Near Field of Horn Antenna
Near Field of Horn Antenna
(OP)
I am trying to find the near_field-far_field boundary for a Horn antenna with a rectangular aperture of 10cm by 20cm. I am using it for frequencies 8 to 10 GHz.
I have read that this boundary is given by 2*D^2/lambda (where D is the largest dimension of the antenna). -However, am I correct in stating that the "D" in this equation is NOT 20cm for this horn? -Surely the aperture is just the end of the tapered-out waveguide, and the "antenna" is really the feed probe in the rectangular waveguide section of the horn.
Any help greatly appreciated.
I have read that this boundary is given by 2*D^2/lambda (where D is the largest dimension of the antenna). -However, am I correct in stating that the "D" in this equation is NOT 20cm for this horn? -Surely the aperture is just the end of the tapered-out waveguide, and the "antenna" is really the feed probe in the rectangular waveguide section of the horn.
Any help greatly appreciated.





RE: Near Field of Horn Antenna
Once you've got it all figured out for your horn, then consider what happens if the horn is then suspended hundreds of feet in the air and positioned at the focal point of the Arecibo dish.
What happens to the near-field / far-field boundary then?
Arecibo info:
http://en.wikipedia.org/wiki/Arecibo_Observatory
RE: Near Field of Horn Antenna
is horn antenna electric or magnetic or both (depending on configuration) in the near-field? I dont know why I was in impression that it can act as magnetic antenna?
sorry about the dumb question. It just came to my mind when I saw this thread.
RE: Near Field of Horn Antenna
-Also, there is the "reactive" near field and "radiating" near field to be considered in this system.
Also, (generally) the bigger the dish, the more closely the waves coming out of it are plane_wave-like, and since the far field is (partly) characterised by being "plane_wave-like", then the big Arecibo dish would have a very "close" far-field (?)
RE: Near Field of Horn Antenna
I've always thought that the definitions of near-field and far-field were man-made, arbitrary and variable (several different definitions). As I've mentioned before, nothing significant happens at the boundary no matter where the committee decides to scratch the line.
RE: Near Field of Horn Antenna
Therefore, I would be most grateful if anybody knew what the equation for the near_field-far_field boundary distance is for all types of antennae. (e.g. Horns, Parabolics, Dipoles, Yagi's, Planar spiral antennae, arrays of patch antennae, log periodic dipole antennae, small patch antenna etc etc)
RE: Near Field of Horn Antenna
Everyone has access to an anechoic chamber: it is outdoors in an empty field (look straight up). The main practical issue is that of suspending one end or the other without causing significant reflections. I guess bad weather might be another practical issue... This also assumes that you can find a quiet frequency and use a narrow band receiver.
Usually, you just need to keep reflections of the main beam out of the sidelobe measurements. That's where the ratio (xx dB down) of the reflections might affect the measurements. Therefore, (outdoors) you aim the source straight up and move everything else. This assumes that you have a directional standard for the source.
I'd think that this would apply in chambers too, where the source should be fixed aimed at the best area of the RAM absorbers. Aiming the source all over the place would ensure that your results would contain garbage from every possible reflection.
There's been a lot of papers published on making near-field measurements and then adjusting the results for the far field. Way over my head, but I've seen the references.
Once you have the 3D pattern (or enough slices), integrate it and make sure it averages out to 0 dBi. It's fairly easy in MS-Excel. Don't forget to weight down the over-represented data (typically near the poles of a Mercator data set). I've seen patterns where the antenna must contain a power source (ha ha) because the published pattern integrated to + several dBi.
Scale models are common so that the experiment fits into the available resources.
SW models are increasingly common.
RE: Near Field of Horn Antenna
RE: Near Field of Horn Antenna
TTFN
RE: Near Field of Horn Antenna
I was thinking that it would be best to get the Source_Antenna and Antenna_Under_Test as close as possible. -That way, the ground reflected ray would be well out of boresight and likely to be just a weak sidelobe ray. However, i must ensure that i do not get the antennas so close that they are in each others' significant near field. -That's why I would like to find some really solid information on the Near Field equation (e.g. what the "D" actually is for all different types of antennas)
To check that I have the source and test antennas in each other's far field, i was thinking of just doubling their separation and noting if the received power goes down by 6dB. (Though this would be time consuming and I would prefer to have a solid equation for the acceptable separation of source and test antennas.)
RE: Near Field of Horn Antenna