The following discussion about the convenience of using deep of flat parabolic dishes took place in the Moon-Net reflector. I found the subject so interesting that I decided to put together all the messages in this page.
On 15-Mar-1999 DL4MEA wrote:
A discussion this weekend brought some confusion. Please help me: Is there any reason - despite availability of suiting feeds - why mostly deep (around f/D=0.5 to 0.33) - dishes are used for EME?
There is a version of a W2IMU feed shown best for f/D=0.8, but I never saw anyone using this togehter with a dish. Most of the dish users have f/D around 0.3 and use the smaller f/D version of W2IMU or even the VE4MA feed when the dish becomes deeper. Is there anything why it should be avoided to use such a high f/D?
On 15-Mar-1999 W1GHZ wrote:
Deep dishes are used by hams because they are readily available. the question is, why are they used commercially?
I think it is because they are more compact - the feed is closer to the dish - and thus easier to mount and balance on the mount. if it were possible to make a good feed for f/D=0.25, that would be very popular since the feed would be level with the rim.
A second factor is that feeds with narrow beamwidth, for larger f/D, have a larger aperture, so feed blockage increases.
Offset dishes change the story. now the usual dimensions call for a feed that would be used for f/D = 0.6 to 0.8 on a conventional dish, and have almost no feed blockage.
All we need is a source of offset dishes big enough for EME!
On 15-Mar-1999 SM6CKU wrote:
Build a half 10 meter dish with f/D .25 and you have an excellent 5 meter offset dish (at least).
On 15-Mar-1999 DF9CY wrote:
Yes there are reasons for deep dishes at f/d = 0.28 .. 0.4:
* low noise because of low spill-over
* easier feed construction (symmetry of H and E plane is a problem, but not a big one)
* a wide angle feed gives a small feed. this reduces size and obstruction
* a high f/d needs a high gain feed for the narrow beam width. this results in possible sidelobes by the feed.
I once read a book on radio-astronomy, where these points were mentioned.
On 15-Mar-1999 G3SEK wrote:
In the old days, an F/D ratio of 0.6 was considered optimum for EME because it gives highest gain. What has changed from the old days is that receiver NFs are now very much lower, so we can make use of the lower noise pickup of a deeper dish. The gain is lower but the G/T ratio on RX is better, and on TX you can increase power to compensate.
On 16-Mar-1999 N2OTO wrote:
Paul Wade W1GHZ (N1BWT) wrote:
> all we need is a source of offset dishes big enough for EME!
In my opinion, if someone were to design, and publish the design, of an offset dish, no one could do it better than you.
(i.e.- a dish that can just do it on 23 cm and work great at 10 Ghz.)
On 16-Mar-1999 W6/PA0ZN wrote:
Re offset feed dishes.
If there is any real interest in building this kind of dish, I wrote a C-program a couple of years ago that calculates the surface curvatures for any parabolic or offset feed dish, big or small. It is basicly based upon the GBT design and if I ever get the time, I intend to build a 1:10 scaled model of this particular antenna. My planned retirement has been delayed and that is the reason that I have not started this project. The real GBT is a 100 * 108 m reflector under construction at Greenbank West VA, since probably 1993 or so. So far I have had 2 problems with it, one is time and the second one is my concern of getting a desigh for feeds for 432 and 1296 MHz. Although I have promises of professional help, if I ever get to that point. The offset feed might be beneficial under circumstances as far as elevations go, although that is a double edged sword.
