Use of offset dishes

The following discussion about the convenience of using offset 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 21-Jan-2002 JR1EDE wrote:
Does any one know of the offset dish design software? Or did anyone build offset dish for EME?

"DISHES.EX"' by Rein W6/PA0ZN http://www.nitehawk.com/rasmit/off_set.html looks useful but I have some difficulty to use it. I'm still studying it ...Thanks Rein!

http://www.ticra.com/index.htm and http://www.gr.ssr.upm.es/sabor.htm are also good tools too.

By the way, I would like to introduce the mount system designed by JA6AHB is the best for the offset dish as far as I know. Visit below (sorry, it's in Japanese but you can reach the pictures): http://member.nifty.ne.jp/JA6AHB/index.html Use a copy command in each picture and paste it in another viewer (Microsoft Photo Editor for example) to enlarge it, maybe up to 200%.

The resolution of the pictures below are not good but I'm sure you can reach his unique idea of the mount: http://www03.u-page.so-net.ne.jp/cb3/ttanaka/EME/index.html

Any other resources will be very much welcomed.

On 21-Jan-2002 DL4MEA wrote:
Hello Kohjin,

also check for a software package called "SABOR". This is made in Spain, and its a general software that provides patterns and surfaces of any kind of dishes. Also I played around with constructing and offset dish. But it is not possible to feed an offset dish properly on 23cm. An offset dish makes sense if you can produce a feed with a narrow opening angle easily, which is easy and small on 10GHz but very large on 23cm. If you make the dish so it is fine for your feed, the dish will get a very large surface and the additional effort is not worth the blockage safed.

Play around with SABOR using opening angles of practical feeds any you will see that by yourself. I therefor decided to use a standard dish and a diagonal waveguide feed.

On 21-Jan-2002 W6GHV wrote:
Kojin checkout W1GHZ's antenna handbook.

On 21-Jan-2002 G4DGU wrote:
Quite a few years ago, Dick Knadle, W2IMU, the inventor and patentee of the dual mode feed antenna, wrote a very good paper on offset dishes - with 1296MHz in mind. This was #18 of the Crawford Hills VHF Club technical reports, and can be found at http://www.tiac.net/users/wade/offset.htm. I rate it as an essential read.

I'm surprised by the comment by DL4MEA that it isn't possible to feed an offset dish on 1296. Although some geometries may be inconvenient, and there are some minor issues when used with linear polarisation, there's no fundamental reason why an offset dish with its distinct advantages can't be used sucessfully on 1.3GHz eme.

On 21-Jan-2002 K2TXB wrote:
The article in question was written by Dick Turrin, W2IMU (SK), not by Dick Knadle K2RIW!

On 21-Jan-2002 DL4MEA wrote:
Hello Chris,

of course I've read this article, and I've had also some talks with others when I did this. Maybe I've overseen something.

So my question to the community: What is the minimum opening angle that a moderate sized feedhorn may achieve on 23cm. Talking about the size like the diagonal waveguide feed, W2IMU-Feed and VE4MA feed. Not a feed that is large as a trash can (e.g. OE5JFL's noise alignment horn)

The problem is where you place the feed. If you do as mostly done in commercial dishes so that the feed is just away of blocking the dish surface, you will get a very large overhang on the upper area. If you then calculate the surface of the dish that additional surface, which does not mean aperture, is much more than a small sized feed can cause to a standard central fed dish.

Imagine that the aperture (where the waves from the moon are coming) is like a big round tube. With a prime focus dish you cut this feed at 90?, it means the shortest line of cutting it. The whole surface is used to reflect as many energy to the feed as possible. If you look at this tube from an offset dishes view, you are cutting steep. In order to get the best relation between aperature and dish surface, you must go as close to the 90? cut as possible. That on the other hand means that the focal length is very long, and the feed must get a narrow opening angle.

This can be very nicely moderated with Sabor. Try to download it and you will see. I have some pictures here which I once made, but my time at the moment is very limited, but if you really want to see them I can put them to my homepage.

On 21-Jan-2002 G4DGU wrote:
Hello Guenter

Like you, I've spent some time looking at possible dish configurations, and my models certainly gave the strong impression that the problems of feeding an offset dish with a large f/D ratio, at 1.3GHz could be overcome. I did say that some geometries might be inconvenient...! I also took the view that the additional surface area was a price worth paying when taking into account the other advantages of the offset reflector. Like many engineering problems, the choice of an antenna depends on your initial assumptions and requirements.

