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Question about mesh size
John Fell
Hi Greg,
You could test the hole effect by putting a low level source onto the dish feed and using a diode detector or suitable low level power meter on a horn behind the dish .
Ideally you should see no or very much reduced RF on the detector ( use an LNB with attenuation on its input if you want to go really low , but will need to watch for overspill sneaking round the edges ) .
Elliptical hole should allow you to find which is the most leaky axis and use the dish in the best orientation - i.e. maybe needs dish turning 90 degrees from Sat arrangement , then dish will need pointing allowing for the offset feed angle - the dish face will be OK in the other axis .
73
John
G0API
On Sun, 26 Mar 2023 at 14:52, Greg - ZL3IX via groups.io <gregandroberta=yahoo.com@groups.io> wrote:
I've just acquired a 2.3 m mesh dish. The holes in the mesh are elliptical with a long axis of 3.5 mm. I know the hole should be < 3 mm for 10 GHz use, but does anyone know if it will still work OK at 3.5 mm? What will be the gain reduction?
Julian, G3YGF
Greg,
Adding to G0API, you could get a diode detector with a small horn, or even just a waveguide transition, which is about 6dBi.
You will be measuring the attenuation of the mesh with an almost plane wave on it.
With the dish feed in place and a few mW or tens of mW into it, place the transition on the front surface of the dish facing the feedand then a few cms behind the dish. Align the transition for max signal - the same orientation in both places. You want the attenuation to be about 10dB or more.
If you can line up the polarisation with the shorter dimension of the holes, that will be better.
The feedthrough will increase exponentially as the hole size increases...
Julian, G3YGF
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Adding to G0API, you could get a diode detector with a small horn, or even just a waveguide transition, which is about 6dBi.
You will be measuring the attenuation of the mesh with an almost plane wave on it.
With the dish feed in place and a few mW or tens of mW into it, place the transition on the front surface of the dish facing the feedand then a few cms behind the dish. Align the transition for max signal - the same orientation in both places. You want the attenuation to be about 10dB or more.
If you can line up the polarisation with the shorter dimension of the holes, that will be better.
The feedthrough will increase exponentially as the hole size increases...
Julian, G3YGF
On 26/03/2023 16:01, John Fell wrote:
Hi Greg,You could test the hole effect by putting a low level source onto the dish feed and using a diode detector or suitable low level power meter on a horn behind the dish .Ideally you should see no or very much reduced RF on the detector ( use an LNB with attenuation on its input if you want to go really low , but will need to watch for overspill sneaking round the edges ) .Elliptical hole should allow you to find which is the most leaky axis and use the dish in the best orientation - i.e. maybe needs dish turning 90 degrees from Sat arrangement , then dish will need pointing allowing for the offset feed angle - the dish face will be OK in the other axis .
73JohnG0API
On Sun, 26 Mar 2023 at 14:52, Greg - ZL3IX via groups.io <gregandroberta=yahoo.com@groups.io> wrote:
I've just acquired a 2.3 m mesh dish. The holes in the mesh are elliptical with a long axis of 3.5 mm. I know the hole should be < 3 mm for 10 GHz use, but does anyone know if it will still work OK at 3.5 mm? What will be the gain reduction?
Iain Crawford
I suspect 3.5mm holes will be fine. Some time back I did some tests at various frequencies with a 'gridpak' antenna designed for 2.4 GHz. The metal diameter was 8.5mm with 21.5mm gaps between the mesh i.e. well over 1/2 wavelength at 10 GHz. Even so, this still blocked 50% of a test signal on 10.2 GHz.
John Lemay
You’ll find “EME Calc” from VK3UM very helpful.
John G4ZTR
From: UKMicrowaves@groups.io [mailto:UKMicrowaves@groups.io] On Behalf Of Greg - ZL3IX via groups.io
Sent: 27 March 2023 08:24
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] Question about mesh size
Thanks Iain. Anecdotes from previous experience were what I was looking for. Also hoping that someone has some theoretical graphs on shielding effectiveness vs hole size in mesh. Come to think of it, I can probably find that somewhere on the net. Will look and report back.
g4bao
The issue you'll have is basic dish shape / rib shape tolerance.
As an example., my 1.2m solid offset dish outperforms my 1.9m (2,7mm mesh) prime focus dish at 10GHz by quite a way.
73
John
On Mon, 27 Mar 2023 at 02:38, Iain Crawford <iain@...> wrote:
I suspect 3.5mm holes will be fine. Some time back I did some tests at various frequencies with a 'gridpak' antenna designed for 2.4 GHz. The metal diameter was 8.5mm with 21.5mm gaps between the mesh i.e. well over 1/2 wavelength at 10 GHz. Even so, this still blocked 50% of a test signal on 10.2 GHz.
In message <nIWW.1679838770729019577.w3lg@groups.io>, Greg - ZL3IX via groups.io <gregandroberta@...> writes
4NEC2 has two models for a parabolic dish path50 which comprises solid patches , and parab50 which uses a wire mesh roughly 4mm square. The dishes are 22cm in diameter and use a circular dipole feed
The gain at 10GHz is 23dB for the mesh dish and 22.88dB for the patch dish.
The radiation patterns in terms of the sidelobes look different. The patch model is more symmetrical horizontally and vertically.
At 20GHz ( 1.5mm) the mesh model gives 29.6dB and the patch 28.68dB
At 50GHz (0.6mm) the mesh model gives 32.83dB and the patch 37.29dB.
Enthusiasts might like to have a go at it.
In my day job I had a little program to work out attenuation of wire grids at both RF and light frequencies . I wanted to keep the RF out and let the light through. I can't find where I got RF screening formula.
I have an ex Ferranti Blue Parrot 10GHz Radar Twist Cassegrain dish, which has wires at 45 deg spaced at 8mm . I commented on this one before on here.
Brian GM4DIJ
--
Brian Howie
I've just acquired a 2.3 m mesh dish. The holes in the mesh are
elliptical with a long axis of 3.5 mm. I know the hole should be < 3 mm
for 10 GHz use, but does anyone know if it will still work OK at 3.5
mm? What will be the gain reduction?
4NEC2 has two models for a parabolic dish path50 which comprises solid patches , and parab50 which uses a wire mesh roughly 4mm square. The dishes are 22cm in diameter and use a circular dipole feed
The gain at 10GHz is 23dB for the mesh dish and 22.88dB for the patch dish.
The radiation patterns in terms of the sidelobes look different. The patch model is more symmetrical horizontally and vertically.
At 20GHz ( 1.5mm) the mesh model gives 29.6dB and the patch 28.68dB
At 50GHz (0.6mm) the mesh model gives 32.83dB and the patch 37.29dB.
Enthusiasts might like to have a go at it.
In my day job I had a little program to work out attenuation of wire grids at both RF and light frequencies . I wanted to keep the RF out and let the light through. I can't find where I got RF screening formula.
I have an ex Ferranti Blue Parrot 10GHz Radar Twist Cassegrain dish, which has wires at 45 deg spaced at 8mm . I commented on this one before on here.
Brian GM4DIJ
--
Brian Howie
Pete - GM4BYF
Mesh aperture should be less than a tenth of a wavelength. Not as
good as solid but not a great deal of loss
73 Pete GM4BYF
On 27/03/2023 08:24, Greg - ZL3IX via
groups.io wrote:
Thanks Iain. Anecdotes from previous experience were what I was looking for. Also hoping that someone has some theoretical graphs on shielding effectiveness vs hole size in mesh. Come to think of it, I can probably find that somewhere on the net. Will look and report back.
--
vry 73
Pete GM4BYF
vry 73
Pete GM4BYF