Supporting the dish feed


John Lemay
 

Morning all

Supporting a dish feed - one arm, three or four ??

I've recently completed a new prime focus mesh dish. I made up the usual
four arms to make a quadruped to support the feed from the rim. But it
looked a bit tricky to adjust the feed position so temporarily I provided
instead a single support close to the axis of the dish. A simple series of
holes on this arm gave me easy options for moving the feed to and fro.

I made a small hole in the mesh next to the hub to pass the cable through.
That makes it obvious that the feed cable can be shorter than one which
follows one of four arms to the rim.

I'm happy with the mechanical side of things (such as the single arm needs
to be stiffer), but what are the pros and cons as far as RF performance is
concerned ?

TIA

John G4ZTR



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Denis Stanton G0OLX
 

John
Thank looks like a very good solution with minimal metal in the way of incoming rf.
Best 73’s
Denis
G0OLX

On 23 Feb 2021, at 09:24, John Lemay <john@carltonhouse.eclipse.co.uk> wrote:

Morning all

Supporting a dish feed - one arm, three or four ??

I've recently completed a new prime focus mesh dish. I made up the usual
four arms to make a quadruped to support the feed from the rim. But it
looked a bit tricky to adjust the feed position so temporarily I provided
instead a single support close to the axis of the dish. A simple series of
holes on this arm gave me easy options for moving the feed to and fro.

I made a small hole in the mesh next to the hub to pass the cable through.
That makes it obvious that the feed cable can be shorter than one which
follows one of four arms to the rim.

I'm happy with the mechanical side of things (such as the single arm needs
to be stiffer), but what are the pros and cons as far as RF performance is
concerned ?

TIA

John G4ZTR



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https://www.avg.com





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John Fell
 

Hi John,
For your example of just the feedhorn to support on the single arm it has the advantages of minimal aperture shadowing and reduced feeder run .

For feed mounted RF systems a tri-or quad allows reduced individual section arms and a closed system of forces , which will improve rigidity and resistance to windage loadings , without undue increased blockage.

Looking at the tower head I guess the dish is around 6Ft diameter and middle range F/d - using at 2.3GHz or 3.4GHz  ? 

73
John
G0API

On Tue, 23 Feb 2021 at 09:24, John Lemay <john@...> wrote:
Morning all

Supporting a dish feed - one arm, three or four ??

I've recently completed a new prime focus mesh dish. I made up the usual
four arms to make a quadruped to support the feed from the rim. But it
looked a bit tricky to adjust the feed position so temporarily I provided
instead a single support close to the axis of the dish. A simple series of
holes on this arm gave me easy options for moving the feed to and fro.

I made a small hole in the mesh next to the hub to pass the cable through.
That makes it obvious that the feed cable can be shorter than one which
follows one of four arms to the rim.

I'm happy with the mechanical side of things (such as the single arm needs
to be stiffer), but what are the pros and cons as far as RF performance is
concerned ?

TIA

John G4ZTR



--
This email has been checked for viruses by AVG.
https://www.avg.com






Mike Willis
 

The metalwork right next to the feed may act as a reflector reducing gain and generating sidelobes. Whether this will be better or worse than the traditional feed is something you would been to simulate. The Shepheard's crook feed has been popular for a long time. Can't be that bad.
--
Mike G0MJW


David Law
 

I have a similar home made feed without the ring (I think it's a loop you have there). It's supported on a 12mm fibreglass rod running from centre of dish to centre of feed. The feed has 4 thin 100lb non stretch braided Dacron fishing lines running as "guys" to the edge of the dish.
Dacron is used for kite line and fishing line. It could be a solution if to feel the arm is a bit wobbly.


John Lemay
 

All

 

Many thanks for the helpful and constructive comments. I think it’s quite likely I’ll make a permanent version of this trial single arm – seems there are more pros than cons.

 

To put some meat on the design; it’s a 1.5m dish with 0.43 F/D. It may not look like that in the picture though. It’s a loop feed with choke, currently with just a 9cms loop, but with 13 and 23cms to be added, based on a design by OM6AA. I’m experimenting with the feed position on 9cms, being the most critical.

 

Bending the ribs is fun, with nothing but a bench vice and a template. The first one took ages but by the time I’d done 11 the last one was just a few minutes work.

 

John G4ZTR

 

From: UKMicrowaves@groups.io [mailto:UKMicrowaves@groups.io] On Behalf Of David Law
Sent: 23 February 2021 14:38
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] Supporting the dish feed

 

I have a similar home made feed without the ring (I think it's a loop you have there). It's supported on a 12mm fibreglass rod running from centre of dish to centre of feed. The feed has 4 thin 100lb non stretch braided Dacron fishing lines running as "guys" to the edge of the dish.
Dacron is used for kite line and fishing line. It could be a solution if to feel the arm is a bit wobbly.


Virus-free. www.avg.com


Dave G6HEF
 

Hi John,

All interesting stuff. Out of interest what are you steering it with? I've yet to get as far as making a dish but my approach was planned to be something similar to yourself. However I was looking at the SPID solutions a lot of people use counter weights seems essential. It seems to me being able to rotate around the centre of gravity would be far more mechanically sensible and I have schemes in my head to achieve this. However, yours looks something like what I had in mind so interested in how you have done it.

Dave G6HEF