1. UKMicrowaves
  2. Topics

Distance from backplate to clamp for a POTY

Conrad, PA5Y
 

Hello all. I understand that I have to find the focus of the POTY and LNB by peaking the NB transponder, I am also aware of skew which should be -15 deg here. However I have seen that there are some issues with clearance of the POTY back plate and the feed arm on offset dishes. There does not appear to be any guidance for this in any of the POTY reference material. Does anybody have a POTY on a Triax TDS110 and if so how far is the backplate forward of the standard feed clamp? I just want an approximate starting point to see if I need to fabricate something before I start. The good news is that the POTY has the expected double resonance and around 22dB RL at 2.4GHz. Then I need to find some way of waterproofing everything. I do understand that adjusting the focal point for the downlink takes priority, I am really trying to determine if I will have issues with clearance for the TX cable and connector.

 

73

 

Conrad PA5Y

 

 

 

 
Dave G8GKQ
 

Hi Conrad

If you are only doing narrowband, the margins are such that it probably won't matter.  If you are aiming for DATV, you should modify the mounting position so that the LNB feed point is at the optimum position, as the margins are much tighter.

Lots of discussion of the problem here: Dish Feeds - BATC Forum  

Dave, G8GKQ
Conrad, PA5Y
 

Thanks again Dave this is a very useful link!

 

Conrad

 

From: UKMicrowaves@groups.io <UKMicrowaves@groups.io> On Behalf Of Dave G8GKQ via groups.io
Sent: 01 February 2023 11:26
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] Distance from backplate to clamp for a POTY

 

Hi Conrad

If you are only doing narrowband, the margins are such that it probably won't matter.  If you are aiming for DATV, you should modify the mounting position so that the LNB feed point is at the optimum position, as the margins are much tighter.

Lots of discussion of the problem here: Dish Feeds - BATC Forum  

Dave, G8GKQ

Andrew G8TZJ
 

Hi Conrad, I went to extraordinary lengths to find the sweet spot for my patch feed. This was initially using the DG7GP feed which was too close to the dish, well forward of the ideal point. When I got it focused, I gained one S point (on the PSK beacon) on my 9700, that's a 2.5 dB increase - useful. I later used a standard POTY and it was fine with the feed pushed as far back as practical, with the LNB mount in its normal positing on the LNB arm.
The attached photos show my 1m dish feed. Also my waterproofing solution.

73 Andrew G8TZJ
Neil Smith G4DBN
 

I chopped off the end of the dish arm and made a dog-leg mount so the reflector was well clear of any metal, then set the axial position to where I could hit the satelite with a carrier at the beacon level with least power. That was about 800 mW on a 95 cm steel pierced offset.  At that I could see the transponder noise floor on 3cm, so I didn't bother optimising for 3cm other than peaking by rotating the LNB. The clamp angle points at the centre of the projected circle seen from the focus of the offset dish.  The rectangular block fits into the sawn-off end of the arm.


That's the Bullseye lens, I didn't notice any change in the noise floor of the transponder when I swapped it for a biconic Rexolite lens or an HB9PZK, also in Rexolite, but the Bullseye needed a support ring soldering to the end of the tube. The Rexolite lenses were fixed with a smear of neutral-cure (oxime) silicone and pressed in.  I don't know how close 800 mW is to the theoretical gain, but in meant I could get in with a watt and a bit from a transverter in the shack.


I bodged another one with the POTY in an offset clamp, so there was a bit of squint/astigmatism. That needed a mighty two watts to reach the beacon level, and the noise floor wasn't as pronounced, but it was still perfectly usable.  On a smaller dish without any machining, I'd optimise for 3cm receive and just run more power on 2.4 until I could match the beacon.

I used a large polystyrene surveillance camera dome to cover the reflector, and fitted a mounting ring made from engineering plastic behind the reflector to fix it. I left a drain hole with a wick, because it kept filling with condensation from diurnal pumping.

I must have had, oooh, SEVERAL contacts from all that engineering fun.  Totally worth it to know the weather in Germany and what radio the nice DL op was using. I might fire it up again to see what the weather's like now in wherever.

http://www.g4dbn.uk/?p=1279
http://www.g4dbn.uk/?p=1363

Neil G4DBN
https://youtube.com/MachiningandMicrowaves

On 02/02/2023 18:44, Andrew G8TZJ via groups.io wrote:

Hi Conrad, I went to extraordinary lengths to find the sweet spot for my patch feed. This was initially using the DG7GP feed which was too close to the dish, well forward of the ideal point. When I got it focused, I gained one S point (on the PSK beacon) on my 9700, that's a 2.5 dB increase - useful. I later used a standard POTY and it was fine with the feed pushed as far back as practical, with the LNB mount in its normal positing on the LNB arm.
The attached photos show my 1m dish feed. Also my waterproofing solution.

