Microwave dish size and shape versus frequency question


KENT BRITAIN
 

Glad it helped.   Kent WA5VJB/2E0VAA/G8EMY

Yes, I know, a G8, but it is a full license.   Really wanted that G Call! hihi

American hams have no idea what the 8 means.



On Sunday, May 9, 2021, 11:35:23 AM CDT, Chris Wilson <chris@...> wrote:


Hello Kent,

Friday, May 7, 2021

Thanks for the reply Kent, appreciated and helpful.


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


KB> Interesting way to think about it,  But the focal points for the
KB> 2.4 GHz and 10 GHz waves are slightly different
KB> since 2.4 GHz waves cannot become an infinitesimal point.    Of
KB> course 10 GHz wave cannot either.
KB> And 2.4 GHz feeds and 10 GHz feeds will have different phase centers.


KB> In the practical world the points are pretty close and just make
KB> your feed support adjustable and move it
KB> in and out a bit for best signal levels.  I have tested hundreds
KB> of dishes on the antenna range and the feed
KB> usually ends up a bit closer to the dish than the calculated focal point.


KB> Frequency limits on the size of the dish.


KB> The dish needs to be a least 5 wavelengths across to bring the waves to a focus.


KB>                  For 23 cm this would be about a 1 Meter dish.    10 wavelengths is preferred.


KB> Upper frequency is determined by the surface accuracy of the dish surface.


KB>                  Most commercial dishes are good to 24+ GHz


KB> The offset dish can be more difficult to work with since the feed point is not as obvious and
KB> pointing can be more difficult.  But it gets the feed structure out of the beam.


KB> Good luck with your project, Kent





KB>   
KB> 
KB>   
KB>                      On Friday, May 7, 2021, 08:28:18 AM CDT,
KB> Chris Wilson <chris@...> wrote:   

KB> 

KB> 




KB>  07/05/2021 14:20



KB> Absolute newbie question coming...



KB> I saw a big microwave dish advertised on Ebay and then by
KB> coincidence saw this advert mentioned on a UK Satellites forum. I
KB> am getting started in amateur radio satellite communications,
KB> initially via a 1.8 meter PF dish and the QO-100 satellite using
KB> SSB and hopefully later, ATV signals.



KB> The up link is on 2.4 GHz and someone extolling the magnificence
KB> of the Ebay dish was shot down by someone saying it may well be no
KB> use at all for 2.4 GHz uplink., for example, and was probably made for a specific purpose.



KB> This surprised me, I naively thought a dish was a dish, with
KB> prime or offset focus, but now thinking deeper I recall seeing
KB> very small, but seemingly deep versus diameter 10GHz dishes on tripods.



KB> So may I ask, how is the size / shape of a microwave dish related
KB> to its intended frequency usage please? Thanks.



KB> This is the link to the dish in question:



KB> https://www.ebay.co.uk/itm/164502970358?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649









Chris Wilson
 

Hello Kent,

Friday, May 7, 2021

Thanks for the reply Kent, appreciated and helpful.


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


KB> Interesting way to think about it, But the focal points for the
KB> 2.4 GHz and 10 GHz waves are slightly different
KB> since 2.4 GHz waves cannot become an infinitesimal point. Of
KB> course 10 GHz wave cannot either.
KB> And 2.4 GHz feeds and 10 GHz feeds will have different phase centers.


KB> In the practical world the points are pretty close and just make
KB> your feed support adjustable and move it
KB> in and out a bit for best signal levels. I have tested hundreds
KB> of dishes on the antenna range and the feed
KB> usually ends up a bit closer to the dish than the calculated focal point.


KB> Frequency limits on the size of the dish.


KB> The dish needs to be a least 5 wavelengths across to bring the waves to a focus.


KB> For 23 cm this would be about a 1 Meter dish. 10 wavelengths is preferred.


KB> Upper frequency is determined by the surface accuracy of the dish surface.


KB> Most commercial dishes are good to 24+ GHz


KB> The offset dish can be more difficult to work with since the feed point is not as obvious and
KB> pointing can be more difficult. But it gets the feed structure out of the beam.


