Re: Microwave dish size and shape versus frequency question


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

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