[time-nuts] Difference in GPS antennas

[Magnus Danielson]

Joseph M Gwinn wrote:
> Bob,
> 
> 
> time-nuts-bounces at febo.com wrote on 08/14/2009 01:05:34 PM:
> 
>> From:
>>
>> Robert Darlington <rdarlington at gmail.com>
>>
>> To:
>>
>> Discussion of precise time and frequency measurement 
> <time-nuts at febo.com>
>> Date:
>>
>> 08/14/2009 02:06 PM
>>
>> Subject:
>>
>> Re: [time-nuts] Difference in GPS antennas
>>
>> Sent by:
>>
>> time-nuts-bounces at febo.com
>>
>> Hrm, I'm looking for a sheet of Eccosorb to make an anechoic chamber (a 
> very
>> small one) for testing 2.4GHz printed (PCB) antennas.  I'd be interested 
> in
>> knowing the specs.   The idea with what I'm doing is that I cansweep the
>> antenna with the network analyzer and still be near the antenna to keep 
> the
>> cables short, while at the same time not interfere with the 
> measurements.  I
>> met a guy from Microchip last week that showed me what he was doing and 
> what
>> works well for him.  He built a box about 8" high and about 5 inch 
> square on
>> the bottom (inside dimensions).  Even though it's pretty parallel on the
>> inside, reflection is a minimum because of the material.
> 
> The main advice I would give is to make sure the box isn't perfectly 
> square or rectangular, so standing waves cannot form.
> 
> So, build it sloppy, with walls visibly askew (not perpendicular to one 
> another).

That's how my studio is built. Part of it was already there, and the 
walls I put up just happend to become skewed. Even floor-cieling 
distance differs significantly. The bare room had no signficant 
resonance and now stuffed with damping material it has no longer intense 
  reflection either, so it is really pleasent. Need to do a propper 
sweep thought.

So I agree totally with this recommendation!

Also recall that near-field responses of an antenna can fool you if you 
want to know something about the far-field response. So you do want to 
be at a sufficient distance from the antenna for the worst of the near 
field response to have started even out to the far-field response. I 
have done alot of speaker measurements, so I am well aware of the 
problems involved in near/far field context. Antenna stuff is similar 
but different.

Cheers,
Magnus