Designing parabolic dish and feedhorn

  • #1
Summary:
Designing parabolic dish and feedhorn
I need to design a parabolic dish. Only requirement is that it's 3m in diameter. Everything else can change.
I may seem like a bit confused. That's because I have 50 tabs open about the topic.

First thing is I need to calculate dish gain. If I'm going to calculate it according to this equation
parabolic_antenna_gain.png

I need to know "k" or the efficiency of the parabolic reflector.
Some pages say I can just put 50-60% real world efficiency in the gain equation and use that. Some say efficiency is directly proportional to f/D ratio. That turned out to be wrong when I read a chapter from the book "Antenna theory" by Balanis. And besides, it makes no sense when f/D is above 1. If I understand correctly, efficiency does depend on f/D ratio, BUT only if we keep the same feedhorn. If we use optimal feedhorn design for that specific dish f/D ratio, then the aperture efficiency is about 70-80 % for any f/D ratio as seen here:

apertureEfficiency.jpg

N is feed directivity. I think.

What I need is an advice. What are the correct steps, what parameters should I determine first before calculating the rest? Honestly, I have no clue if I'm doing anything correctly so I'm asking for your help.

Frequency is 1420MHz, and purpose is for listening to hydrogen line.

I'm going to stop here for now, to not overwhelm you with questions.
 
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  • #2
sophiecentaur
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Summary:: Designing parabolic dish and feedhorn

I need to design a parabolic dish. Only requirement is that it's 3m in diameter. Everything else can change.
That's a strange question. What is the application? The 'rules of thumb' can depend on more than f/D. What frequency are you dealing with?
 
  • #3
That's a strange question. What is the application? The 'rules of thumb' can depend on more than f/D. What frequency are you dealing with?
1420MHz, Hydrogen line. Purpose is listening to hydrogen line. I've also updated original post with this information.
 
  • #4
sophiecentaur
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1420MHz, Hydrogen line. Purpose is listening to hydrogen line. I've also updated original post with this information.
You're another mad Radioastronomer? I did wonder as my mate is just getting into RA.
 
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  • #5
Baluncore
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RAs don't start out mad, but they can get that way.
Why would anyone do astronomy during the day.

1420 MHz with a 3 m dish. λ = 211 mm; Diam = 14.2 λ; Beam width = 4.0 °.
There are still too many variables.
Do you want to maximise energy efficiency, or do you want minimum sidelobes?

What is the reason for doing H-line astronomy?
What is your local EMI environment?
 
  • #6
You're another mad Radioastronomer? I did wonder as my mate is just getting into RA.

I'm definitely mad, but not yet a radio astronomer :)

1) Do you want to maximise energy efficiency, or do you want minimum sidelobes?

2) What is the reason for doing H-line astronomy?
3) What is your local EMI environment?

1) I'm not sure. Minimum sidelobes would give me a signal with minimum noise? Or something else?

There are two types of feedhorn that I found so far that should work fine. First, feedhorn with Kumar choke, good for dishes with f/D around 0.3-0.4

4-Figure3-1.png


and W2IMU dual mode feedhorn that is good for f/D around 0.5-0.7

DeXLi1uX0AADB5d.jpg

Right now I'm leaning towards W2IMU feed and f/D=0.5 but without an objective reasons. f/D=0.5 because it looks the sturdiest for me personally - larger f/D have focal point very far away and there will probably be some movement and problems with feedhorn going out of focal point.
W2IMU feedhorn because it's smaller than the other one, I already read much more about it and looks to be easier to make.


2) Mainly to learn. For a very long time I though radio astronomy is awesome and I want to try it, and learn as much as I can on the way. If it does work, great, if it doesn't I would at least gain some knowledge.

3) In a very small village surrounded by forest for at least 15 km on all sides. So very little interference.
 
  • #7
sophiecentaur
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Do you want to maximise energy efficiency, or do you want minimum sidelobes?
There is no answer at this stage, I think. This would depend on local conditions.
Last week I saw a demonstration by a visiting (experienced and credible) amateur RA at our new club venue (first view of any hands-on RA equipment but I think my observations are valid). He was looking at the21cm Hydrogen line.The plot he obtained on the receiver showed a great festoon of what he described as Bluetooth interference spikes. The low peak (hump) of a very few dB in the 'grass' was at the right 21cm frequency but there was one spike right in the middle. The speaker said that was much worse than reception at his home. With a main beam of say 10 degrees, pointing at the sky between the buildings, I would say that pickup from the buildings would probably have been due to the antenna side lobes. So in that situation, I would say that side lobe performance would be the major factor. Efficiency would help in basic SNR but not with Signal to Interference Ratio. Away from houses, things could be different.

Bottom line is that the best solution would involve actually measuring the pattern of the antenna, using a range of feeds. 21cm has pros and cons for amateur work. The frequency allows reasonably priced electronic equipment but the antennae will be big and not convenient to measure its pattern reliably. It would be feasible to verify what the manufacturers claim about performance (or not).
 
