Use of 120cm SAB dish and POTY

[pa2eon]

Dear OM's,

To receive the ATV signals from QO-100 I'am looking at the combination;
- 120 cm dish from the firm SAB ( link for info - https://www.asat.nl/schotel120cm)
- The DIY made POTY , design Mike Willis G0MJW ( link for info - http://www.hybridpretender.nl/poty.html)

Will this combination work .. to receive D-ATV signals with the minitiouner from the QO-100 ?

Regards,
PA2EON - Eric

[radiogareth]

Yes, perfect
Gareth.

[g0mjw]

Very nice looking dish - pity it is collection only as the F/D of 0.5 is ideal for the POTY.

Mike

[pa2eon]

Dear OM's

The next technical issue I will have.
I get an Triax offset dish with the meassurements of 101 x 112 cm.
I want to use the POTY to receive the QO-100.

I understand that it is possible .. BUT .. the focus ( lens) position with the POTY can be very different then when the standaard LMB is used.
Who can help me to find out .. what I have tot do .. to find the correct focus point of this dish.
In the picture you see the dish .. still with no POTY .. only for the explanation of this question.

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[Basil]

I use a prime focus dish but as far as I know the focal point is the focal point. So put the opening of the tube of the POTY exactly where the opening of the horn of the LNB's metal casting would be when a normal set up was used. Ignore any plastic cover on the LNB. Measure from the entrance of the actual metal horn with the plastic removed. The POTY's tube is just acting as a waveguide.

[g4eml]

Some modification of the support arm may be needed. Sometimes the larger reflector of the POTY will hit the arm, preventing it from being positioned correctly at the focal point.

[g0mjw]

So, this is a 1m dish, not a 1.2m dish. In an offset it is the smaller dimension that counts, what you have is a section of a parabola so think of a circular dish at the satellite elevation projected on to it.

The focus is in the middle of the lens, assuming you are using one from a surplus rocket LNB, or if you are using HB9PZK's design, 14mm in front of the face of the waveguide pipe (i.e. the 22mm water pipe)

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Mike

[radiogareth]

I use the same dish as yours for portable operation. With just the beacon on I get a MER of 9. I mount the dish unside down for less windage and I have added braces as the arm alone system was a bit variable. The POTY is not very critical as to position. Using the original LNB mount with a 3-D printed spacer ring.
Gareth

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[RimaNTSS]

Firstly, the antenna is not 120cm (as advertised), but only 105cm (maybe even 100cm). In fact, it is TRIAX antenna. The main point for installing a feed (standard LNBF or POTY feed) is to co-locate the feed's phase center with the antenna's focal point. Antenna's focal point I marked on the attached picture with the yellow dot. I personally think that simple installing the POTY feed instead of LNBF will not be perfect. The antenna's feedarm will be the main obstacle. I think the one of the ways could be shortening the original feedarm a little and installing the fully adjustable LNB-holder to the end of it. If there is an interest in that, I could design and make one of those.

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[RimaNTSS]

That what I mean by that. Part of original TRIAX feedarm (red X) is cut off. New feed-support (green) is made and attached to the feedarm. So, the POTY feed can be moved away from the dish in such a way that it's phase center (blue dot) is moved to be co-located with the antenna's focal point (yellow dot).

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