[time-nuts] Re: Composite sky GPS - with an added digression

[glen english LIST]

Hi Jim

Seems this question of mine is well covered in  the ether.

Yes, might not be as bad as I think. I'll write a program to compute 
various scenarios and also will be interested to see how the fixes get 
affected  with loss of signal over some AZEL patch of the sky.

The other thing- these narrowband antennas, over 24 MHz ish, I would 
suspect they are NOT phase  coherent, and that a combined pair of 
signals over identical sky would be anythign but flat frequency response.

Yes, - the LNA. some of these have 40dB gain, golly. I have some 1500 
MHz ferrite isolators, also. probably unnecessary as you say depends on 
the S12 for the LNA Â  and thus the match the antenas see into the 
splitter is likely important. Have spec-an to 50 GHz so.....

Time now I think to set up a couple of receivers and get some raw data., 
and compare with single receiver and combined antenna. Patch antennas 
generally suck I find for all the usual reasons, I am a fan of 
quadrafilar helicies.... but patches are a dime a dozen and 3what is in 
use,  so that's what I will use for the tests and report back.

-glen


>
> Assuming the feedlines are equal length, and you combine them. You'll 
> get classic grating lobes where one is exactly in or out of phase with 
> the other.  BUT.. since the pattern is already rolling off (a typical 
> patch has a 90-100 degree HPBW), both of them are fairly far down at 
> the "dividing line" equi distant, so it may not be as bad as you think.
>
> This is easy to model in a spreadsheet - Just assume the antenna has a 
> cos^2(theta) kind of pattern and then calculate the sum of the two 
> signals (including the phase difference for the angle off vertical, 
> which depends on the spacing between the antennas).
>
> There's enough other lumps and bumps in the pattern from surroundings, 
> the grating lobes might be in the same general magnitude.   Or just 
> try it.
>
> One thing to watch out for: if you're using antennas with built in 
> LNAs - it's easy to build an oscillator - the amplified output of 
> antenna 1 goes through the splitter and comes out the port to antenna 
> 2 and if the reverse isolation of the LNA isn't good enough, it 
> radiates back to antenna 1.  Fortunately, the antennas are narrow 
> band, so it might not happen to oscillate.  But if you get weird 
> results, try covering one of the antennas with aluminum foil (or 
> absorber, if you have some).
>
> And these days, you can get a spectrum analyzer that goes up to 2GHz 
> for about $100Â  - it's not awesome, but it sure would see an 
> oscillation at GPS frequencies, and would prevent all the GPS users 
> around you from hunting you down like a rogue wolf.
>