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,