KB> Good luck with your project, Kent





KB>
KB>
KB>
KB> On Friday, May 7, 2021, 08:28:18 AM CDT,
KB> Chris Wilson <chris@chriswilson.tv> wrote:

KB>

KB>




KB> 07/05/2021 14:20



KB> Absolute newbie question coming...



KB> I saw a big microwave dish advertised on Ebay and then by
KB> coincidence saw this advert mentioned on a UK Satellites forum. I
KB> am getting started in amateur radio satellite communications,
KB> initially via a 1.8 meter PF dish and the QO-100 satellite using
KB> SSB and hopefully later, ATV signals.



KB> The up link is on 2.4 GHz and someone extolling the magnificence
KB> of the Ebay dish was shot down by someone saying it may well be no
KB> use at all for 2.4 GHz uplink., for example, and was probably made for a specific purpose.



KB> This surprised me, I naively thought a dish was a dish, with
KB> prime or offset focus, but now thinking deeper I recall seeing
KB> very small, but seemingly deep versus diameter 10GHz dishes on tripods.



KB> So may I ask, how is the size / shape of a microwave dish related
KB> to its intended frequency usage please? Thanks.



KB> This is the link to the dish in question:



KB> https://www.ebay.co.uk/itm/164502970358?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649


Chris Wilson
 

Hello Neil,

Friday, May 7, 2021

Thanks very much to all who replied, here and directly. A special thanks to Neil for the very detailed yet easy to follow explanation, I appreciate your time doing that :)

Just for completeness, if someone bought the dish in the Ebay advert at

https://www.ebay.co.uk/itm/164502970358?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

Is there a risk it might not work at all effectively on 2.4GHz TX or for 10.5GHz RX? That is what was stated on the Sats UK forum, and no one seemed to query the statement. All the best and thanks again 2E0ILY (Chris in Shropshire).


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


NSG> Hi Chris, so long as a dish is large enough relative to a
NSG> wavelength to avoid diffraction loss at the frequency chosen, and
NSG> the surface irregularities are small enough relative to a
NSG> wavelength to prevent loss from phase errors, *and you can
NSG> illuminate it fully without much spillover*, then the depth
NSG> of the dish does not matter much.

NSG> Usual vague rule of thumb is that the dish diameter needs to be
NSG> at least 10 times the wavelength and surface errors need to be
NSG> about a tenth of the wavelength across the entire surface. As a
NSG> dish gets deeper, it gets harder to illuminate the surface unless
NSG> you use a subreflector, so most PF dishes have a focal length
NSG> about 0.3 to 0.5 of their diameter. Offset dishes tend to be
NSG> shallower, perhaps 0.5 to 0.7 f/d ratio.
NSG>
NSG>
NSG> Subreflectors need to be a minimum of 8-10 wavelengths, so a
NSG> very deep dish 2m wide at 13cm would need a subreflector
NSG> over a metre in diameter, and that would block the dish
NSG> very seriously, so deep dishes under f/d 0.3 are generally
NSG> only used where the diameter is 40 or more times the
NSG> wavelength. With an f/d 0.25 dish, the focus is in the
NSG> same plane as the edges of the dish, so the feed has to be
NSG> able to illuminate the edge at 90 degrees to the axis, which is
NSG> challenging. Worse, you usually set the illumination so it is
NSG> about 10-12 dB down at the edge of the dish to get best
NSG> efficiency with least spillover, but in an f/d 0.25 dish,
NSG> the edge is twice as far from the focus as the centre, to
NSG> there is an additional 6dB of taper added (space
NSG> attenuation) and the feed needs to be only 4-6 dB down at 90 degrees.
NSG>
NSG>
NSG> Those 1.8 dishes are probably OK at C-band, and might even work
NSG> at 12 GHz. Definitely OK for 2.4 but very very sharp and
NSG> potentially too gainy at 10 GHz if the surface is accurate. If
NSG> it isn't very accurate, the beam will be less sharp and more useable.
NSG>
NSG>
NSG> For narrowband use on QO100, 800 mW to a 95 cm dish gets me to
NSG> the reference beacon level, so you would probably only need 125
NSG> mW to get to the reference level on narrowband to the 1.8m
NSG> dish. For TV use, obviously you need to be running more
NSG> power, but that giant dish will mean you can under-run a
NSG> big PA to get minimum spectral regrowth and a super clean signal.
NSG>
NSG> I reckon a 1.0 to 1.2m offset dish is about the sweet spot for
NSG> QO100 unless you have loads of space and something very solid to bolt a 1.8m pf dish down to.
NSG>
NSG> Neil G4DBN
NSG>
NSG>

