Antenna phase center
Antenna phase center
(OP)
If you had a small 6 GHz antenna, and had to measure its phase center very accurately, how would you do it. I am especially interested in if the phase center shifts slightly at different angles.





RE: Antenna phase center
RE: Antenna phase center
RE: Antenna phase center
The phase centre of the antenna is found by finding a equi-phase front in the far field. You would move the receiving antenna in a circle centred on the expected (geometric) phase centre of the antenna (your first guess). You would then change both the radius of curvature (slightly) and the centre of curvature to get a better equi-phase front. This sounds like a slow (but highly automated) process. Note that the receiving antenna is moving in an X-Y plane as well as in an azimuth angle just for doing measurements in one plane.
I suppose that at each X-Y position you should also change the azimuth angle slightly to maximise the received signal.
I have never done anything like this, so this is just a theoretical discussion on my part.
RE: Antenna phase center
I've measured a few of them. Do you have an antenna chamber biff44? Typically you rotate the antenna, measure phase, then slide it back or fwd and remeasure phase. Then note where your antenna is compared to the center of rotation. Wide beam antennas are tricky to make a good setup, since things bounce off the absorber around them, narrow beam antennas are easier.
If your antenna isn't a monopole or bicone, then your phase will start moving/shifting at wider angles, say 60 degrees off boresight or more.
You'll typically measure an H plane phase that's longer at wider angles, and an E plane phase that's shorter.
Picture an X band horn antenna, at boresight the energy radiates from say 5 inches inside the antenna. As you move in the Eplane, the radiation is from the front edge as the electrons turn the corner, and hence you have moved 5" closer or shorter. In the H plane, the radiation appears to be from the far side of the horn since it's a flat plate with plain view to the far angle (the near wall outer edge will also radiate a little, but much less than the far wall flat inner surface) you then measure slightly longer phase delay in the H plane at far angles.
Phase center is a sum of all the energy from the antenna. That data is then combined to a single point.
The phase center should be spec'd as a single point, with +/- phase error, over a range of angles.
Are you building a phased array? If it's an array, the elements near the one you're testing affect your phase center results.
kch
RE: Antenna phase center
Leave the receive antenna stationary. Pivot the transmit antenna. You then only need a simple pivot rather than an X-Y-azimuth mount! Move (slide) the pivot point on the transmit antenna until you get a good/acceptable equi-phase front. To fully automate this you need a slide and a pivot on the transmit antenna, but there is no question of having to move radius of curvature as well of centre of curvature as I had suggested previously.
I think you could even do this test manually since it is so simple, provided you had a stick to do the rotation from a distance. You wouldn’t want an “ugly bag of water” near either antenna!
RE: Antenna phase center
RE: Antenna phase center
RE: Antenna phase center
For 'a small 6 GHz antenna' this might not be a real concern.
RE: Antenna phase center
phase vs angle consistency; the primary criteria for use in amplitude phased arrays or phase interferometer arrays.
phase versus angle and phase stability versus frequency, primarily for Feed antennas used on reflectors historically.
Nowadays UWB antennas need phase versus frequency stability or at a minimum phase center location predictable in some simple algorithm (based on new thru-the-wall-imaging antenna requirements I've worked with).
GPS antennas for differential GPS need phase center stability versus angle over their operating band and can be much more difficult to design than just throwing a patch on a ground plane.
kch