[NLRS] Open Grid dishes

[Dr. Gerald N. Johnson]

Two major factors affect the suitability of a reflector for a particular 
band, and one other consideration. The two major factors are dish mesh 
spacing and surface errors, errors from the accurate parabolic surface. 
A graph in the recent International Microwave Handbook (sold by ARRL and 
RSGB and a very fine tome that should be in every microwaver's hamshack) 
figure 3.38 shows that an error of 1/10 wavelength is OK if the 
periodicity is small, but if the periodicity is > 1 wavelength then a 
1/10 wave error can cost 4 dB gain. Likewise loss through the mesh is 
shown in the following figure 3.39 shows that a mesh of wires with 1/10 
wave spacing where the wire diameters are 1/10 the center to center 
spacing the loss will be 0.25 dB. Larger wires will have a bit less 
loss, smaller wires a bit more loss in gain. Of course, wider spacing 
means more loss also.

The third consideration is F/d. The focal length to diameter ratio. The 
lower that ratio the harder to make a good feed. There have been some 
great developments in that arena for the short focal length dishes. But 
they aren't as easy as a long f/d. Short is .25 to .35, long is .6 and 
up. Think of the problem this way, a feed for a .25 F/D dish has to see 
180 degrees but cut off beyond that 180 degree beamwidth because all RF 
beyond 180 degrees misses the dish. Them because the focus is so much 
closer to the dish center than the edges, the square law from the feed 
means the edges naturally get lower power and a more optimum feed has a 
dip in energy straight back to the dish to increase the energy out to 
the sides of the reflector. Its not impossible, but it takes a more 
sophisticated feed than a simple horn to achieve maximum dish gain. 
There are many compromises possible, including underfeeding the dish for 
sometimes a considerably improved pattern (way less sidelobe energy) at 
the cost of gain but not necessarily G/T and having to support a greater 
wind load than longer F/D dish could achieve with a simpler feed.

These feed topics I'm sure are covered on Paul Wade's excelent on line 
antenna handbook pages, and probably the mesh spacing and parabolic 
error considerations as well. Paul has analyzed and reveiwed nearly all 
published feed designs with good software analysis and those results are 
on his pages.

By the way, Kirk, your computer mail clock is off 5 months so when my 
e-mail program stored by date it was hidden a few hundred messages back.

The going market prices for C band mesh dishes is hauling dish and mount 
away, around here. There may be spots with a higher value on them.

73, Jerry, K0CQ

On 5/14/2012 2:37 PM, Mike Kana wrote:
>
>
> Follow up question. I have a prototype aluminum grid dish made for 2ghz. I was considering covering with fine mesh wire. I would suspect this will work for 1.2 and 3.4?  Comments?
> 73 de mike
> Aa9il
>
> Sent from my iPhone
>
> On May 14, 2012, at 2:24 PM, Zack Widup<w9sz.zack at gmail.com>  wrote:
>
>>
>>
>> I may be wrong about this, but I think open-grid dishes are fine as long as
>> the spacing of the grid is smaller than one tenth of a wavelength at the
>> frequency to be used.
>>
>> The C-band dishes I've seen would definitely work on 3456 and bands lower
>> in frequency.
>>
>> 73, Zack W9SZ
>>
>>
>> On Fri, Jan 13, 2012 at 10:55 PM, kp hpjr<inservice2him at hotmail.com>  wrote:
>>
>>>
>>>
>>>
>>>
>>> A friend of mine has several 4, 6, 8, and 10ft grid style dishes.
>>>
>>> Does anyone know where he might be able to sell them?
>>>
>>> This may be a dumb question but are they used for any amateur bands. EME?
>>>
>>> Kirk
>>>
>>>
>>>
>>>