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Post subject: Atnenna measurement variations
Posted: Sun Sep 25, 2005 1:50 pm
I am doing a range set up using three antenna systems, I have a 7.5 foot Dish as Source Antenna and Horn Antenna as Reference , The reading I get for the same frequency with same set up is not consistent, I took about 10 reading continuosly and the range was around 1.5dB difference between the highest and lowest point, I don't understand why it could be so inconsistent? Do you have any idea about it? I would be thankful if you can answer my question.
Posted: Tue Sep 27, 2005 7:08 pm
Joined: Fri Sep 02, 2005 7:25 pm
Location: Hampshire UK
Depending on the frequency in use, you might be doing pretty good to be able to measure variations within 1.5 db anyway.
There are lots of variables here. I take it your source dish is transmitting a signal generator output horizontally, and you switch between the horn and your antenna under test at the other end of the range (is that near, or some kilometres away)?
The gain lobe in the source may be high enough that even a slight wind pressure would be enough. Depending on frequency, rain or air moisture could affect it. If your antenna under test is also a dish, you have a double chance of wobbling the lobes 1.5dB worth. Everything has to be fixed firmly, and not be prone to vibration. I was once careless with a piece of heliax coax cable, and caused more than 1.5dB of problems because of bending it too hard!
All of the above assumes you are using very stable calibrated kit.