Andy:
So it could be a real signal, or an artifact of the measurement system. My
initial guess is that it will be the measurement system, so let’s talk about
what could go wrong there.
1. You said you could see it for an antenna transducer only; it’s not there
when you connect a current probe (or a termination) to the coax inside the
chamber. So that let’s off things like:
a. A real local noise source.
b. Coupling from something like your computer monitor or printer.
c. A parasitic oscillation in your test system.
2. You said you could see it with either active or passive antennas. This
works against the active antenna’s pre-amplifier having a spurious
oscillation, but it also raises a troubling idea. Since the antenna factor of
a passive rod at 12 kHz is maybe 95 dB, any real E-field signal visible with
that passive rod would be huge, probably large enough to overload an active
antenna!
a. Have you tried connecting a small loop antenna in place of either rod, and
do you then see a signal?
b. I wonder what would happen if you connected either rod antenna to your
coax, observed that signal, and the inserted a 3 dB attenuator on the antenna
output?
i. Emission still there?
ii. Is it down exactly 3 dB?
iii. Is this related to the impedance
seen by the receiver?
c. I wonder what you would see if you didn’t have a rod on either antenna?
Try putting on a 50-ohm termination or a short.
d. At last resort, I wonder what you would see if you connected the same
length and type of coax from the receiver to the antenna external to your
chamber?
3. I use an AH System AHS 200-551 active rod antenna with an external PC
controlled HP-8572 pre-selected Receiver System, and I don’t see any
“emissions” in the range of your problem.
Moving back again to what you can see, you should be using an RBW of 100 Hz in
that range. What happens when you try to view the “emission” in a narrower
bandwidth or with a different sweep speed? I’m assuming that you see a
narrowband “emission”, but then you did say that the “excess emission”
is 10 kHz to 13.5 kHz. Did you mean that that whole range is full of noise or
many little peaks? Can you see any modulation of this “signal”?
I keep coming back to your seeing this emission with both active & passive
antennas. I think that observation is a key to the problem.
Ed Price
[email protected] <blocked::mailto:[email protected]> WB6WSN
NARTE Certified EMC Engineer
Electromagnetic Compatibility Lab
Cubic Defense Applications
San Diego, CA USA
858-505-2780
Military & Avionics EMC Is Our Specialty
From: [email protected] [mailto:[email protected]] On Behalf Of Price, Andrew
(SELEX GALILEO, UK)
Sent: Monday, November 09, 2009 7:57 AM
To: [email protected]
Subject: Excess Emission!!!!!
Hi everyone,
I am wondering if anyone in the forum has come across the following problem:
A 10kHz to 13.5kHz excess emission (to both the DEF STAN 59-411 Air and
MIL-STD-461F Air external specs) when using a Rod antenna. This excess
emission occurs with both active and passive rod antennas.
The chamber has been checked to ensure that all supplies are off, lights off,
no cables going in or out of the chamber with the exception of the measureing
cable connected to the receivers (R&S ESIB 7 & ESU26).
The measureing cable has been connected via a through bulk head connector in
the chamber wall (bonded) and isolated connector with no difference observed.
This problem has only recently come to light as previous measurements to DEF
STAN 59-41 and MIL-STD461A-C issues lowest frequency 14kHz and testing to
MIL-STDs Issues D to F there has been so far no requirements to measure down
to 10kHz.
There is no problem with conducted measurements to either MIL-STD or DEF STAN
at this frequency range.
Can anyone shed a light on what may be the cause of this problem?
Regards
Andy
Andrew P. Price
Principle Hardware Engineer, EMC Specialist
SELEX GALILEO, Sensors & Airborne Systems
Christopher Martin Road
Basildon
Essex SS14 3EL
Mail Ref : K160
( Tel Direct : +44 (0)1268 887271
( Tel EMC LAB : +44 (0)1268 883308
) Mobile : +44 (0)7507 854888
P Please consider the environment before printing this email.
SELEX Sensors and Airborne Systems Limited
Registered Office: Sigma House, Christopher Martin Road, Basildon, Essex SS14
3EL
A company registered in England & Wales. Company no. 02426132
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