POTY, DIsh, Bullseye & beam questions....

[radiogareth]

In pursuit of optimising my home station as best as possible (and trying to nail down a stange interference), the following questions have popped up. I'd be grateful for input on them as although able to use Google, I have yet to find the 'answer/truth'......

I recall that the POTY was optimised for a certain size range of offset dish. What is that size? Currently using a 1.05x1.15 offset, mounted upside down.
Using said dish, what is the 'best case' gain figure, assuming the POTY is accurately made and delivering circular polarisation?

I have 3 13cm Yagis acquired over the years, two 24 element home made (by a well-respected pioneer, G8TB now SK) and a commercial 46 element Loop-yagi - very well made but brand unknown. Other than the -3dB loss due to linear polarization, would these compete with a 'dish & POTY'?
I have a larger offset dish, 1.21x1.34, is it worth mounting a POTY on it for TX and/or RX GAIN benefits (my first question may well answer this anyway).
I've 3-D printed the parts for the Helical Feed that appeared at the same time as the POTY. Has anyone used one and is able to offer a comparison against a POTY (before I spend time and build/finish it).
I have treated myself to a Bullseye LNB, nominally for /P NB use when it may not be convenient to take my TXCO 25MHz reference with me. Other than the stability benefits it offers, should I cut the lens off it and place it in the back of a POTY or 'leave well along and use for conventional dish only (no POTY)?
Thanks for any input(s).
Gareth

[g0mjw]

This should all be in the Wiki but:

The POTY is optimised for F/D from 0.4-0.6 st 2.4 GHz so what is your dish F/D? If you don't know then best don't worry. For X-band you just need a proper lens, either one scavenged from a rocket LNB or one of HB9PZKs designs. Google is your friend.

What do you mean by compete? Let's say the real gain of the dish is 26 dBi. What is the gain of the Yagi at 2.4 GHz? Is it more than 29 dBi? If you believe it, then there is your answer. A quick check of available 13cm yagis up to about 2m in length indicate gains of 20 dBi or so. That's not going to beat a 1m dish, though it might be enough if you have a large PA and it can take the power. without meltdown. It would be fine for narrow band but DATV probably not.

A 1.2m dish has more gain than a 1m dish. The difference about 1.5 dB.

Helical feed is not usually as good dual band solution, but if you use separate TX and RX dishes neither needs to compromise, so you probably win with single band feeds over dual band ones. The disadvantage is you need two dishes.

Bullseye LNB: Good value - 20 Euro but the shipping is 12 Euro. Perhaps buy several, before 1st Jan.

They don't usually have a lens to cut off, it's just a plastic cover over a horn.
You don't need to cut off the horn to fit it to the POTY. You just drill a 22mm hole in the plastic cover and poke the waveguide through, or take the cover off and replace with a 3D printed adapter. This is mostly reversible, excepting the hole.

What is most important is to get the feed at the focus. That might require some mechanical work to make space for the larger feed - whichever type it is it's going to be bigger on 2.4GHz.

Mike

[radiogareth]

Thanks for your detailed reply Mike. Very helpful.
I was enlightened by your comments on the net last night about the disadvantages of mounting a dish inverted when used with the POTY. The website where I found the idea of inverted mounting was of course using 'normal' LNBs which I assume would have been optimised for single band use and therefore not have much/any overspill. I found that I was able to receive the Q100 NB beacon at the same S/NR as Martin, G4FKK who has the same size dish as mine but mounted conventionally. So whilst I have a vague understanding about a 'cold' sky and a 'warm' ground, how would this manifest itself in my system?
I would say that in pursuit of trying to eliminate a recent pulsing broadband interference to my Q100 reception, I have tried 'screening' my dish with a cylinder of chicken wire, wrapped around the periphery of the dish but forming a cylinder around the beam axis from Q100. My MER went up by 1, so is this due to me removing some of the warm earth noise?
What a vast and interesting pursuit all this is....

With thanks
Gareth.

[g0mjw]

Hi Gareth - indeed it only matters in the specific situation we have when the lens is over-illuminating the dish. Normally there is a taper to the patters such that is is -10dB or so at the edge. Then overspill really doesn't matter. A normal LNB will be optimised like this. Then there are the first sidelobes and I suspect these are quite high that will be equally looking at ground and sky. At lower latitudes where the elevation is higher the LNB is looking more towards the ground anyway so we only gain where we are looking up with an offset and it's not likely to be a large gain.

Incidentally for terrestrial use it's often best to mount the dish on its side. There will be a bearing offset but at least you will be able to get it to look towards the horizon easily.

MER is not always a good measure. The beacon MER I received here was initially quite poor, that was with a LEO Bodnar external reference to the PLL LNB. The cause was phase noise from the reference. After a minor tweak to the parameters it improved several dB. You might not notice this with a smaller dish but you probably will with a larger one. Unmodified LNBs are best, but they have the problem of drift. The MER is also dependent on the transponder use so it's tricky to use.

Pulsing broadband? What do you mean? I assume not the interference on the narrow band transponder we all see. It's more likely to be getting into the IF than an actual 10 GHz signal. User better cable?

Mike

[m0pnn]

Hi

Gareth

I have a 20 element Yagi and with 12 watts I can get into the narrow band no problem on SSB/CW but my signal is not as strong as the narrow band beacon. Using the POTY with the supplied lens (90cm Dish)my TX signal is as expected much stronger but the RX signal is attenuated on the wideband TV beacon from MER of 7 to 2. When using a 3D printed 3.5 turn Helix the TX signal is the same or maybe a little better than the the POTY the RX is again attenuated. My solution like me is simple use another dish (80cm) to TX for the narrow band. Its to wet and horrible at this time of year to stand on a narrow wall and play with different lens etc.

Cheers

Paul

[g0mjw]

There should be well under a dB degradation with the POTY and a proper lens. Either the lens is U/S, the wrong lens, or it's in the wrong position. A lot of people get it in the wrong place as the larger 2.4 GHz reflector doesn't fit without modification of the feed mount on many dishes. I have given up trying to explain this in words so I have attached Hans Holsink's diagram which makes it very clear what to do.

unnamed.jpg (19.2 KiB) Viewed 2383 times

If it won't fit like this on your dish you have to modify the feedarm. No ifs, no buts.

Mike

(Note this is on the Wiki)

[m0pnn]

Hi

Mike

I have seen your many explanations it must be very frustrating. I know its something I must work on to get right.

Cheers

Paul