David wrote:
I have a 1/2 wave - end fed whip antenna that has been designed for
operation on 915 MHz.

A Signal Generator is attached to a VSWR detector. The reference port is
50 Ohms. The antenna is connected to Z2 port and the detector output to
an oscilloscope.

I have calibrated the output for 50 Ohms and noted the 2:1 VSWR voltage
when
using a 2:1 precision ref at Z2 in place of the antenna.

I have a Spectrum analyzer with short wire as a pickup to observe
radiation from antenna (I image 1/2 the trouble is here ?)

When I sweep manually across say 800 MHz to 1GHz the VSWR gets worse
when the radiated power increases and visa versa.

When I then plug in a commercial antenna the VSWR does the same thing
EXCEPT when it is in the region it is resonant, the VSWR improves as the
level rises, at all other points the VSWR goes down as radiated power
increases.

Is there something silly going on here or is this what should happen.
This is the first time I have attempted to measure an antenna so would
appreciate any help.
I notice the peaks at a reasonably fixed period
(816,879,933,993,1.051G), can I learn something about the antenna from
how apart these peaks ?

Thanks in advance,

If the wire is significantly less than a wavelength on the spectrum
analyzer it shouldn't cause effects that are _too_ weird, and is
probably better than hanging an unknown sorta-tuned antenna on the thing.

You mentioned the Magic Flame War Word when you mentioned VSWR. The
relationship between VSWR, amplifier performance and antenna performance
is complex. I'll get behind this here bunker and say that VSWR
measurements are useful things if you know the effects, and if you're
using components that are designed for 1:1 VSWR at some design impedance
that's what you should do. Beyond that a given amplifier may not have
it's peak power output at 1:1 VSWR, and a given antenna may not have
it's peak effectiveness at 1:1 VSWR, unless the amplifier and antenna
are specifically designed for that. Try to ignore the side threads that
this paragraph will generate -- those folks all voted for either Nader
or LaRouch in the last election.

Did you pay attention to the amplifier power output when you were doing
your VSWR sweep? Did you try moving the spectrum analyzer pickup
around? An amplifier that's designed for a 50 ohm output and run into a
different impedance may indeed generate more power, at least until the
finals overheat. It may be that the amplifier you're using happens to
generate more power into the off-tune load that the antenna presents -
or - that the antenna radiates more efficiently in the direction of your
pickup.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com