POTY Lens for shallow dish

[Pheonix2024]

Hi, I'm sure this has come up before but I am a new member and unable to locate a detailed answer to my problem.

I have purchased a POTY from AMSAT UK and found that the RX signal is attenuated by about 3db from the usual LNB position.

I have checked all the measurements and feed point and skew etc. all to no avail.

My last remaining hope is that the dish that I am using seems to have a F/D of 0.76 which is outside the design range of the POTY feed (0.4 - 0.6 I believe) and suspect that this might be causing the problem.

The lens fitted is the pointy triangular one. Is it possible that a different lens would reduce the attenuation by better focusing the signal into the waveguide ?

Regards
Greg G4CST

[Basil]

It is an OFFSET and not a prime focus dish that you are using? A lens of any type shouldn't be used on a prime focus dish. Sorry if you knew this but as no one had replied I thought it does no harm to mention it

[Pheonix2024]

Thanks Chris

Yes, it is an offset dish.

Passion Radio offer a new alternative but it's not clear why !!

Greg
G4CST

[radiogareth]

If not already done, the POTY will need mounting in such a way that the lens part is in the same place as where a bare LNB 'mouth' would have been. This often means some bracketry to put it further back and slightly off to one side to clear the LNB arm. I was able to hack mine with some sheet alloy in place of the rectangular tube (which I had to cut back a bit). Could you post a picture and dish dimensions etc? Chances are someone has 'been there and done that'.
Gareth

[g0mjw]

Hi Greg,

The POTY was designed for an F/D of around 0.6. You will lose out on both TX and RX with a F/D of 0.76. I would use a rocket lens if you can find one. The lens design from HB9PZK, made from rexolite, will work reasonably well, efficiency over 60%, vs maybe 30% without it. Ideally, use a dish like the channel master. Clearly, you need to have the lens at the focus. Don't just fix the LNB in the same clamp position with the POTY sticking way out of the front and expect it to work! It needs to be at the focus, so the lens needs to be about where the LNB horn was. That will mean clamping to the 22mm tube rather than the LNB collar. I thought that was obvious, but I have seen some some examples that suggest it is not as obvious as I thought.

Note also, some really bad lenses have been sold by some. The AMSAT-UK feed has a good "pointy triangular thing" which if placed at the focus, should work reasonably well.

Mike

[g0mjw]

It is an OFFSET and not a prime focus dish that you are using? A lens of any type shouldn't be used on a prime focus dish. Sorry if you knew this but as no one had replied I thought it does no harm to mention it

Not true! A lens can be used on a prime focus dish, it just needs to be the right lens for the focal length of the dish. Prime focus dishes with long F/D do exist. The reason offset dishes tend to have a longer F/D is because an offset dish is really a prime focus dish, with most of the dish missing and the feed re-pointed at what is left. What is left has a smaller D so the F'/d is greater. A diagram would help, but it is a little late to draw one so have a look at this https://www.qsl.net/oe5jfl/eme_stn.htm

[PA3CRX]

And for some it might be useful to (re)read the article about illumination of dishes in CQ-TV from page 14.
That might provide more insight.

Chris PA3CRX

[radiogareth]

A picture is worth a 1000 words....here is my solution. Untouched for 3 years....you can see where I had to cut away some of the support arm to get it to sit right. Beacon MER 9dB on an empty satellite, its a 1.05x1.0 dish mounted upside down to reduce windage. The pointy lens is one I made from rexolite to a published design. Took HOURS on my lathe!!
Gareth

potyXAT.jpg (104.58 KiB) Viewed 2682 times

[Basil]

Apologies, I stand corrected, I have never seen either in actuality or in a photo a prime focus dish using a lens at the LNB. Has anyone got a photo?

My PF dish was made by Andrew with a shepherd's crook style mount for the feed, on a cast alloy mount in the middle of the dish, and as it didn't suit QO100 reception I turned up an alloy plate to fill the 8 or so inch hole in the dish. In this plate I drilled a 22mm hole, and using that I can align the 22mm bore of the LNB holder (a Teflon turning) perfectly with the centre of the dish using 22m OD tubing. I can also then accurately measure distance to the centre of the dish without assistance and make any required adjustments.

Personally I think offset dishes sound a bit of a PITA when modifying feed points and keeping water out of the LNB. Efficiency be damned

[g6hfs]

Never had a problem with water in over 2 years.

Brian