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Re: ao-40 dish :)



Scott,

Its really too small for 23cm let alone 70cm...remember the "rule-of
-thumb":  a dish needs to be >8 wavelengths in diameter for it to work
right (i.e like a dish vs a dented pizza pan):
13cm (2400) 8-WL = 104cm (41-inches)
23cm (1268) 8-WL = 184cm (6-foot)
70cm (435)  8-WL = 560cm (18-foot)

OK, I know many are using 60cm even 45cm dishes on AO-40 s-band downlink,
but as the dish get small in regard to wavelength the pattern gets
"lumpy"...i.e. exhibits large sidelobes...and if those look at the warm
ground they degrade your noise figure terribly!  Of course, this may not be
so bad in an uplink antenna (but not especially nice).

Next to look at is what kind of gain will you get (disregarding the above)?
 Here is the gain* for a 80cm, 4-foot, 7.5-foot, and 10-foot dish:
80cm:            G(2400) = 24.5 dBi, G(1268) = 19 dBi, G(435) = 9.7 dBi
4-foot (1.2m):  G(2400) = 28 dBi, G(1268) = 22 dBi, G(435) = 13 dBi 
7.5-foot (1.8m): G(2400) = 33.6 dBi, G(1268) = 28 dBi, G(435) =18.8 dBi
10-foot (3.0m): G(2400) =36 dBi, G(1268) = 30.6 dBi, G(435) = 21 dBi

*Note all above assumes a dish efficiency of 0.55 using the gain formula:
G(dBi) = 10*Log [4*pi*eff*(diam)^2/(wavelength)^2]

So even if your 80cm dish would work on 435 MHz, you would not get more
gain than a 3-element yagi (an Arrow would do better).   A 7.5-foot dish is
about equal to the big M2 satellite antenna (436CP42).  On 1268 using a
4-foot dish you do about the same as a 55-element loop-yagi!  Now a 7.5 or
10-foot dish would "shine" on mode-LS and do OK on mode-US.

This is theoretical, now if someone has a working antenna that disputes
this...lets hear about it (on AO-40, preferrably)!  This last comment I
borrow from an old microwaver friend, who hearing all the discussions, grew
weary of it all and said:  "OK meet up on xxx.xxx GHz and we'll talk about
it" 

Ed - AL7EB

so at 70cm your 80cm dish is about twice the size of an ordinary yagi
reflector element in length!  You will be greatly disappointed if you try
this!


At 03:31 PM 9/3/02 +0100, Scott Elliott wrote:
>while we are on the subject of dishes for ao-40.
>
>is it possible to put a duel band setup on a 80 cm dish?
>
>70cms and 2.4ghz. i know i would be able to do with 23cms, but im not sure
>about 70.
>
>thanks for any ideas :)
>
>Speak to you later.
>
>Scott Elliott
>M3SJE / 2E1FJB / M1SJE
>
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