mesh vs solid dishes

**Simba27** · 13-09-2004

Another of my hare-brained ideas...

I read somewhere that a mesh dish reflects 80% of the signal of a solid dish. If that is true then it occurs to me that by filling in the holes on my old sky dishes I could get a 25% improvement in signal.

What would be the best method:

Car body filler, rub down well, metallic spray paint followed by a matt black finish?
Would a metallic filler work better?
Aluminium foil tape?

Would it work? Anybody tried it?

Phil.

***BGonaSTICK*** · 13-09-2004

LOL, sounds like a lot of work for a relatively small return. Maybe you'll only realise 15 - 20%?

Don't think it needs to be metallic, so the P38/matt finish sounds good!

Let us know if it gets through the MOT!

**Llew** · 13-09-2004

Originally Posted by BGonaSTICK

LOL, sounds like a lot of work for a relatively small return. Maybe you'll only realise 15 - 20%?

Don't think it needs to be metallic, so the P38/matt finish sounds good!

Let us know if it gets through the MOT!

Ah, fill in the holes and a paint job.
My Golf GTI MK.1 is calling me...

**rolfw** · 13-09-2004

Originally Posted by Simba27

Another of my hare-brained ideas...

I read somewhere that a mesh dish reflects 80% of the signal of a solid dish. If that is true then it occurs to me that by filling in the holes on my old sky dishes I could get a 25% improvement in signal.

What would be the best method:

Car body filler, rub down well, metallic spray paint followed by a matt black finish?
Would a metallic filler work better?
Aluminium foil tape?

Would it work? Anybody tried it?

Phil.

Unfortunately the loss is likely to be more to do with the irregularity of the surface and lack of precision of dish shape, rather than the holes themselves, as the holes are smaller than the wavelength the signal will not pass through.

**gameboy** · 14-09-2004

My local Lidl still have some cheap 90cm dishes left from the offer a few weeks back. People bought the receiver to attach to their existing dishes. £13.99 if I remember- so it might be a better option.

Wish I was on commission!

They are also doing a complete Analogue system for under £20 on Monday - receiver, cable, connectors, dish and LNB. Only certain areas check the web page...

http://www.lidl.co.uk/gb/index.nsf/p...e_Receiver_Set

**Simba27** · 15-09-2004

Originally Posted by rolfw

Unfortunately the loss is likely to be more to do with the irregularity of the surface and lack of precision of dish shape, rather than the holes themselves, as the holes are smaller than the wavelength the signal will not pass through.

As a newb here, I don't wish to dispute your expert opinion Rolfw. But I have some vague memories of A-level physics which I'd like to enter into the discussion.

When a wave meets a hole smaller than the wavelength, doesn't it get "diffracted", ie. spread out? And with multiple holes, IIRC, this leads to interference patterns.

Surely for the purposes of satellite reception it's better to reflect the wave in the direction of the LNB than to diffract it?

I can see that the precise shape of the dish is very important to direct the signal into the LNB. My rough estimate suggests that, on this size of dish, the surface must be within 2 degrees of the correct angle. I think that's achieveable by simply sanding the filler flush to the metal surrounding it.

I'm actually going to try this in a very scientific way and measure the gain or loss (if any). It won't be for a good few weeks though because I have a long list of more important things to do.

BTW. Am I posting this nonsense in the right area? I've noticed that most of the posts in the alcove are non-satellite related.

Phil.

**rolfw** · 15-09-2004

Be interesting to see your results Simba27, not an expert myself by any means.

**patrick** · 16-09-2004

does this mean that the mesh on the screen of my microwave cooker lets 20% of waves through?

**Channel Hopper** · 16-09-2004

Originally Posted by Simba27

When a wave meets a hole smaller than the wavelength, doesn't it get "diffracted", ie. spread out? And with multiple holes, IIRC, this leads to interference patterns.

go to the front of the class phil

Originally Posted by Simba27

It won't be for a good few weeks though......

you may need some additional peroxide when you mix the p38..

glue and strong foil , neatly applied to the face, will give you some very quick answers to the question .

