1.6m Prime Focus

[Rick]

Just picked up a 1.6m Prime Focus dish. Waiting for a new LNB for it but thought I'd post a couple of pics. Any one able to Identify it ?

 

[Rick]

Thanks to this thread -http://www.satellites.co.uk/forums/fringe-reception-general/169312-precision-antenna-dishes-back-production-qp-reflectors.html, I think I have a Precision dish. It's definatley aluminium, and you can see the rings from when the dish was spun. I've emailes QP reflectors to see it they can confirm it for me.

 

[futumsch]

Hi Rick,
Precision did a 1.5m and a 1.8m dish, not sure they did a 1.6m. Could your dish be an imperial measurement i.e. 5 foot?
The Precision dishes had a tubular support on the back of them as shown on the attached pic of a Precision 1.2m dish and the rim of the dish is knurled right round. Also the LNB (or LNA) polar rotator is held on with four support rods not three.

Nice project you got there in any case.

 

[futumsch]

Rick,
Do the DH satellite antennas look a better match for what you have got?
_http://www.setileague.org/photos/cons/kc9bdl.jpg
_http://www.dhsatellite.com/images/AZ-EL%20BACK%20EXP.JPG
_http://www.dhsatellite.com/gallery.htm

There's a nice pic of their 'spinning mandrel' on their home page.

 

[satelliteman]

A nice dish Rick, are you keeping it for yourself?

 

[ralphmagno]

looks good
hello,
the feed horn needs work.
i see no plastick cap to keep the bugs out.
they build nests inside sometimes in the summer time.
should work well
ralph
ralph

 

[Rick]

I did manage to find pics of a Precision dish with 3 arms, and the Scaler and Feed horn look Identical. Looks closer to the Precision than the DH. If it's imperial it measures 63"

I'm planning on refurbing the dish, not sure if I will keep it or sell it though. There is lots to put right. I've already changed the LNB and Polariser for a Invertro C120. Getting 20%-30% increases in gain over the TD110, still no 7w v's. Though I think that the Feedhorn isn't aligned with the dish, so when I get it on the bench I will string the dish and calculate the focal length, as from my rough measurments it's about 20mm too close atm. The 18" Jager Actuator is siezed solid, though the motor still spins, so am going to strip that and see if I can get it going again. Channel Hopper has kindly provided some pointers there.

 

[Rick]

Well it's not a Precision :sad: Though it is possible that it was spun by them and the mount and feedhorn added my another company, impossible to say.

QP Reflectors, have been Very Helpful, and may be able to supply a feedhorn better suited to the dish and kU band. :smile:

 

[hexah]

IRTE used to do a range of spun dishes they called 'Precision'. If you look at the back the rings are more like tide marks. The name 'precision' was a bit of a joke because they weren't! I am fairly sure they did a 1.6m and a 1.3m among other sizes.

 

[Rick]

I always thought IRTE dishes are pretty good. Any ideas who I'd contact to ask ?

 

[hexah]

Looking very closely at the first picture I can see the little ridges by the backring. The dish face with the IRTE was fine, it was just very rough at the back, not what you would expect from a 'precision' dish, you would expect it to be smooth as well. If well cared for an IRTE should continue to give more years of good service. I am not certain it is an IRTE dish though.

I still have an LNB exactly the same as the one in the photo though I used it with a mechanical polarizer. It was sold under a range of brand names, it covered the low band and I think the noise figure was around the 2dB mark. It wasn't exactly cheap when I bought it maybe 20 years ago and did not give a visible improvement over my first LNB! I wasn't exactly happy at that! :-Nooo

 

[Channel Hopper]

Apologies, just tuned in

That dish is almost certainly a Levick, ancient but not bad at all.

The major problems are the corrosion on the polarmounts, and the scalar feedhorn which did not work well at the higher frequencies.

 

[Rick]

Thanks Channel Hopper.

Thanks for the ID, would that be John Levick ?

The polar mount looks worse than it is, shouldn't be too much of a job to sort out. QP Reflectors might be able to supply a feedhorn which will work better. I expect the current one didn't need to peform well at high frequencies. The dish was fitted with a Low Band LNB when I got it.

Saturdays job is to clear a space in the garage for the dish while I am working on it.

 

[hexah]

Good point Channel Hopper.

Equipment of that vintage is optimized for the low frequencies. If it does not perform as well as expected on some transponders and they are at the top of the band, this may be why.

A good example is the earliest _Sky perforated 'mesh' dish which is 90x85cm. The signal strength is the same across the band as a modern dish of about the same size but the signal quality is worse. There is a clear roll off of signal quality on the high frequencies.

If you put the old dish beside a modern perforated 'mesh' dish you will see the old one has a bigger hole size and the the modern one has smaller holes to improve the response on the shorter wavelengths. I suspect the parabola on the old one is also optimized for the longer wavelengths though that may not make as much of a difference.

This may be another reason why some people thought perforated 'mesh' dishes were worse than solid dishes - they aren't worse it is just that these ones were not optimized for the entire wave band.

