On Monday, May 25, 2026 at 1:09:30 AM UTC-7 Andrew Thornett wrote:
Can a horn be made larger simply by adding an extension to end of same flare angle, or does ot need to have different flare angle?
Could I, for example, if I so wished, just add a bit to end of my Horn of Plenty to make it bigger? Give it some Viagra, as it were?
Andy
SIMPLER MORE CONCISE EXPLANATION, THAT GIVES PRACTICAL ADVICE FOR AMATEUR RADIO TELESCOPE CONSTRUCTOR.
On Mon, May 25, 2026 at 2:56 AM Adrian Clausall wrote:
Andrew, absolutely.
Think of it like an optical telescope, the larger the primary mirror or reflector then the smaller field of view and the more light gathered so the fainter the objects able to be observed.. Keeping the same flare angle preserves the same impedance matching gradient and the only things that are changed is the horn length, width, weight, beamwidth and gain but what doesn’t change then is the VSWR which stays essentially unchanged. Actually, also the framing around the periphery of the horn aperture might also provide some reduction in the side lobes as the “edge curvatures” reduce the rim knife edge refractions somewhat that contribute to side lobes. See figures on pp 21 of this high end but excellent reference of horn antennas electromagnetic theory (https://www.ece.mcmaster.ca/faculty/nikolova/antenna_dload/current_lectures/L18_Horns.pdf)
The above is a concise response, where I have tried to avoid going into details, and is based on the assumnption that your intention is to only extend thre horn a “reasonable” length < 1m. So yes, extending a horn while keeping the same flare angle increases the aperture and can increase gain—but only up to the point where aperture efficiency begins to fall due to excessive amplitude taper. Horns have an optimum length for a given flare angle. Beyond that, gain increases only slowly or even decreases and so the more detailed explanations of the theoretical implications as have been provided are thus accurate.
Adrian
MORE COMPLEX EXPLANATION, FOR AMATEURS WHO WISH TO GAIN A MORE DETAILED UNDERSTANDING OF THE PHYSICS INVOLVED.
From: Marko Cebokli.
Sent: Tuesday, May 26, 2026 1:23:25 PM
[Marko sent me a much more detailed explanation. The comments in capitals or square brackets are my own, and represent my attempts to understand Marko’s explanation.]
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 SPECIAL ABOUT THAT VALUE?
If you just want to add 30% or so to the aperture, it might still work with the original flare angle.
THIS FITS IN WITH ADRIAN’S COMMENTS ABOVE.
There are a lot of online horn calculators, just search for “horn antenna calculator”.
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).
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