Mount for SETI Horn of Plenty
Andrew Thornett
Stephen Arbogast
Stephen Arbogast
Stephen Arbogast
Stephen Arbogast
Andrew Thornett
Sent: Sunday, May 24, 2026 1:52:52 AM
To: Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Subject: [SARA] Re: Mount for SETI Horn of Plenty
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Mike Otte
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Stephen Arbogast
Andrew Thornett
To: Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Subject: Re: [SARA] Re: Mount for SETI Horn of Plenty
Adrian
Andrew Thornett
Sent: Monday, May 25, 2026 2:21:56 AM
Cathal O'Donghaile
Andrew Thornett
Sent: Monday, May 25, 2026 9:21:02 AM
Cathal O'Donghaile
Adrian
Andrew Thornett
Ayushman Tripathi
Hi Andrew,
Really nice build!
Quick question: how do you think this'll perform vs a 1m dish with a cantenna at H-line?
Also, I found the original Horn of Plenty image from the SETI League site:
https://setileague.org/photos/wghorn.htm
Andrew Thornett
Sent: Monday, May 25, 2026 9:17:15 pm
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Stephen Arbogast
Stephen Arbogast
Stephen Arbogast
Andrew Thornett
Marko Cebokli
A horn with a significantly bigger aperture must have a smaller opening angle, otherwise the phase error at the aperture will be too big. Imagine a sphere with the center at the horn throat, and its deviation from a flat plane at the horn mouth. You want that to be less than lambda/4 or so. High gain horns tend to be quite long!
If you just want to add 30% or so to the aperture, it might still work with the original flare angle.
There are a lot of online horn calculators, just search for "horn antenna calculator".
Marko Cebokli
25.05.2026 13:01, je 'Andrew Thornett' via Society of Amateur Radio Astronomers napisal
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Andrew Thornett
Sent: Tuesday, May 26, 2026 10:03:02 AM
To: sara...@googlegroups.com <sara...@googlegroups.com>
Marko Cebokli
Hello Andy!
For maximum gain (at boresight), you want a plane wave (flat wavefront) with constant amplitude across the aperture of your antenna. For lower sidelobes, you still want a flat wavefront, but with an amplitude taper towards the edges, which will cost you gain (a tradeoff).
With a flat wavefront, the contributions from all parts of the aperture will add in phase at infinity. If the wavefront is not flat, the sum will not be maximum, because the "arrows" will not point in the same direction (remember vector addition - or adding horizontally shifted sinewaves).
Wavefronts emanating from a point (throat of your horn) are spherical. The longer the horn with the same aperture, the flatter they will be at the aperture - the less difference there will be between the path from the throat to the center and edges of the aperture.
You can also imagine the situation on receive. The wavefronts from the source in the far field (which condition is true for astronomical sources) are flat (means the same as the rays are parallel). Once they perfectly simultaneously arrive across your horn aperture, they must now reach the horn throat, and the ones impinging near the edges will have a longer path than those that came in at the center. You want to minimize this difference, so they add as much "in step" as possible. With a simple horn, these differences will never be zero, but with a sufficiently long horn, you can reduce them to acceptable values. You can correct the phase error of a short horn with a lens at the aperture, but that is practical only for small horns (X band and up), with a L band horn with 1m aperture, the lens would probably weigh 100 kg...
The required length increases very fast with the aperture, so horns with more than 20..25dBi of gain are really not practical (well, maybe at mm waves).
Marko Cebokli
26.05.2026 11:12, je 'Andrew Thornett' via Society of Amateur Radio Astronomers napisal
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Andrew Thornett
Thabks Marko for making such a large efffort to explain it to me. My attempts to understand what youre saying in capitals below - please correct me if Im wrong.
Andy
Hello Andy!
For maximum gain (at boresight), you want a plane wave (flat wavefront) with constant amplitude across the aperture of your antenna. For lower sidelobes, you still want a flat wavefront, but with an amplitude taper towards the edges, which will cost you gain (a tradeoff).
THE BEST HYDROGEN HORN IS ONE WHERE THE RADIO WAVES ARE PARALLEL AT THE FRONT END OF THE HORN - A BIT LIKE WHEN I AM DOING SOLAR ASTROPHOTOGRAPHY USONG A DAYSTAR QUARK FILTER WHICH NEEDS PARALLEL WAVES OF LIGHT.
