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Antenna patterns have glitchs in them.

Antenna patterns have glitchs in them.

Antenna patterns have glitchs in them.

Dear Engineers,

Whilst pattern testing various antennas in a very remote part of Ireland, it has been noticed that the pattern often has a “glitch” on boresight (i.e. the glitch position corresponds to when the antennas are "peaking" to each other -pointing into each others boresight). The glitch takes the form of a very sharp downward spike which goes down by several dB.  When the glitch appears, we usually keep re-taking the pattern until we get a pattern without the glitch. The glitch sometimes happens slightly off boresight also, but is usually on boresight.

The antennas under test are usually horns or planar spirals. The antennas under test are always used as receivers. The transmit antenna is always a horn antenna. Frequencies used are in the 1 to 18 GHz range. The antenna_under_test is rotated 180 degrees. Patterns are taken at intervals of 1 GHz or ½ GHz. The received power is sampled at a certain rate as the antenna is rotated. Patterns are taken outdoors and it is sometimes windy. The distance between antennas is always in the far field, but is never measured exactly. It is usually between 2 and 4 metres.

I am wondering if the glitch could be caused by a reflected ray, -perhaps the transmitter radiates toward the test_antenna, and the received ray reflects back to the transmitter and has a cancelling effect at the transmitter, (-due perhaps to a misfortunate placement of the antenna at a distance which makes the path length of  such a reflected ray equal to an odd integer number of half-wavelengths).

-Though it is confusing that the glitch often disappears when the pattern is re-taken. (Sometimes five or so re-takes have to occur before a glitch-free pattern occurs) –However, perhaps the motor drive which rotates the antenna does not always go at precisely the same speed  and perhaps received power sample points are then  not always taken at exactly the same point of rotation.

It has been said that the glitch could be due to some software fault. The received power is read by an analyser which feeds the data to a PC, which in turn produces a graph showing the antenna pattern.

-would be very grateful for any thoughts on this.

RE: Antenna patterns have glitchs in them.

"...five or so re-takes..."

I find that funny. Anyway...

Maybe try taking the patterns STARTING with the boresight (confirmed by careful peaking manually) and then working out in various directions. If this eliminates the problem, then it indicates that it is probably a measurement problem. If the problem remains (even sometimes), then it might help to provide clues. After all, if you've just peaked, then how could it go up?

RE: Antenna patterns have glitchs in them.

That sounds odd.

I've seen two spiral antennas so close to each other that round trip RCS bounce produced ripple. i.e. the energy from the transmit to receive antenna is both the direct ray and the double bounce off the receive/test antenna and the transmit antenna again. Add some absorber in your tx antenna aperture and see if your problem goes away.

Are you using a rotary joint? sometimes they get stuck and look bad at a particular point.

How bout maybe you're saturating your receiver only at boresight of the antenna? That wouldn't make that problem.

If you have a rotary joint, I'd bet on that being the culprit.


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