I didn't use SABOR in my analysis, however, I know the software, and I'd also recommend it.

As it happens, I'm not altogether sure that I'll choose an offset reflector when, (hopefully) in the next few months, circumstances allow me to start building the new antenna. I have more-or-less worked-out how to make an offset reflector good to at least 10GHz, but it turns-out to be a lot less of a risk to make a symmetrical dish.

On 22-Jan-2002 DL4MEA wrote:
Sorry, that I must add one more comment:

Of course the offset dish is better than a prime focus dish. There are two advantages: * The feed does see cold sky * The feed is not in the path of the aperture, a so called non-blockage situation

But now: on 23cm it is a problem, it may be one on 13cm, and it is no longer above these, you have the problem of getting a feed which can illuminate a dish with a equivalent f/D of 0.7. (By the way: on an offset dish one must differ between the f/D the feed has to illuminate and the f/D of the basic dish. An offset dish is a section of a very low f/D, say numbers below 0.25, dish but the feed needs to feed only the section and so the feed has a lets call it equivalent f/D. Usual equivalent f/D the feed has to illuminate are around 0.7-0.9 at 10GHz TVRO dishes.

Below this value you will loose all benefit of an offset dish. * The feed for the biggest f/D I know is the extended diagonal waveguide feed, that can feed a dish with f/D of around 0.5-0.6. My current prime focus dish is f/D 0.475 and it is just properly fed by the largest sized diagonal waveguide feed without extension.

* If you construct a well illuminated offset dish for a feed with f/D of 0.5, you will need mechanically around 30% more surface than what is electrically used (the so called aperture), this surface will cause wind load, icing, also just weight, all the problems you have

* In order to have the bargain of the feed pointing to cold sky, your feed must be mounted on the bottom end of the dish. Imgaing what you have at low elevation angles: Your feed support is pointing downward, so your dish must be mounted high on the tower. For hiding of a dish, also wind load and so, it is much better to have the dish as close to ground as possible. Also, at 0? elevation, you have a very big overhang, so mechanically weight balancing is really tricky then. Of course, you can mount the feed sideways (as shown on some of W1GHZ's or N1BUG's pictures, but then it a) looks ugly and b) looses the bargain of cold sky overlap.

* If you have this dish running at high elevations, your feed will again point to the earth, and so the this benefit is also lost.

Believe me, I have spent many hours, with all my experience in electrical and also mechanical engineering. I had and have many possiblities to do mechanical jobs as well as the skills, but even so I did not get convinced that an offset dish is usable and worth the effort on 23cm.

As described, I decided to better spend that additional size to a prime focus dish. Until somebody can show me a feed which is not the size of a trash can (or OE5JFL's horn (http://www.qsl.net/oe5jfl/moonecho.htm) which is able to illuminate a f/D of 0.8 poperly. Then I would go and built an offset dish immediatly.

On 22-Jan-2002 IK2MMB wrote:
Chris, I believe Guenter was meaning that feeding an offset dish for 23cm isn?t too easy due to the important size of the feeder. It is certainly possible to be done from a theoretical and also from a practical point of wiew. Anyhow from a practical point of wiew I have to say that I share Guenter thoughts. Considering that an offset dish has to have a long f/d to be worthwhile to work with, ususally longer than the typical 0.50-0.55 that most of us work with with prime focus w2imu feeder, the appropriate feeder for a 0.70 or so f/d will be kind of big. By rule of thumb its mouth will have to be more or less as big as a camping table in diameter, hi! Maybe more. Can you imagine to sustain in front of a dish such a beast appropriately too. Shall we add some little snow and ice during the winter.... I believe this was the message but yes, it can be done.

On 22-Jan-2002 AL7EB wrote:
I looked at OE5JFL's 2.3m long conical mesh horn. It has a gain of 20 dBi which is equivalent to a beamwidth of 20 degrees. I don't believe you need to achieve this narrow a beamwidth/gain for feeding an offset feed dish...in fact I use a 33-inch offset feed dish on 2400 MHz for reception of AO-40. It is my observation that most of the satellite-TV offset feed dishes all use the same curvature (approx. a primary F/D of 3.5)(since a standard feedhorn is used for all diameters) which requires a feed with approx. 10.5 dBi gain. This is equivalent to feeding a normal center-fed dish with a F/D of 0.7 (granted most of the info is derived from W1GHZ). So what is my feed? A simple five-turn helix! Using a 0.6 dBNF preamp that is sufficient to see the transponder noise floor, and 3 dB of sun noise. Yes, I realize that is not eme performance, but the antenna design considerations are the same for satellite as they are for eme at microwave freq, just larger dish diameters needed for eme.