HOWEVER, TO GET RID OF THESE SIDE LOBES (A BAD THING AS THE MORE ENERGY THAT IS WASTED IN SIDE LOBES THEN THE LESS THE TELESCOPE CAN COLLECT AT THE CENTRAL WAVEGUIDE) THE WAVES SHOULD NOT BE PARALLEL - SO THERE HAS TO BE A COMPROMISE.
With a flat wavefront, the contributions from all parts of the aperture will add in phase at infinity. If the wavefront is not flat, the sum will not be maximum, because the "arrows" will not point in the same direction (remember vector addition - or adding horizontally shifted sinewaves).
I GUESS THST IS AN EXPLANATIOJ OF THE FIRST BIT, SO IF I DO NOT UNDERSTAND IT FULLY, THEN I CAN STILL PRODUCE A WORJING HORN, AS LONG AS I APPLY THE PRINCIPLES.
Wavefronts emanating from a point (throat of your horn) are spherical. The longer the horn with the same aperture, the flatter they will be at the aperture - the less difference there will be between the path from the throat to the center and edges of the aperture.
OH RIGHT, SO IF THE HORN IS TRANSMITT9NG THEN IT HAS TO TURN SPHERICAL WAVES FROM THE MONOPOLE IN THE WAVEGUIDE INTO FLAT WAVES AT THE FRONT OF THE HORN.
CONVERSELY, IF THE HORN IS RECEIVING AS WE DO, THEN FLAT WAVES NEED TO BECOME SPHERICAL WAVES - IS THAT RIGHT?
You can also imagine the situation on receive. The wavefronts from the source in the far field (which condition is true for astronomical sources) are flat (means the same as the rays are parallel).
THAT MAKES SENSE TO ME - LIGHT INCLUDING RADIO WAVES - COMES FROM SKY SOURCES EFFECTIVELY AT INFINITY AS THEY ARE SO FAR AWAY SO THEIR RAYS ARE FLAT WHEN THEY REACH MY HORN ANTENNA.
Once they perfectly simultaneously arrive across your horn aperture, they must now reach the horn throat, and the ones impinging near the edges will have a longer path than those that came in at the center.
MAKES SENSE - RAYS ON OUTSIDE HAVE TO TRAVEL FURTHER TO MEET AT A CENTRAL POINT THAN CENTRAL RAYS - IF THE HORN IS 4M LONG AND 2M WIDE THEN CENTRAL RAYS TRAVEL 4M TO THIS POINT BUT OUTSIDE RAYS MUST TRAVEL SQUARE ROOT OF (2 x 2 + 4 x 4) = 4.47m [Pythagorus theorum].
You want to minimize this difference, so they add as much "in step" as possible. With a simple horn, these differences will never be zero, but with a sufficiently long horn, you can reduce them to acceptable values.
I SEE - THIS IS THE PHASE ERROR - BASICSLLY, THE PHASE DIFFERENCE ORODUDED BY DIFFERENT PATH LENGTHS FROM CENTRE TO OUTSIDE OF HORN.
You can correct the phase error of a short horn with a lens at the aperture, but that is practical only for small horns (X band and up), with a L band horn with 1m aperture, the lens would probably weigh 100 kg...
THIS BIT ABOVE I DONT UNDERSTAND - HOW CAN YOU CORRECT IT?
The required length increases very fast with the aperture, so horns with more than 20..25dBi of gain are really not practical (well, maybe at mm waves).
SMALLER WAVELENGTHS MEAN SMALLER HORNS SO YOU CAN EFFECTIVELY BUILD LOT LONGER HORNS - LONGER RELATOVE TO WAVELENGTH THEY ARE MEASURING.
Marko Cebokli
Previously, you said the following - can O also ask you the questions below:
A horn with a significantly bigger aperture must have a smaller opening angle, otherwise the phase error at the aperture will be too big. Imagine a sphere with the center at the horn throat, and its deviation from a flat plane at the horn mouth. You want that to be less than lambda/4 or so. High gain horns tend to be quite long!
WHY LAMBDA/4 = WHAT IS MAGICAL ABOUT THAT VALUE?
If you just want to add 30% or so to the aperture, it might still work with the original flare angle.
A. C.
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Adrian
"My God, it's full of stars!"
Dave Bowman
andrew....@googlemail.com
Hi Adrian,
I appreciated your concise responses! Easier to understand.
I didn’t understand what I was asking so my questions developed as I got more information back – and I asked for mor information as a result.
Andy
On Saturday, May 23, 2026 at 4:58:13 PM UTC-6 Stephen Arbogast wrote:
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