I am not an antenna
range expert, but have done some work in the
field (no pun intended). You did not mention
the frequency or bandwidth you were measuring
or the power levels, which could be helpful
to me or someone else trying to help you figure
out the source of the variation.
that you are not making these measurements inside
an anechoic chamber, then one source of variation
can be due to multipath if there are many reflective
objects in the area, especially vehicles that
could be in different places for each measurement.
Your dish probably has a fairly narrow beam
with lots of sidelobes in the proximity of the
dish, so changes in geometry within a few feet
of the dish can change the main lobe shape,
especially if the dish is being pointed close
to the horizon where it might not be designed
Also, having the receive horn
too close to the dish can increase sensitivity
to geometry changes. There is probably a rule
of thumb like having at least 10 dish diameters
between the dish and the receive horn. I will
defer to experts for that rule.
source could be if you are using a power detector
at the low end of its sensitivity.
you confirmed that the power from the source
is stable? Are you measuring a single tone or
a modulated signal?
An unlikely but
possible cause could be if the dish just happened
to be pointed toward the sun and the noise floor
could vary enough over a short period of time
if all the conditions (frequency, bandwidth,
etc.) were worst case.
Anyone else have
- Kirt Blattenberger