  • #8
marcusl
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Efficiency and sidelobe levels are set primarily by the radiation pattern of the feed horn, so you will get what you get when you use someone else's design. I wouldn't worry about it, since you won't be able to change it. If you are building your own dish, rigidity and accuracy are important. Your surface should follow the theoretical paraboloidal surface shape to within lambda/10 to work, while lambda/50 or better is typical in commercial dishes to achieve good sidelobe control.
 
  • #9
Baluncore
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Bottom line is that the best solution would involve actually measuring the pattern of the antenna, using a range of feeds. 21cm has pros and cons for amateur work. The frequency allows reasonably priced electronic equipment but the antennae will be big and not convenient to measure its pattern reliably. It would be feasible to verify what the manufacturers claim about performance (or not).
RA is a big challenge with many facets. There is a limit to how much one person can do. A beginner, working by themselves, needs a quick result that can then be built on. The antenna beamwidth and pattern can be predicted without need for calibration.

It may be easier initially to allow the radio source to pass through the beam, (which may also yield the beam pattern), than to try tracking or mapping an area of sky. Accurate aim of a 3m dish with a 4° beam should not be a problem, it is done for satellite TV all the time.

The first problem is knowing what in the sky they will be able to see with a couple of warm amateur FET pre-amps and a satellite down converter. Get a copy of Radio Astronomy by John Kraus, it has lots of useful information and an appendix with RA,Dec radio source listings.

@GhostLoveScore
There are a limited number of bright sources. Have you gone through a list of RA sources, to identify those that will be above the noise floor and that pass high overhead at your latitude. What is your latitude?

Will you build or buy your dish? Can you find one or more second hand dishes from a terrestrial microwave link? Do you need to be picky about f/d ratio in your location?

Have you worked out the maximum size of the holes in wire mesh that will reflect H-line wavelength. 210mm / 10 = 21mm diam. For H-line work, you could use 1/2" = 12 mm wire mesh to make a cheap, larger than 3 m dish. You could stretch and pull back to adjust the mesh surface on an approximate dish framework to get the required surface accuracy, 210mm / 20 = ±10 mm.

If a dish was so deep it contained the focus, then the focus will be screened from local interference. You may also place a strip of 377 ohm space-cloth around the lip to prevent knife-edge diffraction. But your particular isolated location should not need that.
 
  • #10
1) It may be easier initially to allow the radio source to pass through the beam, (which may also yield the beam pattern), than to try tracking or mapping an area of sky.

2) The first problem is knowing what in the sky they will be able to see with a couple of warm amateur FET pre-amps and a satellite down converter. Get a copy of Radio Astronomy by John Kraus, it has lots of useful information and an appendix with RA,Dec radio source listings.

@GhostLoveScore
3) There are a limited number of bright sources. Have you gone through a list of RA sources, to identify those that will be above the noise floor and that pass high overhead at your latitude. What is your latitude?

4) Will you build or buy your dish? Can you find one or more second hand dishes from a terrestrial microwave link? Do you need to be picky about f/d ratio in your location?

5) Have you worked out the maximum size of the holes in wire mesh that will reflect H-line wavelength. 210mm / 10 = 21mm diam. For H-line work, you could use 1/2" = 12 mm wire mesh to make a cheap, larger than 3 m dish. You could stretch and pull back to adjust the mesh surface on an approximate dish framework to get the required surface accuracy, 210mm / 20 = ±10 mm.
1) That's what I plan to do. I wrote a computer program that shows me EQ coordinates from Alt Az coordinates. Then I only have to wait for the source to pass over the dish.

2) I did get the book, and I'll be getting hydrogen line preamp from ebay. It's about 50 bucks, so I don't know if it goes into "amateur FET preamp" category. I don't plant using satellite downconverter. Preamp, then filter, then into SDR radio. Maybe one other amp before SDR, I'll see if there will be a need for it.

3) yep, Cassiopeia A and Cygnus A and some other. I'm at 45N latitude

4) I'll be building it. I can get that TV offset dishes about 90cm in diameter. They could serve for proof of concept.

5) yep, I plan to use a wire mesh
 
  • #11
Bottom line is that the best solution would involve actually measuring the pattern of the antenna, using a range of feeds.

The problem is that there really isn't a good guide on how to design feedhorns. Yes, I can simulate them in NEC program, but it such a pain, you need to make a fine mesh in the shape of feedhorn. I'll do it one day, but not right now.

building your own dish, rigidity and accuracy are important. Your surface should follow the theoretical paraboloidal surface shape to within lambda/10 to work, while lambda/50 or better is typical in commercial dishes to achieve good sidelobe control.
Yes, I'll see how much accuracy I will get when I build it. This is all just a big experiment.
 