73 Andrew G8TZJ
Marek Szuba
 

On 2023-02-01 09:37, Conrad, PA5Y wrote:

Then I need to find some way of waterproofing everything.
If you are not averse to 3D printing, have a look at what DM4DS has
published on Thingiverse. On my current dish I use the short-front
variant of https://www.thingiverse.com/thing:3736069 in conjunction with
the long version of https://www.thingiverse.com/thing:3779729, both
printed in translucent PLA (to make it harder for sunlight to heat them
up) and painted over with clear acrylic varnish (to keep humidity out).
I have also cut out a gasket out of PVC to place between the two halves
of the LNB/waveguide cover, and squirted some silicone sealant on both
of its sides for good measure, to hinder potential rain egress from the
side. So far (i.e. about a year), so good!

--
Marek M0JUR
Conrad, PA5Y
 

Thanks Andrew the photos are useful. Hopefully I will get the RX side up and running this weekend and the TX done the following weekend. Progress will be interrupted by some EME on 144 and 432. There is a new DXCC available. I still chase them 😊

 

Conrad

 

From: UKMicrowaves@groups.io <UKMicrowaves@groups.io> On Behalf Of Andrew G8TZJ via groups.io
Sent: 02 February 2023 19:44
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] Distance from backplate to clamp for a POTY

 

Hi Conrad, I went to extraordinary lengths to find the sweet spot for my patch feed. This was initially using the DG7GP feed which was too close to the dish, well forward of the ideal point. When I got it focused, I gained one S point (on the PSK beacon) on my 9700, that's a 2.5 dB increase - useful. I later used a standard POTY and it was fine with the feed pushed as far back as practical, with the LNB mount in its normal positing on the LNB arm.
The attached photos show my 1m dish feed. Also my waterproofing solution.

73 Andrew G8TZJ
Conrad, PA5Y
 

I don't have a 3D printer but I know a man who does,

Conrad

-----Original Message-----
From: UKMicrowaves@groups.io <UKMicrowaves@groups.io> On Behalf Of Marek Szuba via groups.io
Sent: 02 February 2023 23:37
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] Distance from backplate to clamp for a POTY

On 2023-02-01 09:37, Conrad, PA5Y wrote:

Then I need to find some way of waterproofing everything.
If you are not averse to 3D printing, have a look at what DM4DS has published on Thingiverse. On my current dish I use the short-front variant of https://www.thingiverse.com/thing:3736069 in conjunction with the long version of https://www.thingiverse.com/thing:3779729, both printed in translucent PLA (to make it harder for sunlight to heat them
up) and painted over with clear acrylic varnish (to keep humidity out).
I have also cut out a gasket out of PVC to place between the two halves of the LNB/waveguide cover, and squirted some silicone sealant on both of its sides for good measure, to hinder potential rain egress from the side. So far (i.e. about a year), so good!

--
Marek M0JUR
Conrad, PA5Y
 

Great stuff Neil, I’ll have one 😊

 

Conrad

 

From: UKMicrowaves@groups.io <UKMicrowaves@groups.io> On Behalf Of Neil Smith G4DBN via groups.io
Sent: 02 February 2023 20:29
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] Distance from backplate to clamp for a POTY

 

I chopped off the end of the dish arm and made a dog-leg mount so the reflector was well clear of any metal, then set the axial position to where I could hit the satelite with a carrier at the beacon level with least power. That was about 800 mW on a 95 cm steel pierced offset.  At that I could see the transponder noise floor on 3cm, so I didn't bother optimising for 3cm other than peaking by rotating the LNB. The clamp angle points at the centre of the projected circle seen from the focus of the offset dish.  The rectangular block fits into the sawn-off end of the arm.

 

 

That's the Bullseye lens, I didn't notice any change in the noise floor of the transponder when I swapped it for a biconic Rexolite lens or an HB9PZK, also in Rexolite, but the Bullseye needed a support ring soldering to the end of the tube. The Rexolite lenses were fixed with a smear of neutral-cure (oxime) silicone and pressed in.  I don't know how close 800 mW is to the theoretical gain, but in meant I could get in with a watt and a bit from a transverter in the shack.

 

 

I bodged another one with the POTY in an offset clamp, so there was a bit of squint/astigmatism. That needed a mighty two watts to reach the beacon level, and the noise floor wasn't as pronounced, but it was still perfectly usable.  On a smaller dish without any machining, I'd optimise for 3cm receive and just run more power on 2.4 until I could match the beacon.

 

I used a large polystyrene surveillance camera dome to cover the reflector, and fitted a mounting ring made from engineering plastic behind the reflector to fix it. I left a drain hole with a wick, because it kept filling with condensation from diurnal pumping.

 

I must have had, oooh, SEVERAL contacts from all that engineering fun.  Totally worth it to know the weather in Germany and what radio the nice DL op was using. I might fire it up again to see what the weather's like now in wherever.

 

 

 

On 02/02/2023 18:44, Andrew G8TZJ via groups.io wrote:

Hi Conrad, I went to extraordinary lengths to find the sweet spot for my patch feed. This was initially using the DG7GP feed which was too close to the dish, well forward of the ideal point. When I got it focused, I gained one S point (on the PSK beacon) on my 9700, that's a 2.5 dB increase - useful. I later used a standard POTY and it was fine with the feed pushed as far back as practical, with the LNB mount in its normal positing on the LNB arm.
The attached photos show my 1m dish feed. Also my waterproofing solution.

