Circular Offset Dish?

[PaulR]

Well, it is clear that your dish has a higher rim at top and bottom of the dish, and the rim might well be exactly circular.
But what signs do you have that the reflector shape is in fact multi-focus? Is it in the specs, or in the manual?

The dish was bought second hand at a radio rally several decades ago. No manual with it I'm afraid. My recollection of the theory of how the dish behaves comes from a dish review in the much missed What Satellite magazine, also several decades ago. I recognised the type of satellite dish from this article.

When in fact placing the 28E LNB at the center of the arm gave a real improvement, then the multi-focus property of the dish would be questionable.

My recollection may be wrong, I might have gone straight to a larger dish. But I did say that the dish design is a compromise which spreads the compromise across the satellite arc.

Note the curved arm that the LNBs sit on. This came with the dish and, while it may have been added later on, is indicative of a multi-focal dish. Not conclusive I'll agree. It was also a poor design as the arm would repeatedly droop down if the weight of LNBs on one side was notably different to the other side. The cables in the photo gave a certain amount of support to the arm.

The photos are from 2011. I'd had the dish for many years prior to that.

 

[unk266]

Well, the multifeed dishes that I know, are wider than high, so not circular, as yours.
So maybe @PaulR can give more specifics of his dish, if it is similar?
I'm not convinced yet, that yours is a multifeed dish.

When you don't get signal equivalent to an about 110 cm dish, something is wrong, I'd say.
(height 127cm, estimated offset=26 degree, would give corresponding width = 114cm.)

With what method/procedure did you find your focal spot? Did you also vary the dish's elevation angle, during your hand-positioning test?
And does the mounting bracket have an elevation scale?

When it still is just a non-flat paraboloid dish, a measuring procedure for my calculation method I wrote here:

Measuring and calculating zone-1 and zone-2 dish

Yesterday I managed to adjust my excel satellite dish specs calculator, to calculate non-flat paraboloid dishes (such as the zone-1 and -2 dishes in the UK) as well. Of course you could measure them with the water method (is that from John Legon?), but that measurement procedure is a bit of a...

www.satellites.co.uk

Greetz,
A33

Well, the multifeed dishes that I know, are wider than high, so not circular, as yours.
So maybe @PaulR can give more specifics of his dish, if it is similar?
I'm not convinced yet, that yours is a multifeed dish.

When you don't get signal equivalent to an about 110 cm dish, something is wrong, I'd say.
(height 127cm, estimated offset=26 degree, would give corresponding width = 114cm.)

With what method/procedure did you find your focal spot? Did you also vary the dish's elevation angle, during your hand-positioning test?
And does the mounting bracket have an elevation scale?

When it still is just a non-flat paraboloid dish, a measuring procedure for my calculation method I wrote here:

Measuring and calculating zone-1 and zone-2 dish

Yesterday I managed to adjust my excel satellite dish specs calculator, to calculate non-flat paraboloid dishes (such as the zone-1 and -2 dishes in the UK) as well. Of course you could measure them with the water method (is that from John Legon?), but that measurement procedure is a bit of a...

www.satellites.co.uk

Greetz,
A33

Well I first mounted the DIY helical on the stock mount and positioned the dish freehand. Signal strength peaked at 27dB so i mounted the dish and made sure it achieved 27dB of signal again.No It did not have an elevation scale.

 

[a33]

@PaulR :
I'm afraid that the presence of a multifeedrail is by no means a real indication that the shape of the reflector is indeed circular, horizontally. For instance the Triax dishes (60, 78, 88, 110) often have Triax multifeedrails on them, and are paraboloid.
Even wider than high Zone-1 and zone-2 dishes are paraboloid, not multifeed dishes.
The shape of the reflector surface is the determinant....

Many multifeed dishes that I know have the multifeedrail integrated in the dish design: T90, Big Bisat, one specific Triax with integrated rail I believe?, SMW OA-1600, ...
I know only of three that don't: Maximum E80 or whatever it is called, 'normal' Visiosat BiSat, and the big Equatorial dish (as RimaNTSS has).

So at the moment I would stick to my assumption, that a perfectly round offset dish would be a 'normal' paraboloid dish.

Greetz,
A33

 

[PaulR]

I'm afraid that the presence of a multifeedrail is by no means a real indication that the shape of the reflector is indeed circular, horizontally. For instance the Triax dishes (60, 78, 88, 110) often have Triax multifeedrails on them, and are paraboloid.

I know. I said that as well.

Although, the type of arm and multi-LNB arm are of a matched type that I've not seen elsewhere. Conclusion, the bracket was designed to only fit this dish.

Even wider than high Zone-1 and zone-2 dishes are paraboloid, not multifeed dishes.

I know that as well. It's only the special feed horn of Sky LNBs that match the shape of the mini-dish.

Many multifeed dishes that I know have the multifeedrail integrated in the dish design: T90, Big Bisat, one specific Triax with integrated rail I believe?, SMW OA-1600, ...

Another one I know. In fact I have such a setup here in the UK.

So at the moment I would stick to my assumption, that a perfectly round offset dish would be a 'normal' paraboloid dish.

Then I can do no more. I have no chance of finding the WhatSat article from all those years ago and no-one else has chipped in to say they remember these dishes either.

 

[Terryl]

Well, every upper L-Band Weather Satellite I can reach, Elektro-L2, Elektro-L3, Fengyun-2H, EWS-G1, and polar orbiting sats like NOAA15 ,18,19 ; Meteor-M2 and M2-2 and Metop-B and C.

If your going for satellites in low Earth orbit, then a dish is not the best choice, a circular polarized omni directorial would be the best choice as some of these satellites are moving and would not be in the same path all the time.

To use that dish setup, you would need a elevation and azimuth motor, that type would be able to track a moving satellite.

 

[Channel Hopper]

If your going for satellites in low Earth orbit, then a dish is not the best choice, a circular polarized omni directorial would be the best choice as some of these satellites are moving and would not be in the same path all the time.

To use that dish setup, you would need a elevation and azimuth motor, that type would be able to track a moving satellite.

I asked similar in another thread.

Wanted - Wanted - 90cm Prime Focus in Essex

Greetings, would any of you kind folk have a 90cm PF for sale? Needed for Meteor weather satellite reception on 1.7ghz. Mounting hardware isn't important but must have the three LNB stays please. Can collect most of Essex or around Basildon. Thanks. de Spence M0STO

www.satellites.co.uk

 

[unk266]

If your going for satellites in low Earth orbit, then a dish is not the best choice, a circular polarized omni directorial would be the best choice as some of these satellites are moving and would not be in the same path all the time.

To use that dish setup, you would need a elevation and azimuth motor, that type would be able to track a moving satellite.

Well, you see, since that setup would cost big money, I just flip the dish upside down and handtrack the sat free hand, worked quite well
NOAA 19 HRPT 4600 lines.
Image Here

 

[a33]

what actually interests me the most is this dishes beamwidth. I need to match it as closely as possible with the DIY Helical L band feed.

So, what I would do, if I were you (but of course I am not you):

Measure the 4 measures for the non-flat paraboloid dish calculation, with precision (depth to the mm precise!).
Then check the outcomes: see if the resulting focal position gives improved results.
If No, then that's bad luck (maybe due to it not being a paraboloid dish).
If Yes, then you have an improved starting point for the needed illumination angle.

Greetz,
A33