Greetings Harry:
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.
Assuming 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 to work.
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.
Another source could be if
you are using a power detector at the low end of
its sensitivity.
Have 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
ideas?
- Kirt Blattenberger