NSG>
NSG>
NSG> On 07/05/2021 14:27, Chris Wilson wrote:
NSG>
NSG>


NSG> 07/05/2021 14:20

NSG> Absolute newbie question coming...

NSG> I saw a big microwave dish advertised on Ebay and then by
NSG> coincidence saw this advert mentioned on a UK Satellites forum. I
NSG> am getting started in amateur radio satellite communications,
NSG> initially via a 1.8 meter PF dish and the QO-100 satellite using
NSG> SSB and hopefully later, ATV signals.

NSG> The up link is on 2.4 GHz and someone extolling the magnificence
NSG> of the Ebay dish was shot down by someone saying it may well be
NSG> no use at all for 2.4 GHz uplink., for example, and was probably made for a specific purpose.

NSG> This surprised me, I naively thought a dish was a dish, with
NSG> prime or offset focus, but now thinking deeper I recall seeing
NSG> very small, but seemingly deep versus diameter 10GHz dishes on tripods.

NSG> So may I ask, how is the size / shape of a microwave dish
NSG> related to its intended frequency usage please? Thanks.

NSG> This is the link to the dish in question:

NSG> https://www.ebay.co.uk/itm/164502970358?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649


NSG>
NSG>


KENT BRITAIN
 

Interesting way to think about it,   But the focal points for the 2.4 GHz and 10 GHz waves are slightly different
since 2.4 GHz waves cannot become an infinitesimal point.     Of course 10 GHz wave cannot either.
And 2.4 GHz feeds and 10 GHz feeds will have different phase centers.   

In the practical world the points are pretty close and just make your feed support adjustable and move it
in and out a bit for best signal levels.   I have tested hundreds of dishes on the antenna range and the feed 
usually ends up a bit closer to the dish than the calculated focal point. 

Frequency limits on the size of the dish.

The dish needs to be a least 5 wavelengths across to bring the waves to a focus.

                 For 23 cm this would be about a 1 Meter dish.    10 wavelengths is preferred.

Upper frequency is determined by the surface accuracy of the dish surface.

                 Most commercial dishes are good to 24+ GHz

The offset dish can be more difficult to work with since the feed point is not as obvious and
pointing can be more difficult.  But it gets the feed structure out of the beam.  

Good luck with your project, Kent



On Friday, May 7, 2021, 08:28:18 AM CDT, Chris Wilson <chris@...> wrote:




  07/05/2021 14:20

Absolute newbie question coming...

I saw a big microwave dish advertised on Ebay and then by coincidence saw this advert mentioned on a UK Satellites forum. I am getting started in amateur radio satellite communications, initially via a 1.8 meter PF dish and the QO-100 satellite using SSB and hopefully later, ATV signals.

The up link is on 2.4 GHz and someone extolling the magnificence of the Ebay dish was shot down by someone saying it may well be no use at all for 2.4 GHz uplink., for example, and was probably made for a specific purpose.

This surprised me, I naively thought a dish was a dish, with prime or offset focus, but now thinking deeper I recall seeing very small, but seemingly deep versus diameter 10GHz dishes on tripods.

So may I ask, how is the size / shape of a microwave dish related to its intended frequency usage please? Thanks.

