Use of 120cm SAB dish and POTY

[g0mjw]

Rather than modify anything, you can use a 40mm to 22mm adapter that is offset to raise the feed a little to clear the feed arm. Very easy to 3D print.

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OffsetFeedClamp.zip

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Mike

[Basil]

Once you have written the file how long does a hobby type 3D printer take to create such a thing? I have never played with one, so have no idea.

Given a round bar of turnable plastic that would take well over an hour on the lathe and then the Bridgeport. Assuming I didn't get distracted and do something rash...

[RimaNTSS]

use a 40mm to 22mm adapter that is offset to raise the feed a little to clear the feed arm.

That will not allow co-locating the feed's phase with the antenna's focal point. So, the system will not provide maximum performance.

[g0mjw]

That is not correct. It will work fine, the offset in elevation angle can be compensated for easily, it will have negligible impact on the gain and XPD. The focal distance is much more critical. This is how those multi lnb dishes can work looking at more than once satellite. Within reason of course.

Mike

[g0mjw]

Once you have written the file how long does a hobby type 3D printer take to create such a thing? I have never played with one, so have no idea.

Given a round bar of turnable plastic that would take well over an hour on the lathe and then the Bridgeport. Assuming I didn't get distracted and do something rash...

Depends on settings, about half an hour.

[RimaNTSS]

It will work fine

Sure, it will work. But having the feed too high and too close to the dish will not provide the performance of the system as it could if the feed is placed properly. Who needs to install 105cm if it performs not better than 90cm one?

[Basil]

Once you have written the file how long does a hobby type 3D printer take to create such a thing? I have never played with one, so have no idea.

Given a round bar of turnable plastic that would take well over an hour on the lathe and then the Bridgeport. Assuming I didn't get distracted and do something rash...

Depends on settings, about half an hour.

OK, thanks, impressive!

[g0mjw]

Sorry, but I do know do what I am talking about, having designed the thing, on where you can compromise and where not. If it is 2cm higher it will make no significant difference, apart from to the elevation, which needs to be corrected, ditto to the left and right, but the distance in/out has to be correct. There is always an optimum point, at the phase centre, but there is a tolerance around that point, where the losses are minimal, that we can use. What you will lose at off axis angles, is mostly the cross polarisation discrimination, but as long as it is still OK, maybe -20dB instead of -40dB, it will be OK. For QO100 especially so because there is no signal on the other polarisation. The gain will be only marginally impacted over quite a large range of angular offsets. However, the focal distance along the axis is much more critical.

W1GHZ covers this nicely https://www.w1ghz.org/antbook/chap4.pdf page 15+

Mike

[Basil]

Is focal distance similarly critical to signal strength with prime focus dishes? (I suspect a yes is the answer, but suspicion doesn't add knowledge ) Thanks.

[PA3CRX]

In CQ-TV 273 I wrote an article about Dishes and their feeds. Maybe worthwhile to reread it?

Depending on the axis that the feed is moved in an offset dish, it could influence the take off angle of the dish.
So not just slide the feed over the axis to maximum signal strength but try different positions and chang the elevation angle of the dish to look for the maximum.

Said this, I think when you place the feed in (about) the same position as the original feed, it will be fine.
With a Poty, for 13 cm the wavelength could accept some misplacement. For 10 GHz you have more displacement but with this size dish you have likely a lot of downlink signal, even if the efficiency is not the highest.

Chris PA3CRX