Yes! Yes! I know the helix won't work for 23cm eme where you must reverse polarity sense between Tx and Rx. My point is that I don't think the feed problem is quite that hard. Basically you need a feedhorn with about 80 degree HPBW or 10 dBi gain. For good G/T on rx of course you would like the dish edge illumination to be down less than -15 dB, so a little under-illumination is good (for receive). Probably only need about 10% under illumination to accomplish this using a 10 dBi feed. I computer modeled the helix design for my 33-inch offset dish.

Your point regarding the loss of the advantage of having the feed look at cold sky is correct for higher elevation angles, but this is not any worse than a center fed dish which also looks at the ground at those angles (BTW I see a 1 S-unit noise rise when pointing below 7 degrees elevation with my 2400 MHz antenna...it never rises for elevations 7-thru-90 degrees). The offset feed points roughly 27 degrees up when the dish is pointed at the horizon, so from maybe 10 to 27 degrees elevation the advantage is there. The increased dish efficiency by elimination of feed blockage is always present.

I suppose it may be more difficult mechanically to design for 23cm, than the higher frequencies. I think it is interesting that VE4MA is using a 2.5m (8-foot) diam. offset dish for 24 GHz eme. I know that commercial offset dishes are made up to ten-foot diameter (I wonder if there is a 12-foot offset dish available). The improvement in efficiency due just to no feed blockage is about 1.5 dB plus/minus a bit. That is like gaining a couple foot of diameter on a normal 10/12 foot dish!

So I guess someone will have to try it. Maybe the lower practical limit is 13cm...maybe 23cm.

As far as dish mounting considerations, mine is mounted on an az-el system, only requiring the dish to be rotated downward from the normal center-fed dish elevation position. It has survived 45 mile wind and Alaskan ice/snow. Its a problem of scale as far as I can see. Of course I would not want to build an offset feed dish from scratch (but, I have no interest in building any dish from scratch)...so I guess if you are considering a big 10-45m offset dish project then I might agree.

I am thinking of using a single 10 or 12-foot offset dish* for use from 23 thru 3cm eme. For me finding the dish will be the challenge.

Ed *Note if I move from Alaska this coming year, moving my 16-foot (23cm) and 8-foot (3cm) center-fed dishes will not be cost effective, so I hope to move the az-el system for the 8-foot ku-band dish and use it (modified) with a new 10 or 12-foot dish...maybe an offset feed type...who knows?

On 22-Jan-2002 JR1EDE wrote:
Thanks for the many inputs to my query, (Subject was changed though) I'm not yet ready to summarize nor post my own comment but wish to let all know below at present time.

Allow me most of all previous follow-up deleted here and there.

At 00:29 02/01/22 -0900, Edward R. Cole wrote:
>I know that commercial 
>offset dishes are made up to ten-foot diameter (I wonder if there is a 
>12-foot offset dish available).

3.8 meter C & KU BAND T/R Offset Dish available from Prodelin. http://www.prodelin.com/pdf/1383.pdf

>I am thinking of using a single 10 or 12-foot offset dish* for use from 23 
>thru 3cm eme. For me finding the dish will be the challenge.

I've been looking for a 2.4m Prodelin offset dish searching around the net and finally located the same one VE4MA uses. Because of it's one piece reflector, the shipping cost is too expensive I gave up in vane. The two piece recent model was located recently and I'm considering to import one for 1.3GHz through up to 24GHz EME. This two piece reflector shows 1.5dB moon noise at 24GHz I was told. Above mentioned 3.8m should be much better, maybe in future?

By the way, nobody gave any comment for Grasp8W Student Edition. It's free and comes with a well written manual together worth to read. http://www.ticra.com/swindex.htm

On 22-Jan-2002 S57UUU wrote:
Feeding a 0.7 f/d dish shouldn't be a problem, the horn aperture has to be maybe 30..40% bigger than the standard IMU horn. The 'Potter horn' would be ideal here. It is basically like an IMU horn with an added conical section. The design in not trivial, since you have to change the length of the cylindrical section to get the proper relative phase of the modes at the aperture, but it could be probably easily found experimentally on a 10GHz scale model.

