transmission line between the generator and antenna). But if you don't
have a perfect match at the generator, the antenna might or might not be
resonant at the point where the match at the generator is best. In
practice, for a 1/4 wave antenna, it'll be very close. But as I said in
the beginning, that's of really no importance in determining the antenna
performance.

If you were to put an adjustable matching network between the signal
generator and antenna so you could assure that the signal generator
always sees 50 + j0 ohms, you'd find that the antenna length adjustment
(over a moderate range) has virtually no effect on the received signal
strength as long as you adjust the matching network for maximum received
signal after each length adjustment. But most VHF transmitters and
receivers are designed to work with a 50 ohm load and don't have any
built in adjustable matching capability, so the adjustment you've done
will result in the best overall performance when connected to such a
transmitter or receiver.

Roy Lewallen, W7EL

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?