How to estimate aperture efficiency
Eduard Mol
Marko Cebokli
The Sun is pretty crazy lately, so I suggest the Moon. You will probably have less than a dB ratio, but that can be measured.
Note that with this method, you are measuring G/T, not efficiency. Which is actualy what you want to optimize in a receiving system.
Marko Cebokli
2023-12-04 10:41, je Eduard Mol napisal
...of my 1 meter dish?
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Eduard Mol
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Marko Cebokli
Hello Eduard,
G/T is gain over (noise) temperature. It is a ratio that tells how good you are at receiving.The Moon/cold sky ratio will increase both with higher gain and lower noise (total noise, LNA + antenna), therefore you want to maximize G/T.
Optimizing for (dish) antenna efficiency might increase its noise pickup and in fact reduce your sensitivity. Therefore, for sensitivity, it sometimes makes sense to sacrifice some gain for a lower antenna noise, like with a lower edge illumination / deeper dish.
If your LNA noise is high, this is not so important, and you can just go for gain (efficiency). But with a super low noise LNA, antenna noise pickup (300K ground...) will become a significant part of your total noise, and reducing the gain a bit to reduce noise-sucking sidelobes can improve your sensitivity.
For example, if you could reduce the antenna noise from 60K to 30K by sacrificing 1dB of gain:
300K LNA: (300+60)/(300+30)=1.09=0.37dB noise reduction : does not make sense, G/T reduces by 1-0.37 = 0.63dB
30K LNA: (30+60)/(30+30)=1.54=1.87dB noise reduction: does make sense, G/T improves by 0.87dB
Marko Cebokli
2023-12-04 20:31, je Eduard Mol napisal
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B & MR Randall
Marko, Eduard,
Another point. All the noise from digital devices, etc can be a lot greater than 300K, so giving up gain for a cleaner pattern can be helpful. The digital noises of the world come and go as devices are turned on and off. G/(T+Digital_noise)??
Different antenna aiming can change both T and Digital_noise. Measurements at elevation of 90 degrees and 2 o3 beamwidths above the horizon may tell us something.
Nearby cell sites making overload effects in a preamp is another problem that is helped with a cleaner antenna pattern. Note that the antenna pattern may be different at the cell site frequencies.
Maybe I am a little grumpy from all the extra noises in my environment!
Bruce Randall
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Alex P
Eduard Mol
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Marko Cebokli
Hello Eduard!
To check the shape of your dish, a pattern measurement is the best. In fact, you only have to measure a few beamwidths around boresight (the rim moves a couple of wavelengths). There are some satellite beacons in the 20GHz band, so you could probably do it in a simple way. A well focused perfect antenna will have a clean main beam and deep nulls between main beam and first sidelobes. Note that the second requirement is good focus. Even a mm in or out will start to fill the nulls at 20GHz. So you should do many measurements while moving the focus axially, in say, 0.5mm steps.
Beacons:
https://uhf-satcom.com/satellite-reception/ka-band
The most detrimental is a big ("propeller") distortion of the dish. With a symmetrical dish (non offset) you can check for that by sighting from the side, and see if front and back rims match. Do this from two 90 deg apart directions.
Marko Cebokli
2023-12-05 14:17, je Eduard Mol napisal
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