Joseph M Gwinn wrote:
Bob,
[email protected] wrote on 08/14/2009 01:05:34 PM:
From:
Robert Darlington <[email protected]>
To:
Discussion of precise time and frequency measurement
<[email protected]>
Date:
08/14/2009 02:06 PM
Subject:
Re: [time-nuts] Difference in GPS antennas
Sent by:
[email protected]
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
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