This is the link to the dish in question:


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







Neil Smith G4DBN
 

Hi Chris, so long as a dish is large enough relative to a wavelength to avoid diffraction loss at the frequency chosen, and the surface irregularities are small enough relative to a wavelength to prevent loss from phase errors, *and you can illuminate it fully without much spillover*, then the depth of the dish does not matter much.

Usual vague rule of thumb is that the dish diameter needs to be at least 10 times the wavelength and surface errors need to be about a tenth of the wavelength across the entire surface.  As a dish gets deeper, it gets harder to illuminate the surface unless you use a subreflector, so most PF dishes have a focal length about 0.3 to 0.5 of their diameter. Offset dishes tend to be shallower, perhaps 0.5 to 0.7 f/d ratio.

Subreflectors need to be a minimum of 8-10 wavelengths, so a very deep dish 2m wide at 13cm would need a subreflector over a metre in diameter, and that would block the dish very seriously, so deep dishes under f/d 0.3 are generally only used where the diameter is 40 or more times the wavelength.  With an f/d 0.25 dish, the focus is in the same plane as the edges of the dish, so the feed has to be able to illuminate the edge at 90 degrees to the axis, which is challenging.  Worse, you usually set the illumination so it is about 10-12 dB down at the edge of the dish to get best efficiency with least spillover, but in an f/d 0.25 dish, the edge is twice as far from the focus as the centre, to there is an additional 6dB of taper added (space attenuation) and the feed needs to be only 4-6 dB down at 90 degrees. 

Those 1.8 dishes are probably OK at C-band, and might even work at 12 GHz.  Definitely OK for 2.4 but very very sharp and potentially too gainy at 10 GHz if the surface is accurate.  If it isn't very accurate, the beam will be less sharp and more useable.

For narrowband use on QO100, 800 mW to a 95 cm dish gets me to the reference beacon level, so you would probably only need 125 mW to get to the reference level on narrowband to the 1.8m dish. For TV use, obviously you need to be running more power, but that giant dish will mean you can under-run a big PA to get minimum spectral regrowth and a super clean signal.

I reckon a 1.0 to 1.2m offset dish is about the sweet spot for QO100 unless you have loads of space and something very solid to bolt a 1.8m pf dish down to.

Neil G4DBN


On 07/05/2021 14:27, Chris Wilson wrote:

  07/05/2021 14:20

Absolute newbie question coming...

I saw a big microwave dish advertised on Ebay and then by coincidence saw this advert mentioned on a UK Satellites forum. I am getting started in amateur radio satellite communications, initially via a 1.8 meter PF dish and the QO-100 satellite using SSB and hopefully later, ATV signals.

The up link is on 2.4 GHz and someone extolling the magnificence of the Ebay dish was shot down by someone saying it may well be no use at all for 2.4 GHz uplink., for example, and was probably made for a specific purpose.

This surprised me, I naively thought a dish was a dish, with prime or offset focus, but now thinking deeper I recall seeing very small, but seemingly deep versus diameter 10GHz dishes on tripods.

So may I ask, how is the size / shape of a microwave dish related to its intended frequency usage please? Thanks.

This is the link to the dish in question:

https://www.ebay.co.uk/itm/164502970358?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

-- 
Neil
http://g4dbn.uk


Chris Wilson
 

07/05/2021 14:20

Absolute newbie question coming...

I saw a big microwave dish advertised on Ebay and then by coincidence saw this advert mentioned on a UK Satellites forum. I am getting started in amateur radio satellite communications, initially via a 1.8 meter PF dish and the QO-100 satellite using SSB and hopefully later, ATV signals.

The up link is on 2.4 GHz and someone extolling the magnificence of the Ebay dish was shot down by someone saying it may well be no use at all for 2.4 GHz uplink., for example, and was probably made for a specific purpose.

This surprised me, I naively thought a dish was a dish, with prime or offset focus, but now thinking deeper I recall seeing very small, but seemingly deep versus diameter 10GHz dishes on tripods.

So may I ask, how is the size / shape of a microwave dish related to its intended frequency usage please? Thanks.

This is the link to the dish in question:

https://www.ebay.co.uk/itm/164502970358?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

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