Why do feed-horns ever have to be so big?
OK - so I expect that something this size would be used on a really big dish, like maybe 25m or more. If we estimate the lady is about 170cm tall (without high heel shoes), and roughly measuring on the picture, the feed is about 85/110 of her height.
Then feed is about D = 1.3m.
At S-Band 2.3GHz, the aperture directivity is about 29.9dBi, and at X-Band 8.0Ghz , it is about 40.7 dBi. You can lower these as much as you like by the horn efficiency (take a guess)!
There might be other tricks inside the funnel, like corrugations.
My point is, with a gain so high, most things you could point it at would be under-illuminated with just a very hot spot in the center. Surely any dish can be fed efficiently with a much smaller horn. Maybe it has to illuminate a tiny sub-reflector very far away?
If you know, then do tell, because I just do not know enough about what might motivate such a huge feed.
Apart from the fact that a horn for 2GHz is going to be big anyway to achieve hi gain
You are assuming its feeding a dish, and that may be correct
But it could also be a direct feed in a point to point system ( no dish, just horns at each end of the link)
cheers
Dave
Any "aperture antenna" like a horn or a dish is designed with respect to signal wave length, to obtain a desired radiation pattern and directive gain.
If your horn look too big to you, you can try adding a lens or another trick to get a similar gain by a smaller structure.
This is how antennas work. Using a shorter wave length is often preferred to get a smaller antenna, then such device requires more precision to achieve good parameters, also related to the wavelength.
Hi Darktrax
Do you know what is its use?
I'm wondering whether it is aimed for far-field radiation (communications, radiometry, etc) or for measurement (e.g. in a compact range facility).
Regards
Z
Hi EDN folk.
OK - after a little searching, I got lucky. Since the weight and economics of even a point-to-point link would be saying "Use a dish ya fool", I kept looking among the very big dishes.
The big feed comes from the DSN (Deep Space Network), and feeds a 34m dish as part of an array which includes the 64m and the 34m standard antennas, and it has a whole lot of special features, including trick insides that mean the aperture size at the horn entrance is no longer the main measure to calculate the maximum dBi.
Both frequencies are at one focus!
This feed is Dual-Band S and X, feeding a dish from a common Cassegrain focus. This becomes possible because the huge dish size comes with an appropriately huge sub-reflector which is large enough to fit a suitable number of S-Band sized wavelength across. This seems to be deliberate slight under-illumination to make rear spill-over noise as low as possible. Even before the ray met the sub-reflector edge, it was at a taper spilling less than 2%.
Using the more sane dish sizes we normally encounter for communications, the S-Band feed has to go at the prime focus, and if we want dual-band, we have to use a frequency selective surface sub-reflector that looks somewhat transparent to S-Band signals
It is corrugated inside with tricks!
High efficiency feeds recovering HE11 use corrugations, but these ones are special in that the corrugation depths are tricky by using odd number multiples of λ/4.
Depths greater than λ/4 and less than λ/2 in S-Band, and depths greater than 5λ/4, but less than 3λ/2 for X-Band. I guess it helps that X-Band wavelength is about one fourth of that for S-Band. With these controlling features inside, as the horn is made longer, so getting larger aperture, a point is reached where further increase in length does not increase the gain any more, nor make the beam-width any smaller. The effective aperture is not the size at the front.
The pattern shapes would differ with frequency, but not so much the gain or beam-width. They wanted to be able to use the same feed for various dish sizes in the network, so they worked to make the optics compatible.
The dishes are shaped!
Departing from the parabaloid-hyperboloid in a controlled way to mess with the shape of the sub-reflector to re-distribute the energy across the main dish more uniformly, and not at all in the center blocked area, and then also alter the shape of the paraboloid to "un-mess" the effect on the phase has been standard ever since Voyager1. This arrangement takes it a step further. A ray from the sub-reflector edge does not arrive at the main dish edge, but instead lands about 0.65m inside. The spill-over noise from the ground, the side-lobes from past the sub-reflector, and the way these beam-feeds deliver into waveguides below are all part of it. Probably the heat-insulation barriers for cryogenic kit may also feature in the design.
They really go after gain!
The addition of one new 34m antenna to the network adds about 0.8dB to the 64m and 34m standard array. The outer parts of the dishes seem to be mesh. I guess this must be for reasons of weight and wind loading.
I am sorry- my message appears a bit above.
Under" See? Making good antennas is tricky..."