You might ask, why do you want to do an offset feed dish? Well, I studied a paper by W2IMU written in the early 80's and his arguments for this sort dish made a lot of sense. Besides that I believe no amateur has done it at least that large although PA0AVS constructed a horn for 432 and above. In the early TVRO period, people constructed reflector surfaces. One of those designs was actually published in Radio / Electronics and during a recent visit to Capetown I actually noticed one on top of the astronomy department of the university there. I have also seen in Japan and Korea dishes on gouvernment buildings ( offset feeds ) that are probaly in the 3 to 4 meter diam estimating from the ground. You can also see larger and larger offsets on uplink trucks for TV. Beautiful machines with very nice mounts and feed structures. all folding down during non use. Recently I noticed a large offfset feed on a truck in use by the California Emergency Agency ( some other name, but I forgot )
Freq. coverage presents the same problems as for parabolic dishes. What to use as a surface, windload, regidity of the structure, surface accuracy etc etc and feeds!. The mount!! My idea's are just on how to build one, that large and it is not a ready design. In my humble opinion, one needs to design those structures with one's resources and capabilities in mind. There are also the mount issues that one tends to overlook thinking about dishes, but mounts are a crusial part of the antenna. As said if there is any interest in the matter I will help via thw WEB.
On 16-Mar-1999 G4IUG wrote:
Here in the UK offset dishes are very common for TV use. At 10GHz a feed is simple. At 1296MHz a feed for an offset dish becomes very large, or does it ? Anyone actually got an offset feed at 1296 that provides a reasonable efficiency ?
On 16-Mar-1999 G3SEK wrote:
Earlier, I wrote:
>In the old days, an F/D ratio of 0.6 was considered optimum for EME
>because it gives highest gain. What has changed from the old days is
>that receiver NFs are now very much lower, so we can make use of the
>lower noise pickup of a deeper dish. The gain is lower but the G/T ratio
>on RX is better, and on TX you can increase power to compensate.
Barry, VE4MA has pointed out that is an over-simplification, because shallow dishes come back into their own on the higher bands, for three good reasons:
1. Surface accuracy becomes more and more of a problem at higher frequencies. It is much easier to build a shallow dish accurately, since the departure from flat is smaller.
2. On the higher bands it becomes easier to make feeds that illuminate shallow dishes well, and do not cause much blockage in a prime-focus dish.
3. Offset dishes are even better, and these are always high F/D. Feed blockage is not an issue, the offset feed can be made large, to give better illumination taper with equally good sidelobes, and also the spillover is pointed more towards the sky.
Barry points out that RW3BP is running a 1.6 m (40") offset dish for EME on 1296 (300w), 5.6 & 10 GHz and by all accounts he has an outstanding signal for such a small antenna. His system works at least as well as a conventional prime focus 8 ft dish.
On 16-Mar-1999 SM6CKU wrote:
If a 3m offset dish is a large dish I don't know but you may see one on http://www.parabolic.se/antennas.shtml
On 17-Mar-1999 OH2AUE wrote:
I should imagine that using fairly short focal distance dishes for EME ( and for any space communications for that matter ) results in lower antenna noise temperature. It is fairly difficult to design and mass produce a feed system for longer focal distances that have very low sidelobes ( sidelobes seeing past the dish - spillover from sidelobes, not the main beam - and thus increasing noise temperature ). For radio astronomy purposes the antenna f/D's are usually guite low - I think - maybe some of our professionals have some input.
In radio astronomy even a few K difference is worth working hard for and small things like support strut design etc. are key issues that contribute to Tant. In this respect offset antennas are gaining popularity at a high rate because the aperture is not blocked with the feed as with prime focus antennas. I recall Tom Clark mentioning using a cone at the bottom of a prime focus dish to reduce the effect of the prime focus feed seeing itself as an hotter noise source due to ambient temperature/ reflection of hot earth. Offset antennas of 1 - 5 meters are very popular in Europe nowadays for satcom applications because of the lower noise temperature primarily, but also because the reflector being more vertical ( keeps off the snow more easily with the aid of antenna de-icing systems needed up here in Scandinavia ). A lot of research has gone into even minute details like the shape of the fee struts and also their oriention to the hot ground. In the basic three strut design one can actually improve the noise temperature by quite a few K by setting the tripod design with two struts at the top and one at the bottom for antennas elevated to the sky.
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