[Moon] offset superiority on 3cm eme
Edward R Cole
kl7uw at acsalaska.net
Mon Oct 27 16:06:04 CET 2014
Hans,
Just my way of respectfully disagreeing. So easy for e-mail to
impart the wrong impression.
I am basing my observation on the study of conic surfaces which is a
topic in solid geometry. The ellipse is the shape that develops when
a flat surface (a plane) intersects either a cone or a cylinder at an
angle to the axis of the former.
An offset feed dish can be synthesized by illuminating a part of an
ordinary center-fed dish by using a narrower beam feed angled off the
main dish focal line to the side of the dish so it illuminates from
center to edge. The area illuminated will describe an elliptical
shape, but the shape of the beam that reflects from the dish is
parallel with the original dish "rays" and if cut by the plane of the
original dish will describe a circle. In other words the resulting
beam looks like a cylinder coming from a part of the original
dish. This is easy to see using light on a shiny dish and a piece of
paper for the reflected light to shine upon. I believe VE4MA used
this principle to illuminate three panels of a standard center-fed
dish at his winter qth in AZ (clever I thought).
One of the advantages of the offset feed is that the feed generally
looks at an upward angle at the sky so any spill-over is seeing cold
sky vs ground. This is true if the antenna is aimed for signals
coming from the horizon (dish looks like it's pointing downward. But
this advantage disappears if the dish is looking for signals in the
sky above the offset angle (typically about 27-degrees). Actually
the feed will be looking at the horizon when the dish is looking at
2x 27 degrees in the sky. Probably a reason why the offset feed dish
is used for satellite-TV.
Reduction of feed blockage is another advantage, especially on small
dishes (in terms of wavelength).
Also it is easier to design feeds with more gain than is optimum on
deep dishes F/d <0.4. I once upon a time had a 1.8m dish with
f/d=0.25; the focal point was right in the plane of the dish opening
so a board across the dish could support it. But it had to
illuminate with a beamwidth of 180-degrees. I used a simple
cylindrical waveguide horn that was more like 90-100 degrees so I
under-illuminated the dish severely. Lost 6-dB gain of the
dish. But it worked fine for my purpose using only about the center
3-foot of the dish on 3300-MHz for a terrestrial shot.
I have considered mounting my 9cm feed to offset feed my 4.9m dish
but hate to lose the gain by doing that. So instead I will co-mount
it with my 23cm feedhorn so the 9cm will be slightly off center from
the focal point but pointed to the dish center. There will be a
resulting offset angle of radiation and some aberration loss due to
not being at the focal point.
Advantage will be that multiple feedhorns will be mounted to avoid
feed change out with band change. Something to avoid in the middle
of winter on a starry night in Alaska. ;-)
73, Ed - KL7UW
At 02:28 AM 10/27/2014, Hans van Alphen wrote:
>Hi Ed,
>
>Don't need to say sorry for an honest and open discussion. I also
>would like to learn from it.
>
>Perhaps the word apperture is not the correct one, I always found it
>a strange word which I did not realy understand.
>What I do understand is that the total reflecting surface of a 1.8
>mtr offset dish is larger than the reflecting surface of a 1.8 mtr
>prime focus dish.
>More reflecting surface, if manufactured correctly, means more
>gathering of energy and that gives us more gain.
>
>Comparing a 1.8 mtr offset dish to a prime focus one, should in my
>opninion be done by comparing the total reflecting surface of these two.
>I am not a mathematics sientist, so I don't know (yet) how to
>calculate the reflecting surface of the two different types, but I
>am sure that an offset dish has more reflecting surface.
>I am too busy now in my garden, perhaps later today I will try to
>find a way (using Google, Hi) how to calculate.
>
>Thanks for the discussion.
>
>73's Hans PA0EHG
>----- Original Message -----
>From: <mailto:kl7uw at acsalaska.net>Edward R Cole
>To: <mailto:h.v.alphen at planet.nl>Hans van Alphen ;
><mailto:faustocoletti at alice.it>Fausto Coletti ;
><mailto:f6dro at wanadoo.fr>Dom DEHAYS ;
><mailto:moon at moonbounce.info>moon at moonbounce.info
>Sent: Monday, October 27, 2014 10:39 AM
>Subject: Re: [Moon] offset superiority on 3cm eme
>
>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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73, Ed - KL7UW
http://www.kl7uw.com
"Kits made by KL7UW"
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