Re: [amsat-bb] BBQ Dish Sanity???

[Anthony Monteiro <aa2tx@xxxxxxxxxxx>]

Hi Guys,

More comments as follows:

 From Jerry, K5OE:
Regarding the observation of less than "calculated 24 dBi gain" of these 
MMDS dishes, I did a geometric analysis of these dishes a couple of years 
ago for a presentation at amsat-na (Atlanta). An excerpt from that report 
follows:
...
With all that said, nothing is implied here that 21 or 22 dBi is not enough 
gain to work AO-40 under good conditions. The 18" offset dish I took to 
amsat-uk last week only tested out to have 17 dBi gain (I predicted 18), 
yet I have had both CW and SSB QSO's with it sitting in the bed of my truck.


At 09:59 AM 8/8/03 -0400, BobsImsai8800@aol.com wrote:

>Jerry and Tony,
>
>A lot of grid dish complaints are probably due to lack of "local repeater 
>full quieting" level signals.  I think that the people who are used to 
>chasing DX think that it works better.  I always recommend a bigger dish 
>or for those that can't, a ClearSpeech speaker to give the necessary 
>improvement in gain -- the speaker can be used in other applications also.
>
>Your numbers may be close, check out the CSVHF results in 2001 using a 2 x 
>3' dish and a 1.5 by 2' dish with almost exactly one half the 
>area.  Jerry's screened dish is there also.
>
><http://www.csvhfs.org/CSANT01.HTML>http://www.csvhfs.org/CSANT01.HTML


---------------------------------------

Hi Jerry, Bob, and the group,

Thanks for the reminders. I would never say that you couldn't work AO-40
with only 17 dB to 20 dB of gain. I set up our field day station with
Cardboard boxes that are only about 17 dBi (net - after RHCP) and we made
lots of contacts pretty easily. (for pics see arrl.org: NS1RA - Satellite ST)

It is very interesting that the grid dish at CSVHF measured 20.8 dBi
versus my 20.5 dBi. I should note that I am only using the
stock feed rather than the patch feed used at CSVHF, nonetheless,
these results are very similar.

With Cliff's prompting, I checked the feed location and asked
Jerry,  K5OE to check my work (Thank you Jerry!) I found that
the feed on my dish appears to be off focus by almost 6 inches.
Here is the really interesting part, on W1GHZ's web site, he
has a nice chart showing loss versus focus error. It indicates
the loss for 6" focus error at 2.4 GHz  would be around 3-4 dB
which is really close to the difference between the spec and
what I measured.

I am wondering if I have the wrong feed postioning arm for this dish?

Anyway, I intend to make a new arm and re-run the tests.
I will let you know if anything interesting comes up.

Please note that I am not ignoring the G/T issues. As Lee
indicated, the loss from the pigtail makes a pretty
bad dent in the G/T. I just haven't gotten around to
addressing these yet since the gain is so far down from
what it should be.

Perhaps a proper feed could really make a big difference in
both gain (efficiency) and G/T performance?

Thanks everyone and 73,

Tony AA2TX









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