***prof*** · 17-09-2004

Bloody hell we've been short changed, just how much material have they saved over the years, sat dishes, ring doughnuts, polomints,(no offence Polomint.)

***Old Satellite*** · 17-09-2004

OK nice discussion - for those that enjoy the real understanding of the loss due to inaccuracies in the either the size or the deformity of the surface there are some simple calculations.

Any one still want to continue to follow ? no thought not so here are some general help lines

RMS based (Root Mean Square) deviation- normal method of measurement for an average surface irregularities. KU band frequencies 10-12 GHZ

An error of 10mm within a surface area of a dish in a random aspect through the surface area of the dish surface would result an loss of signal equal to approx 30 %

An error of 1mm within a surface area of a dish in a random aspect through the surface area of the dish surface would result an loss of signal equal to approx 3 %

Normally the professional dishes will be made to approx 0.15- 0.40mm resulting a loss of less than 1 % signal from surface irregularities.

a normal pressed Astra / Sky mini mesh dish will be normally be around the 0.60-0.75mm area.

So in short if you want to see some improvements then make sure your foil etc is better than 0.5mm otherwise I would no bother.

And by the way if you bend the dish out of shape with the extra work then most likely the extra bends will make it worse.

Anyway dishes are cheap

so good look and have some fun

regards

Old Satellite

**gameboy** · 17-09-2004

That's exactly what I was going to post!

Only I thought it was more (approx 0.15- 0.41mm!)

**Simba27** · 20-09-2004

OK. I think I understand what you are saying.

If every point on the surface of the dish is slightly wrong by an average of 1mm, then I lose 3% signal. Is that right?

On that basis, I think that it would be avantageous to clean off the rust, flaking paint and bird crap for a start.

Suppose that I am not altering the existing surface, just adding extra surface by filling in the holes. I don't see how I could get less signal by doing that. I gain a theoretical 25% but then I am going to lose some of that due to errors in the added surface.

To lose 100% of that extra signal, the average error in the extra surface would have to be 33mm. Given that the holes are only about 2.5mm in diameter, surely an average error of <33mm should be achievable?

Let's suppose that the average error of the extra surface is 3mm. That's 9% of the 25% improvement lost - which results in a net gain of over 22% on the original unaltered dish.

As BGonaSTICK said, maybe I'll get 15-20% improvement.

Or have I misunderstood?

Phil.

***BGonaSTICK*** · 20-09-2004

Yeah, but I was talking out of my butt!

***Old Satellite*** · 20-09-2004

In simple terms you would have to reduce the surface errors close to 0.1 mm to see any benifits. with the price of new dishes simply the best option is to upgrade the dish.

regards

Old Satellite

**Simba27** · 21-09-2004

I accept your expert opinion but I don't understand the reasoning behind it. I'm a newbie at this and I'm trying to learn the theory rather than solve a particular problem. I have several spare dishes and don't need to buy another one. I just wondered if it was possible to gain anything and if not then why not?

When I look at this filthy old dish, covered in rust and algae, I have difficulty believing that it's average surface error is even close to 0.1mm. Yet it gave a reasonable signal until I took it down a few weeks ago. I know a few people have spray painted their dishes, wouldn't that introduce surface errors of more than 0.1mm.

Like I said, if I don't change the existing surface, then it will continue to reflect the same amount of signal that it always did. I don't think I can lose any signal.

There are at least 5000 holes in the dish, and having looked at it again I reckon they are more like 1mm in diameter. I think the smaller diameter will make it easier to fill them accurately flush with the surrounding metal - no sanding/painting to damage the existing surface.

If even one of my hole fillings was accurate within tolerance, then I should get a little bit more signal. If I could manage an average accuracy of 1mm, then 500 of those fillings would be accurate within 0.1mm. Which should equate to an extra 2.5% signal.

You said that a normal sky dish is built to between 0.60 and 0.75mm accuracy. So if that's good enough for the existing surface why would my hole fillings need to be accurate to 0.1mm?

