I found these things (they're aerials I presume) with six "spokes" at the Perth Airport. Does anyone know what they're for? Thanks.
Source: Google Maps
Source: Google Maps
Source: https://www.youtube.com/watch?v=WjJYSquq4jU&ab_channel=RowinDreef
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\$\begingroup\$ That particular pattern, and the fact that there are two near each other (perhaps for each runway?) and the unique angle they make with the ground plane, I cannot explain. But my first guess that I'd want to eliminate would be beamforming direction finders (phased arrays.) That said, if you can, get out there with a tape measure and get some measurements for us!! If they let you do it, then that eliminates certain thoughts. If they don't let you, then they might tell you why they are dragging you away. Either way, please report back. ;) \$\endgroup\$– jonkCommented Oct 2, 2021 at 4:34
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\$\begingroup\$ Yeah I'm actually in Melbourne, and in case you don't already know, we've been in lockdown for the last 56 days, so there's no chance of me going there anytime soon I'm afraid. If you want to know how big the things are, you'll probably just have rely on the trees around it for reference and a map with satellite images. Here's the coordinates if that'll help: -31.928984, 115.974288 \$\endgroup\$– kene02Commented Oct 2, 2021 at 5:23
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\$\begingroup\$ That area of the airport seems related to the Bureau of Meteorology. They release weather balloons just north of there. Maybe a UHF wind speed radar or something like that. \$\endgroup\$– Spehro 'speff' PefhanyCommented Oct 2, 2021 at 5:33
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2 Answers
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Any array of aerials can be used, with sufficient extra hardware and processing, for doing any phased array or beamforming application.
However, certain arrays lend themselves to very low cost implementation. That radial pattern is consistent with a phase modulated direction finding receive antenna array (which is not saying that's what it is).
In use, such an array would switch reception from one to the next aerial at typically audio frequencies, making a whole circuit round the circle in a millisecond or so. This would impose phase modulation on the carrier of the received signal. The phase of the phase modulation (are you still with me?) with respect to the switching pattern would directly give the direction of arrival of the signal.
The hardware needed is minimal. A controllable antenna switch, to switch the inputs into a single, standard, audio receiver. A phase sensitive detector, which could be analogue, or a program running on a PC with an audio input, to resolve the direction.
The system is entirely passive, and needs no particular signal or cooperation from the sender.
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Going by the dimensions obtained from Google maps, they appear to be two 7-element ESPAR antennas for HF, located 170 m apart.
ESPAR - Electronically Steerable Parasitic Array Radiator.