Also it is not impossible to make a 0.55 f/d offset dish that an IMU horn could feed efficiently.

The two main reasons why ofsset dishes aren't popolar with hams in my opinion are:

1. you can't easily see where it's pointing (mainly a psychological problem)
2. it is harder to measure/adjust the surface shape

On 22-Jan-2002 RW3BP wrote:
I use offset dish since 1997. It is 1.65m dish (section of 3.3m 0.25 f/D). So it's equivalent f/D is less then 0.7. Had my first EME QSO in 1997 on 3cm band. Then scanned all allowed here five bands from 3cm to 70cm. Last was 70cm in June 2000 (DL9KR, PA3CSG, SM2CEW). Never heard my own echo but hope to get it on 24GHz hi. For 3 and 6cm I used rectangular horn; for 13and 23cm IMU horn; for 70cm 2x4el Yagi.

In my opinion it is no big difference between offset and normal dish but anyway I like it. In 1999 I purchased bigger one. It is 2.4m one piece solid offset dish with equivalent f/D abt 0.7. I installed it only few month ago and now use it for 24GHz rx tests. Best result for this moment is 2.4 dB of moon noise and 14dB of sun. Sometimes copied Al and Barry with RST level.

On lower bands (13cm & 23cm) I plan to extend this dish by mesh up to 3.5m or more to get equivalent f/D close to 0.55. It will be semioffset dish but I dont see problem to move feed from -15dB to -10dB zone. Blockage will be still very small. By the way one more plus for offset is absence of back energy from dish to feed and no influence to SWR. One else plus is lot of space under feed for big PA etc.

On 22-Jan-2002 S57UUU wrote:
Edward R. Cole wrote: 
> 12-foot offset dish available). The improvement in efficiency due just to 
> no feed blockage is about 1.5 dB plus/minus a bit. That is like gaining a 
> couple foot of diameter on a normal 10/12 foot dish!

I think that is too optimistic, the real gain gain is more like 0.15dB.

The main advantage is probably the lower noise due to less scattering, here a 1.5 dB reduction of noise is quite realistic.

On 23-Jan-2002 DL4MEA wrote:
> On lower bands (13cm & 23cm) I plan to extend this dish by mesh up to 3.5m 
> or more to get equivalent f/D close to 0.55. It will be semioffset dish but 
> I dont see problem to move feed from -15dB to -10dB zone. Blockage will be 
> still very small. By the way one more plus for offset is absence of back 
> energy from dish to feed and no influence to SWR. One else plus is lot of 
> space under feed for big PA etc. 

Semi-offset with these values is what makes sense, I absolutly agree with that. And these values seem to be promising, a equivalent f/D of 0.55 can be fed with moderatly sized feeds.

Semi-offset means you move away the blockage from the most efficient area of your beam, and thats fine. Even better, with a nicely designed mechanic you can make a feed changing mechanism that moves unused feeds into the even less illuminated area. That is also promising. I will be interested in your ideas, but at the moment I am just dealing with bricks, concrete, steel, water pipes and so on...

On 26-Jan-2002 JR1EDE wrote:
Believe or not, here is the conclusion stolen from MMA Memo 135, NRAO.

The offset reflector design has better sensitivity than the minimum blockage conventional design at about the 10% level, but only under conditions of extremely low atmospheric opacity and receiver noise temperature. The offset design has poorer performance than the conventional design in the areas of polarization purity and field-of-view. If these problems with the offset design are cured using additional reflectors in the optics path then the sensitivity advantage of the offset design is lost.

The analysis is performed for two frequency bands, 86 GHz and 230 GHz, each at the two elevation angles 90 and 30 degrees.

http://www.nrao.edu/almamirror/memos/html-memos/alma135/memo135.html

"Electromagnetic Performance Comparison of an Offset and a Conventional MMA Antenna Design"

On 28-Jan-2002 OH6EH wrote:
> The offset reflector design has better sensitivity than the minimum 
> blockage conventional design at about the 10% level, but only under 
> conditions of extremely low atmospheric opacity and receiver noise 
> temperature.

This is certainly true but they are making the comparison to Cassegrain optics and the apertures are very large in wavelengths. As amateurs use normally huge prime focus feeds and small diameter main apertures (in wavelengths of course), the offset feed has its advantages. A testimony of this is the excellent performance of RW3BP. Using a different illumination/feeds for RX and TX (which is easy and has no disadvantages when using offset feed) is also an excellent idea...     


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