If 0.75mm tolerance is acceptable and I can manage to fill the holes accurate to an average accuracy of 1mm. Then 3750 of the 5000 hole fillings will be within tolerance and the signal gain could be as much as 18%.

Sorry to labour the point, but I still don't fully understand why this wouldn't work.

***Old Satellite*** · 21-09-2004

When I look at this filthy old dish, covered in rust and algae, I have difficulty believing that it's average surface error is even close to 0.1mm. Yet it gave a reasonable signal until I took it down a few weeks ago. I know a few people have spray painted their dishes, wouldn't that introduce surface errors of more than 0.1mm.
Normally pâint thickness is normally around 25-50 microns

Like I said, if I don't change the existing surface, then it will continue to reflect the same amount of signal that it always did. I don't think I can lose any signal

Yes but it is not so simple to keep to the maintain the same shape or surface level - please remember that the shape of an offest dish is very complex.

There are at least 5000 holes in the dish, and having looked at it again I reckon they are more like 1mm in diameter. I think the smaller diameter will make it easier to fill them accurately flush with the surrounding metal - no sanding/painting to damage the existing surface.

The actual filling of the holes will not make any real differnace as a Ku band signal is approx 2.50 mm which means the signal does not pass through the 1.0mm holes.

You said that a normal sky dish is built to between 0.60 and 0.75mm accuracy. So if that's good enough for the existing surface why would my hole fillings need to be accurate to 0.1mm?

This was not refering to the hole filling but that of the improvement in the suface tolerance of the whole dish. This would require complex geometrical software and a Co-ordinate measuring machine etc to achieve this level of improvement.

If 0.75mm tolerance is acceptable and I can manage to fill the holes accurate to an average accuracy of 1mm. Then 3750 of the 5000 hole fillings will be within tolerance and the signal gain could be as much as 18%.

reletively as above the holes are not important as long as they are consistant and not above 2.5 mm in thier size.

Sorry to labour the point, but I still don't fully understand why this wouldn't work.

If you are still interested then work with the dish as you suggest and see what happens.

There are several books on dish design and effeciency - may be would be a good investment if you wish to understand further.

regards

Old satellite

***Old Satellite*** · 21-09-2004

When I look at this filthy old dish, covered in rust and algae, I have difficulty believing that it's average surface error is even close to 0.1mm. Yet it gave a reasonable signal until I took it down a few weeks ago. I know a few people have spray painted their dishes, wouldn't that introduce surface errors of more than 0.1mm.

Normally pâint thickness is normally around 25-50 microns

Like I said, if I don't change the existing surface, then it will continue to reflect the same amount of signal that it always did. I don't think I can lose any signal

Yes but it is not so simple to keep to the maintain the same shape, or surface level - please remember that the shape of an offest dish is very complex.

There are at least 5000 holes in the dish, and having looked at it again I reckon they are more like 1mm in diameter. I think the smaller diameter will make it easier to fill them accurately flush with the surrounding metal - no sanding/painting to damage the existing surface.

The actual filling of the holes will not make any real differnace as a Ku band signal is approx 2.50 mm which means the signal does not pass through the 1.0mm holes.

You said that a normal sky dish is built to between 0.60 and 0.75mm accuracy. So if that's good enough for the existing surface why would my hole fillings need to be accurate to 0.1mm?

This was not refering to the hole filling but that of the improvement in the suface tolerance of the whole dish. This would require complex geometrical software and a Co-ordinate measuring machine etc to achieve this level of improvement.

If 0.75mm tolerance is acceptable and I can manage to fill the holes accurate to an average accuracy of 1mm. Then 3750 of the 5000 hole fillings will be within tolerance and the signal gain could be as much as 18%.

reletively as above the holes are not important as long as they are consistant and not above 2.5 mm in thier size. If you are still interested then work with the dish as you suggest and see what happens.

