Re: Horns

Alan Melia
 

I worked in the same office as some of the BT staff involved in the Goonhilly Telstar project. I believe it was thought in the US that a big dish could not easily be used to track a low orbit sat like Telstar, but we had some of the only experience in the world at the time, having successfully commisioned a bigger dish at Joderall Bank, which could track 90min orbit sats. The Holmdel and Lanion horns were much lighter structures but much lower gain.
 
Yes nearly all the ''aerials'' on the BT tower were of this type and were very much easier to install, and work on than conventional prime focus paraboloids. The easier instal made the road closure much shorter, I believe. and probably less windage?
 
Alan
G3NYK 

----- Original Message -----
Sent: Thursday, January 30, 2020 2:47 PM
Subject: Re: [UKMicrowaves] Horns

Hello Paul,

'... interesting that the higher quality images of the Andover Telstar horn antenna reveal that the reflecting plate you mention may actually be a parabolic surface.  It has always been called a horn but was it really an early offset fed dish?'

Yes, but no, but yes, but no, but yes ... etc! There's a very good Wiki on the the Holmdel Horn Antenna used by Penzias and Wilson in Wikipedia. That article specifically states that the reflector was essentially an offset parabola. My suspicion is that the original designer's thought that they could 'get away' with putting a 45 degree plane reflector on the top of a simple pyramidal horn, but they discovered that currents induced in the edges of the aperture (a problem with any 'simple' horn) resulted in the production of sidelobes, and they used the reflector as a way of focusing the energy. That could be regarded as a form of phase correction for the aperture, but equally, as you say, the antenna could be regarded as an early offset reflector with a screened feed system. If I remember, I'll try to get a copy of the original paper in the Bell System Technical Journal from the IET Library.

Certainly, there is a problem caused by edge currents with simple horns - and I'd include the hoghorn as a 'simple' horn. There are a number of solutions, but for narrowband horns, one of the best approaches was invented(?) by Dick Turrin W2IMU. In this he excited a higher-order waveguide mode, and arranged for that and the original first-order mode to arrive at the edge of the horn 180degrees our of phase, thus cancelling the edge curents. A problem with this is one of construction, as it needs a mode converter to generate the higher order mode. W2IMU used a flare in the waveguide to achieve that. Later work by Potter, in the US, and Skobelev in Russia introduced step changes in the WG to perform the mode conversion, but the principle remains the same. That work has been used by a number of designers, including yours truly , as the basis for the design of feedhorns, often including EM simulation as a method of refining their work.

73 - Oll an gwella

Chris


On 30/01/2020 12:11, Paul Randall G3NJV wrote:
Chris,  interesting that the higher quality images of the Andover Telstar horn antenna reveal that the reflecting plate you mention may actually be a parabolic surface.  It has always been called a horn but was it really an early offset fed dish?



Sent from my Samsung Galaxy smartphone.



-------- Original message --------
From: Chris Bartram G4DGU <chris@...>
Date: 29/01/2020 21:22 (GMT+00:00)
Subject: [UKMicrowaves] Horns

Andy

'I remember some antennas on the larger microwave towers that looked like horns, but mounted vertically, effectively pointing upwards. As far as I could make out, where the mouth would normally be, was a plate at 45 degrees, with the actual aperture on the side at the directly opposite this 45 degree panel, now pointing horizontally.'

You're talking of the 'hoghorn'. That was used in microwave relay systems because of its low sidelobe performance. However, the 'flat panel' wasn't flat. It effectively formed a phase-correcting structure allowing the optimisation of sidelobe performance.

A very large hoghorn was used by Penzias and Wilson in their discovery of the cosmic microwave background.

73

Chris G4DGU

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