73 Andrew G8TZJ

 

 
John Fell
 

Your results are very similar to mine Neil , but I use a pair of 90cm offsets .
Would be interested to see how you interface the Bullseye to the 22mm O/D 10GHz feed .
Do you decap the LNB and insert into the copper  and fit the lens at the other end ?

The circa £9 McLean LNB from Amazon is my current favorite for frequency locking - plenty of space , 2 Rs and 2 Cs and a little twist of 1x0.25mm to link onto the Xtal - locks at 25MHz down to approx -30dBm .Sees 0.7dB Moon Surface on an old 1.9m offset .

73
John

On Thu, 2 Feb 2023 at 19:29, Neil Smith G4DBN <neil@...> wrote:
I chopped off the end of the dish arm and made a dog-leg mount so the reflector was well clear of any metal, then set the axial position to where I could hit the satelite with a carrier at the beacon level with least power. That was about 800 mW on a 95 cm steel pierced offset.  At that I could see the transponder noise floor on 3cm, so I didn't bother optimising for 3cm other than peaking by rotating the LNB. The clamp angle points at the centre of the projected circle seen from the focus of the offset dish.  The rectangular block fits into the sawn-off end of the arm.


That's the Bullseye lens, I didn't notice any change in the noise floor of the transponder when I swapped it for a biconic Rexolite lens or an HB9PZK, also in Rexolite, but the Bullseye needed a support ring soldering to the end of the tube. The Rexolite lenses were fixed with a smear of neutral-cure (oxime) silicone and pressed in.  I don't know how close 800 mW is to the theoretical gain, but in meant I could get in with a watt and a bit from a transverter in the shack.


I bodged another one with the POTY in an offset clamp, so there was a bit of squint/astigmatism. That needed a mighty two watts to reach the beacon level, and the noise floor wasn't as pronounced, but it was still perfectly usable.  On a smaller dish without any machining, I'd optimise for 3cm receive and just run more power on 2.4 until I could match the beacon.

I used a large polystyrene surveillance camera dome to cover the reflector, and fitted a mounting ring made from engineering plastic behind the reflector to fix it. I left a drain hole with a wick, because it kept filling with condensation from diurnal pumping.

I must have had, oooh, SEVERAL contacts from all that engineering fun.  Totally worth it to know the weather in Germany and what radio the nice DL op was using. I might fire it up again to see what the weather's like now in wherever.



On 02/02/2023 18:44, Andrew G8TZJ via groups.io wrote:
Hi Conrad, I went to extraordinary lengths to find the sweet spot for my patch feed. This was initially using the DG7GP feed which was too close to the dish, well forward of the ideal point. When I got it focused, I gained one S point (on the PSK beacon) on my 9700, that's a 2.5 dB increase - useful. I later used a standard POTY and it was fine with the feed pushed as far back as practical, with the LNB mount in its normal positing on the LNB arm.
The attached photos show my 1m dish feed. Also my waterproofing solution.

73 Andrew G8TZJ



2023-02-03.png

Neil Smith G4DBN
 

I made a tapered brass transition from the sawn-off LNB tube to 20.2 mm with recesses to fit the outside diameters, then made a new ring for the original lens and soldered it to the 22 mm tube so the lens snapped on.
Neil G4DBN

http://www.g4dbn.uk/?p=1279


On 03/02/2023 10:36, John Fell wrote:

Your results are very similar to mine Neil , but I use a pair of 90cm offsets .
Would be interested to see how you interface the Bullseye to the 22mm O/D 10GHz feed .
Do you decap the LNB and insert into the copper  and fit the lens at the other end ?

The circa £9 McLean LNB from Amazon is my current favorite for frequency locking - plenty of space , 2 Rs and 2 Cs and a little twist of 1x0.25mm to link onto the Xtal - locks at 25MHz down to approx -30dBm .Sees 0.7dB Moon Surface on an old 1.9m offset .
John Fell
 

Thanks Neil.
73
John

On Fri, 3 Feb 2023 at 12:25, Neil Smith G4DBN <neil@...> wrote:
I made a tapered brass transition from the sawn-off LNB tube to 20.2 mm with recesses to fit the outside diameters, then made a new ring for the original lens and soldered it to the 22 mm tube so the lens snapped on.
Neil G4DBN



On 03/02/2023 10:36, John Fell wrote:
Your results are very similar to mine Neil , but I use a pair of 90cm offsets .
Would be interested to see how you interface the Bullseye to the 22mm O/D 10GHz feed .
Do you decap the LNB and insert into the copper  and fit the lens at the other end ?

The circa £9 McLean LNB from Amazon is my current favorite for frequency locking - plenty of space , 2 Rs and 2 Cs and a little twist of 1x0.25mm to link onto the Xtal - locks at 25MHz down to approx -30dBm .Sees 0.7dB Moon Surface on an old 1.9m offset .

1 - 12 of 12