There are several books on dish design and effeciency - may be would be a good investment if you wish to understand further.

regards

Old satellite

**RedDevil_UK** · 17-01-2005

well simba, im presuming you didnt follow this through??

i for one would like to know what would happen, ive read what people think in theory.... but as we all know, what happens in theory and in practice is two different things.

as a decorator, i fill things every day

let me at it with ready mixed polyfiller haha

**Simba27** · 18-01-2005

I haven't given up on the idea. It's still in the back of my mind, I just haven't had the time to dedicate to it. I want the results to be scientifically valid with no room for doubt. The experiment plan is as follows:

1) Set up a test dish and a control dish. Fit each with an LNB.
2) Using the same receiver and cable (swapping it between LNBs), align both dishes for maximum signal on the same satellite. It's important to spend time at this stage to ensure the absolute maximum signal.
3) Write down the signal strengths for both dishes on various transponders. Perhaps High-Horizontal, Low-Horizonal, High-Vertical, Low-Vertical (not that it should make a difference).
4) Work out how much better or worse the test dish is compared to the control dish. Take the ratio of signal(test) to signal(control) for each transponder.
5) Clean the all the dirt, rust and algae off the test dish.
6) Repeat steps 2, 3 & 4. Calculate any improvement or loss.
7) Fill in holes on work dish.
8) Repeat steps 2, 3 & 4. Calculate any improvement or loss.

I think that by following this procedure, the only variable will be the signal reflected by the dish. Using the same receiver and cable will ensure the measurement is constant. Measuring relative to a control dish will ensure that any deviations due to weather, transmission strength or dish positioning are eliminated. The LNB's don't have to be the same because they are constant between measurements.

To do it properly will need a whole day of my time, and I'm way too busy at the moment. Hopefully I'll have some time during the summer.

Simba.

**RedDevil_UK** · 18-01-2005

and the signal strengths will jump to 100% and get thousands of channels, then everyone on this forum will be racing to B&Q for filler hahahah

ok, will wait till summer

***Artist 2004*** · 10-08-2005

Well, it's more than half way through summer now - I'm dying to know what happened!!

**Simba27** · 04-07-2006

Last summer was rather busy, but I came across the old dishes today while clearing out some junk. I remembered this thread and decided it was time to find the answers.

I setup a 60cm mesh dish on a pole in the garden and fitted a 0.8db universal LNB that I got from a bargain bin. I pointed it at 19E which seemed fitting because it was originally a Sky Analogue dish.

Using my 80cm solid dish on the roof with it's 0.6db LNB I recorded the signal strength reading on GoTV (12663H 22000 5/6) as 99%. Then I connected the same receiver to the 60cm mesh dish and recorded 81%. An unexpected result: The 60cm disih is getting about 80% as much signal as the 80cm dish despite only having 56% of the surface area and thousands of holes in it.

First I gave the dish a good clean with a brush and some sugar soap. I got all the guano and algae and loose plasticote flakes off it. I checked the signal strength again. No change, still 81%. I guess all that filth on the dish wasn't really affecting the signal too much.

Next I decided to try and remove some of the rust, so I used my drill with a wire brush attachment. After a good 5 minutes of brushing, still no improvement in signal, 81%.

Time to fill in the holes. I had a look in my shed for a suitable filler. Something not too labour intensive. Ready mix tile grout - perfect. I spread it on and wiped it smooth with a damp rag. Disappointingly the signal strength was still 81%. I was expecting some change, better or worse.

Looking at the dish up close, I could see that due to the size of the grains in the grout, the "hole fillings" weren't completely smooth. So I gave it a light sanding and a coat of spray paint to smooth it over a bit. Still no change. 81%.

Just for completeness I checked the 80cm "control dish", still 99% - so no change in transmission power or atmospheric conditions.

I can't explain this result. I would have expected some change even if for the worse. I can still see faint dimples in the dish, I should probably have spent more time and made sure to get it completely smooth.

