[Moon] offset superiority on 3cm eme

Edward R Cole kl7uw at acsalaska.net
Mon Oct 27 10:39:26 CET 2014


Hans,

Sorry to disagree but the effective aperture of an offset fed dish is 
circular if viewed from the direction that radiation is received or 
transmitted.  Yes, the dish is oval in shape with one axis longer 
than the other but radiation does not fall upon that surface 
perpendicular to the plane of the ellipse.  Therefore, gain for 100% 
aperture efficiency (which is never realized in reality) is the same 
4piR^2 formula of a circle of diameter = the short axis length.

Greater efficiency is achieved with offset fed dishes which accounts 
for the greater achievable gain.  I defer to W1GHZ or WA5VJB for 
expert explanation how and why better efficiency is achieved.

73, Ed - KL7UW

At 01:15 AM 10/27/2014, Hans van Alphen wrote:
>Hi all,
>
>When using an offset dish we try to compare apples to pears in 
>saying I have an offset dish of for instance 1.8 mtr. The dish is in 
>fact formed the dimension measured horizontal, this is the value we 
>say the dish is. But in fact the dimension vertical is larger. Most 
>times about 1.23 times the horizontal dimension.
>
>An offset dish has in total a larger apperture than a prime focus 
>dish of the same dimension so therefore it should have more gain.
>Main advantage of an offset dish is that compared to a prime focus 
>dish is that is has some more gain, off course this also has to do 
>with the larger apperture.
>Other advantage is that the feed should have a smaller opening angle 
>and looking to the this is aimed a bit upward. The smaller opening 
>angle makes that one can better illuminate the dish and prevent overspill.
>Furthermore the overspill is aiming to the cold sky and not 
>partially to the hot earth as a prime focus does.
>Also in ham radio there is an important mechanical advantage for a 
>large dish, the feedpoint is close to the ground and it's more easy 
>to work on the feed to change and adjust.
>For small prime focus dishes the blocking caused by the feed is also 
>an isue causing a bit lower gain.
>
>If we compare the offset dish to a prime focus it has a bitt less 
>than 1 dB more gain. In my opinion about 0.9 dB more gain. Most of 
>this extra gain is caused by the larger apperture. If we calculate 
>the gain of a prime focus dish lets say 1.8 mtr diameter and then 
>calculate the dish for 1.23 times 1.8 mtr is 2.214 mtr and take half 
>of the difference we get a good comparison to the gain of an offset dish
>
>73's  Hans PA0EHG
>
>----- Original Message -----
>From: <mailto:faustocoletti at alice.it>Fausto Coletti
>To: <mailto:f6dro at wanadoo.fr>Dom DEHAYS ; 
><mailto:moon at moonbounce.info>moon at moonbounce.info
>Sent: Monday, October 27, 2014 9:28 AM
>Subject: Re: [Moon] offset superiority on 3cm eme
>
>Hi Dom,
>
>It is out of the question that offset dish under equal conditions
>is more efficient than prime focus dish for small size antenna.
>For large dish this difference is much smaller due to the small
>size of the feed compared to the dish diameter.
>A small advantage is still present in favor of the offset dishes
>as they normally have a F/D ratio favorable to achieve a more
>efficient illuminator.
>The problem is that it is not easy to find a large offset dish  on 
>surplus market.
>Here in Europe, there is only one manufacturer of large offset dishes for
>TV-SAT but it is not very cheap, it costs around 3000 Euros for a 
>270 cm dish.
>
>73, Fausto IK4NMF
>
>----- Original Message -----
>From: <mailto:f6dro at wanadoo.fr>Dom DEHAYS
>To: <mailto:moon at moonbounce.info>moon at moonbounce.info
>Sent: Sunday, October 26, 2014 6:05 PM
>Subject: [Moon] offset superiority on 3cm eme
>
>Hello ,
>
>I've MEASURED here on a 1m dish , 0,6/0,7dB NF waveguide system 
>9,7dB of sun noise and 0.2dB of moon noise (SFI 110)
>On a 120cm offset , same system , I get 10,6dB* sun noise and 0,3dB 
>moon noise. With the 120cm , the system is a bit below optimal , 
>normaly 11dB of sun noise should be possible.
>With the 1m system , I've worked 4 different stations  IN CW and 
>heard some more.
>
>73
>Dom
>
>
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73, Ed - KL7UW
http://www.kl7uw.com
     "Kits made by KL7UW"
Dubus Mag business:
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