[amsat-bb] Re: 2 ghz antenna question

Chris Huber cahuber at ucdavis.edu
Wed Aug 8 22:31:23 PDT 2007


Thanks for the information. The antennas would be stacked on the same 
mast and would be easy to put the edges next to each other.I was not 
aware of interferometer and its effects. Thanks for the heads up. That 
would have been frustrating to figure out.

Chris N6ICW

Scott Townley wrote:
> Chris,
> It depends... :-)
> Wide spacing (presumably to gain path diversity) only is effective 
> post-detector...meaning, you need two RX paths and you "choose" the 
> best demodulated path.  If you wide-space at RF (using your power 
> divider) you get an interferometer which is most definitely what you 
> *don't* want.
> Although IIRC in commercial installations the diversity antenna can be 
> RF-combined with the main antenna through a 10dB coupler (with the 
> diversity being -10dB).  That keeps the interferometer effects down 
> while limiting the path fades to near 10dB max...but I'm working from 
> memory on that one.
> For arraying the dishes ("close spacing") you should put them nearly 
> edge-to-edge (no more than about 6 or 7 inches apart at the rims).  
> There are several mechanical issues however:  first, the dishes must 
> be co-aligned very well (a 3' dish at 2GHz will have a beamwidth of 
> around 5 degrees, so the two antennas have to point in azimuth *and* 
> elevation within 2.5deg of each other, or you will have gained 
> nothing).  Second, the antennas together will need to be pointed twice 
> as accurately as before.  And stay that way in the wind.
> At 16:56 2007-08-08, you wrote:
>> Hello,
>> I know this is off subject, I am looking for ideas on stacking 2 
>> antennas.
>> I am doing a 25 mile WiFi link on 2 GHz using 1  3 foot grid dish with
>> horizontal polarity. I have a power divider and a second dish that I
>> want to mount directly above the first antenna.
>> Is the vertical spacing critical?  Is there any advantage over close
>> spacing,  ie. increase gain or wide spacing to give better path fade
>> protection.
>> Thanks for your time.
>> Chris N6ICW
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> Scott Townley NX7U
> Gilbert, AZ  DM43di
> http://members.cox.net/nx7u

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