The only explanation I can think of for the constant value is that the old 0.8 LNB was somehow "maxed out". Does that even make any sense? The signal reflected by the dish being enough to drive the LNB at 100% capacity, but the signal out of LNB being only 81% of what the modern decoder is expecting. I'm making this up as I go along. Somebody tell me if I'm talking bollocks here.

Anyway, there's the result. No change for better or worse. Bummer.

spiney · 05-07-2006

Some confusion here!

Where an electromag wave travels through an aperture smaller than its wavelength, then the far field beyond the aperture is much decreased, and the wave cannot propogate. Propogation isn't actually zero (which would need an infinitely small aperture, in effect a solid surface), but it becomes so close to zero as makes no difference.

With a sat dish, if the energy can't propogate through, then it must be reflected back instead (conservation of energy!).

As suggested, the lower efficiency of a mesh dish is due to surface irregularity. If the holes were perfect apertures, they would have to be razor sharp, so instead they're made rounded, so reflection becomes more diffuse, and also the apertures are no longer perfect.

See: http://www.physlink.com/Education/AskExperts/ae176.cfm .

(Added, rather than just quote equations, a better way to follow this is via more familiar Fourier optics, which is exactly the same physics, except the wavelengths are much smaller!

The far field pattern, beyond an aperture, is an Airy disc:

http://en.wikipedia.org/wiki/Airy_disk .

As the aperture size decreases, the central spot also decreases in intensity (reduced propogation), corresponding to a smaller value of I in the Frauenhoffer intensity. But, when k becomes very large, effectively you get just a single very bright spot, corresonding to "straight through" propogation).

PS, here's a nice interactive demo, but your web browser will need Java:

http://micro.magnet.fsu.edu/primer/j...ize/index.html .

PPS, filling in dish holes won't work! But, for entirely different reasons. Where two different surfaces meet, there's a boundary, hence an impedence mismatch.

spiney · 06-07-2006

Ho hum, sorry, I'm probably not explaining too well here (Spiney kills off previously interesting thread ......... ok, so what's new?).

For each hole in a metallic mesh (including sat dish), if the aperture (hole diameter) is significantly less than the electromagnetic wavelength going through it, then the other side of the hole you get an Airy disc pattern, so there's as many discs as there are holes. The pattern intensity corresponds to the Frauenhoffer I value as given by Wiki (the Java interactive demo doesn't show this intensity variation, since that's really about photographic lenses and photography!).
So, yes, there's some energy gets through the holes, but only a tiny amount!

It gets different if you start to deliberately EXTRACT energy from this tiny field, since then you've created a "conduit". In waveguuides, there's a tiny slot in the waveguide wall - much smaller than a wavelength - but a "probe" going through this tiny slot can extract nearly all the energy (and that's exactly how lnbs work!).

( http://www.fnrf.science.cmu.ac.th/th...heory%209.html

www.tpub.com/content/et/14092/css/14092_75.htm

www.tpub.com/content/et/14092/css/14092_74.htm ).

It gets different again, if you start making the aperture LARGER than the wavelength, as then you've made an "aperture antenna" (common for terrestrial tv, and newer type plate arrays of these are used on some Freeview transmitters).

( http://www.ee.surrey.ac.uk/Personal/.../aperture.html ).

( http://www.grc.nasa.gov/WWW/RT1999/6...zakrajsek.html ).

Which takes us back to the BSB squarial, see retro-tech section above ...

(added) the physics of fourier optics is the same - at much smaller wavelengths - as radio waves going through appropriately larger apertures (ignoring energy coupling, which admittedly is different, due to the relative size of atoms!). Strangely, the radio wave version is usually presented in mathematical equations, but the optical version in pictures. Maybe because of the photography connection? Anyhow, although both versions are alternative presentations of exactly the same thing, of course we find it much easier to "understand the pictures"!

eg, both maths and (some very pretty!) pictures at: www.opticalimaging.org/fourieroptics.html

Never ever let the maths put you off ........ !

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mesh vs solid dishes For those of you with an appetite for the scientific and technical, whether it be lofty theory or more practical discussion.

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