Where is the focal point of an LNB?


Chris Wilson
 

01/04/2021 12:18

I want to re-purpose an elderly pressed aluminium 1.8 metre dish last used for terrestrial satellite reception at my old house. I will initially use it for SSB RX / TX to es hail, although it's a bit overkill! My question is, on the attached link to photos of the old LNB, is the focal point the centre point on the innermost annular ring of the feed "horn" or is the the actual antenna deep within the gubbins? I need to fabricate a mount for a 10GHz LNB and a POTY TX antenna, and am not sure where to measure to. Thanks 2E0ILY

Pics are at http://www.chriswilson.tv/satellite/satellite.zip

--
Best Regards,
Chris Wilson.
mailto: chris@chriswilson.tv


Mike Willis
 

Typically around the aperture but it would need a simulation - why not work out (or measure with mirrors and a laser pointer) the focal length?
--
Mike G0MJW


Mike Willis
 

PS - 6 photos of the feed but not one of the actual dish? Given it is a tripod feed I would suggest you make the mount adjustable. Then you can optimise it in situ.
--
Mike G0MJW


Neil Smith G4DBN
 

With unknown dishes, I don't trust the current location of the feedpoint, I make a temporary bracket which can slide along the LNB axis and use sun noise to find the optimal position, verify using a longish optical path, then measure the position and make a permanent clamp with a couple of cm adjustment.

Neil G4DBN



KENT BRITAIN
 

Neil, I 101% agree with you.  I have NEVER found to focus to be at the calculated point!
The vast majority of the time gain improves when the feed is a bit closer to the dish.
Test if you can!   Kent G8EMY/W5

On Thursday, April 1, 2021, 8:25:54 AM CDT, Neil Smith G4DBN <neil@...> wrote:


With unknown dishes, I don't trust the current location of the feedpoint, I make a temporary bracket which can slide along the LNB axis and use sun noise to find the optimal position, verify using a longish optical path, then measure the position and make a permanent clamp with a couple of cm adjustment.

Neil G4DBN



G0FVI
 

Chris, I also use a 1.8m TVRO dish for Es'Hail. I'd agree with what has been said that the focal point is not usually where cacluated. My POTY fits nicely inside a purchased LNB holder using two 22mm plumbing nuts that fit around the body of the POTY, enclosed by the cut off neck from the modified LNB. This 'minimum' engineering solution allows moving focal point in or out a fair bit to achieve maximum signal from Es'Hail (see pic). The focal point for 2.4GHz uplink is not the same as 10GHz downlink but with a dish this size for Es'Hail you really don't have to worry too much (you'll be S9+ in and be getting a decent downlink signal). With your dish you'll just need to modify the cental strut ring to allow a POTY to fit in there.


  


Keith
 

Neil, 
Is that feed one of the RF Hamdesign horn feeds ?
I was originally looking at that design for 8.45 ghz, but have since gone to a squeezed tube polariser.
Cheers
Keith 
VK6KB / VK6EME


Neil Smith G4DBN
 

Hi Keith, it's a Pickett-Potter I made to the WA6KBL design for 3cm, linear dual-mode, fed with WR90 through a tuned oval iris

Neil

On 02/04/2021 14:14, Keith wrote:
Neil,
Is that feed one of the RF Hamdesign horn feeds ?
I was originally looking at that design for 8.45 ghz, but have since gone to a squeezed tube polariser.
Cheers
Keith
VK6KB / VK6EME


Chris Wilson
 

Hello Neil,

Thursday, April 1, 2021

This is what I was afraid of Neil. The set up worked fine, but terrestrial satellites and a 1.8 meter dish gives a lot of leeway, so I will see how I can give things some adjustability, thank you.

Best regards,
Chris 2E0ILY mailto:chris@chriswilson.tv


NSG> With unknown dishes, I don't trust the current location of the
NSG> feedpoint, I make a temporary bracket which can slide along the
NSG> LNB axis and use sun noise to find the optimal position, verify
NSG> using a longish optical path, then measure the position and make
NSG> a permanent clamp with a couple of cm adjustment.

NSG> Neil G4DBN
NSG>
NSG>
NSG>
NSG>


Chris Wilson
 

Hello Mike,

Thursday, April 1, 2021

Thanks Mike and to all who replied, making it adjustable seems the easiest and most practical way, so that's what I will do :)


Best regards,
Chris 2E0ILY mailto:chris@chriswilson.tv


MW> PS - 6 photos of the feed but not one of the actual dish? Given
MW> it is a tripod feed I would suggest you make the mount adjustable.
MW> Then you can optimise it in situ.