It may also be the reason why transparent dishes do not perform well for their size - they have the same large size mesh as the early _Sky dish embedded in them, larger than the modern perforated 'mesh' dishes.

So I think you may find there is still a relatively lower performance on the top end of the band even with a modern feedhorn.

However, it may have a big enough margin in practice on the transponders you want, so it may not be problem. If motorizing, optimize it on the weakest transponders that you want.

Of course, you might be lucky and it will work fine for your proposes with the old feed horn - it is worth trying as it will save a bit of money.

For the mount Hammerite, new washers, etc will make it as good as new. Perhaps spray the dish face with Pasti-kote type matt spray. I would say it should last another 20-30 years with yearly maintenance.

The old LNB may still work if you take off the polarotor and use a WR75 - C120 transition or just hold it in place. Then keep it as an antique!

 

[Rick]

TBH I pretty much get everything I want on my current setup. I got the dish as a Project really, I'd like to see how much I can get out of it, and how like new I can make it look. Certianly some interesting points you make. Would it be possible to calculate what frequencies the dish will work best on ? I plan to measure the exact dimensions of the dish to check to try and match a better feed horn anyway, and from my rough measurments I thik the focal point is about 20mm out, so that will need correcting.

The Old LNB is goosed, tried it both with and without the polariser, no joy.

 

[hexah]

Also, I recommend getting a cheap Inverto White Tech C120 LNB as backup for whatever other LNB you get, in case the other LNB fails just before your favorite programmes start!

Unless you already have some old C120 LNBs, of course.

I have some that are perhaps 15 years years old but they still work. They have a long shelf life.

 

[Rick]

Funnily enough, I got the Invertro White once I discovered that the LNB was bust.

 

[hexah]

TBH I pretty much get everything I want on my current setup. I got the dish as a Project really, I'd like to see how much I can get out of it, and how like new I can make it look.

It is a fun project, it will look good when finished.

Certainly some interesting points you make. Would it be possible to calculate what frequencies the dish will work best on ? I plan to measure the exact dimensions of the dish to check to try and match a better feed horn anyway, and from my rough measurements I think the focal point is about 20mm out, so that will need correcting.

I have forgotten all the maths about parabolas. My brain didn't want to remember it anyway. Going by my fiddling with the old Sky dish it was about 12.4-12.7GHz that was affected but you may have better results due to the solid dish.

There typically is only one focal point with maximum signal and a signal meter would be the easiest way to find it. As has been proved again and again on these forums, fiddling is the best way to go. Trial and error will deliver the goods.

The Old LNB is goosed, tried it both with and without the polariser, no joy.

Oh well, it is a door stopper then!

 

[Rick]

Well I have finally got the dish seperated from the mount and into the workshop. I had to take a grinder to the bolts to take the dish of the mount, but I was planning on replacing them anyway.

I've strung the dish and found that it is slightly out of shape, one of the strings is 9mm out from the other 3, though the other 3 are spot on. Will that make that big a differance to the effectivness of the dish ? Also the arms are holding the feedhorn slightly off centre, but that is an easy fix.

I've a couple of Ideas how I will pull the dish back into shape, but If anyone has a tried and tested method i'm all ears.

I'll post up a couple of pics later on.

I'd also be thankfull if someone could check my calculations, With a Diameter of 1600mm, depth of 220mm I make the f/d to be 0.45 and the ideal focal length to be 727mm. Want to comfirm as the focal length at the moment is 690mm. Of course this would explain why I got an increase in signal when I sild the LNB almost all the way out of the feedhorn.

 

[hexah]

I've strung the dish and found that it is slightly out of shape, one of the strings is 9mm out from the other 3, though the other 3 are spot on. Will that make that big a difference to the effectiveness of the dish ? Also the arms are holding the feedhorn slightly off center, but that is an easy fix.

Yes, it will affect performance quite a bit. There is a cure.

I've a couple of Ideas how I will pull the dish back into shape, but If anyone has a tried and tested method I'm all ears.

The old fashioned way of doing this was to get a friend to help, lay several planks of wood across the dish face, and twist the distortion out of it.

Apply firm pressure and watch it does not slide out, or fall to the floor.

Dents at the rim can be gently beaten out with a rubber mallet. Dents that are within the face can be left to pop out over a few days once the dish is mounted and installed, the flexing during installation will encourage that. If it is still there after a few weeks, beat it out gently with a rubber mallet.

It was common enough for metal dishes to get a bit warped in transit because they generally did not come in individual cardboard boxes with polystyrene inserts. Delivery people knew nothing about being gentle with them either (they still don't!).

I'd also be thankfull if someone could check my calculations, With a Diameter of 1600mm, depth of 220mm I make the f/d to be 0.45 and the ideal focal length to be 727mm. Want to comfirm as the focal length at the moment is 690mm. Of course this would explain why I got an increase in signal when I sild the LNB almost all the way out of the feedhorn.

_www.satlex.de and _www.satellite-calculations.com confirm your estimates.

Going by the signal you are getting, 727mm is right.