Re: Mesh size for 10GHz

Neil Smith G4DBN
 

There is a loss calculator for mesh reflectors on the W1GHZ site at http://www.w1ghz.org/antbook/conf/Mesh_Reflector_Loss_Calculator.xlsx

There is a lot of useful stuff elsewhere in http://www.w1ghz.org/antbook/preface.htm

There is also a mesh loss calculator on the VK3UM (SK) EMECalc program.  I see that the VK3UM site is currently suspended by the ISP, I thought it had been adopted by WIA to preserve it for posterity? Still available in Google cache and on the web archive, but link rot is starting to set in. There is a copy at https://www.vk5dj.com/doug.html


If the mesh is not welded, there are additional losses, but the mathematical treatment of those is very tough indeed.

Bigger issue is if the surface is not an accurate parabola, with flats and creases.  Again, the maths are horrible on the research papers I found.

The mesh loss is bidirectional, so you start to see leakage from behind the dish as the reflectivity drops.  Probably not important if the dish is up on a pole pointing at the horizon, but not good for EME, as the ground is rather hot compared with cold sky.

Main point is that a mesh dish up in the air works 100% better than a solid one in a crumpled heap on the ground, although above 40mph or so, a mesh dish has almost the same wind load as a solid one the same size.

Neil G4DBN


On 21 Oct 2019, at 06:44, Dom Baines M1KTA via Groups.Io <dombaines@...> wrote:

Thanks all for collective info looks like a single layer of the larger mesh should be fine.

Anyway where did the 'rule of thumb' bit come from that is what taxes/puzzles me most ... Ive seen it several times but what is the underneath reasoning/calculation? How much loss might I expect in that direction?
72
Dom
M1KTA

On Saturday, 19 October 2019, 19:48:35 BST, KENT BRITAIN <WA5VJB@...> wrote:


30/4 would be more like 7-8 mm and the usual 'Rule-of-Thumb' is 1/10 wavelength or 3 mm.

Kent

On Saturday, October 19, 2019, 1:37:49 PM CDT, John E. Beech <john@...> wrote:


I don't think it will make any difference. The criterium for a mesh dish is that the mesh size must be smaller than a quarter wavelength at the frequency in use ie < 15 mm for 10 GHz.

de John G8SEQ

>  -------Original Message-------
>  From: Dom Baines M1KTA via Groups.Io <dombaines=yahoo.com@groups.io>
>  To: UKMicrowaves@groups.io <UKMicrowaves@groups.io>
>  Subject: [UKMicrowaves] Mesh size for 10GHz
>  Sent: Oct 19 '19 18:27

>  Was suggested ask here so here goes.

>  Bit of a rookie so might be a daft question:

>  If using a mesh reflector for a dish with size 2.5mm x 4.5mm does
>  this mean it will be fine in one plane but might be very lossy in the
>  other at 10Hz as 1/10th is c3.0mm. But by how much?
>  Should I cross the mesh so the centre (opposite prime focus) at least
>  has no opening larger than 2.5mm ?

>  Solid dish had some wind-age issues and moved (understatement) so got
>  to rethink.

>  72

>  Dom
>  M1KTA





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