Hi,
I have not used a carousel and would not do so without a lot of data to show
that it dose not affect antenna behavior. It has been shown that the terminal
impedance of an antenna changes as it is moved up from 1m above the ground
plane. Similarly, at 3m, it is possible for the presence of a table top
product with a power cord dropping to the ground plane to affect the antenna
impedance. So anything close(r) to my antenna would be a problem for me.
Remember the goal of any measurement technique is that the measurement should
not affect the data and for us the object be measured should not re-calibrate
the measurement device (antenna).
The effects described above are only 1 or 2 dB and only at low or specific
frequencies. However a closer spacing would increase this. Some studies I
have done recently looked at bringing a cable close to the apertures in a
shield showed emission increases of up to 12db - now this was a very close
spacing 10 cm or so, but the coupling mechanism is there.
In CISPR 25 you refer to the spacing from the walls of the ANECHOIC chamber.
These walls are nominally non reflecting, an antenna is reflective and will
have some very frequency dependent characteristics that will change with
location (height). So I believe the coupling would be hard to predict or
measure but would add "a few dB" to the uncertainty of your measured data.
This would be mostly a low frequency problem where one of the antennas is not
very directional. Back to back logs might be fine, but with a log bicon pair
the bicon would se the log as a parasitic element (two element, poorly designed
yagi).
For a quick preliminary scan this might be a perfect solution but for high
accuracy measurements I doubt it.
My 2c,
Colin..
Et Al:
I am soliciting opinions from anyone who has attempted using an antenna
carousel that aids in doing automated EMI measurements under CISPR 25.
The CISPR 25 specification provides requirements in terms of spacings of
the antenna elements from the walls of the room, size of the room, use
of anechoic material, etc but not pertaining to adjacent antennas. In
addition, certain automotive companies place further requirements in
terms of expected performance relative to Round Robin results of a Test
Artifact taken at several labs before allowing a new lab to be listed as
AEMCLRP accredited.
I look forward to hearing from those involved in the Automotive Industry
who have information either good/bad relating to this issue.
Regards,
Jeffrey W. Greilich
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