  • #12
sophiecentaur
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The antenna beamwidth and pattern can be predicted without need for calibration.
That's only when you know the pattern of the feed; unlike with lower frequencies, for 21cm, the gains need to be higher and so the spec is more challenging when you are in other peoples' hands.
It may be easier initially to allow the radio source to pass through the beam, (which may also yield the beam pattern)
True for many user of dishes but optical astronomers who have got as far as moving to RA will often have a rigid and calibrated mount with remote control. The struggle with a camera tripod wouldn't be a problem for them. Also, using the Earth's rotation involves a fair amount of time (a few hours) and background interference can change.
The problem is that there really isn't a good guide on how to design feedhorns.
This link will give you some ideas about DIY feeds. Very cheap and cheerful but, bear in mind that the results from small, low gain dishes seem to be more to show that you can do it than to get fab results. My point is that you are likely to want to move on up (as with all other Astronomy activity) and buying / making bigger and better. These guys have produced a list of useful information sources.
 
  • #13
This link will give you some ideas about DIY feeds. Very cheap and cheerful but, bear in mind that the results from small, low gain dishes seem to be more to show that you can do it than to get fab results. My point is that you are likely to want to move on up (as with all other Astronomy activity) and buying / making bigger and better. These guys have produced a list of useful information sources.

Thanks, but can antennas have pretty low efficiency. What I'm looking at now (VE4MA and W2IMU feedhorns) are advanced versions of it. I know, I'm probably complicating things too much. I should try first with can antenna, but I really want to start with best feedhorn that I can.
 
  • #14
tech99
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All the design agony you are going through will only make a small difference to gain, in the order of 1dB. You can lose that in the connecting cable. A small difference in diameter will compensate for efficiency variations.
 
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  • #15
sophiecentaur
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Thanks, but can antennas have pretty low efficiency.
Do you mean can fed antennae? That's not surprising because the ideal illumination of the dish would be almost uniform over the whole area with none spilling out. A simple feed can't do that sort of thing so the 3dB beam width has to be pretty narrow and some of the dish area is 'wasted' and the overall beam width will be wider and the gain will be less. There is an optimum and I think that's where the 'best' f/D will vary.

I'd bet you'll find that you will need several passes through the problem unless you spend a lot of money on ready made systems from a good supplier. (as with HiFi etc.) But it's the journey as much as arriving so why not have several different attempts? You would learn a lot.
 
  • #16
All the design agony you are going through will only make a small difference to gain, in the order of 1dB. You can lose that in the connecting cable. A small difference in diameter will compensate for efficiency variations.

I suspected that, and I agree with you. The thing is, I get obsessed with making something perfect on the first try, but I know it well it doesn't work that way. I promise I'll stop complicating things this much :)
Best course of action would be to get it to work as simple as possible and then make it better and better over time.

Do you mean can fed antennae? That's not surprising because the ideal illumination of the dish would be almost uniform over the whole area with none spilling out. A simple feed can't do that sort of thing so the 3dB beam width has to be pretty narrow and some of the dish area is 'wasted' and the overall beam width will be wider and the gain will be less. There is an optimum and I think that's where the 'best' f/D will vary.

I'd bet you'll find that you will need several passes through the problem unless you spend a lot of money on ready made systems from a good supplier. (as with HiFi etc.) But it's the journey as much as arriving so why not have several different attempts? You would learn a lot.

Yes, I meant "can fed antennae".

I agree. As I wrote above, I complicate things too much. I know I will have to try several different feedhorns, and make a lot of modifications to get it to work good. But you know, can't blame a guy for wanting to start with the best possible design.
 
  • #17
sophiecentaur
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But you know, can't blame a guy for wanting to start with the best possible design.
Perhaps you are too young to have developed the right level of pessimism for these things. I always expect to need at least three tries at anything. lol
 
  • #18
Perhaps you are too young to have developed the right level of pessimism for these things. I always expect to need at least three tries at anything. lol

I don't think 30 is that young haha
 
  • #19
sophiecentaur
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I don't think 30 is that young haha
No'but a lad (as they say in Yorkshire). :wink:
 
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  • #20
jrmichler
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I always expect to need at least three tries at anything. lol
Then you are better than I am. I find that getting from a concept to a solid, reliable design suitable for shipping to a customer takes about ten iterations. Although, I once had a project that worked the first time. It was a small, simple project, and that was only once in a lifetime.
 
  • #21
Baluncore
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Best course of action would be to get it to work as simple as possible and then make it better and better over time.
That is the only way to do it.
If you try to polish every electron, you will retire as a theoretician.

Perfection is the enemy of progress.
https://en.wikipedia.org/wiki/Perfect_is_the_enemy_of_good

Assemble a "third best" system from available modules this month. Later, when you know what you are doing, you can improve it to be what you would now call "second best". The "first best" is perfect and can never be. By definition, no matter what you do, anything built or manufactured must always be "third best".

The development of RADAR by Watson-Watt followed his "cult of the imperfect".
That RADAR led to the components needed for radio astronomy.
https://en.wikipedia.org/wiki/Robert_Watson-Watt#Aircraft_detection_and_location
I quote the last sentence;
Watson-Watt justified his choice of a non-optimal frequency for his radar, with his oft-quoted cult of the imperfect, which he stated as "Give them the third-best to go on with; the second-best comes too late, [and] the best never comes".
 
  • #23
Baluncore
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Those are a really nice quotes.
And you should put some up on the wall of your workshop and study.
They make it possible to actually do Radio Astronomy.
 
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