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Home based EMC measurements

J

Joe.G

Jan 1, 1970
0
Hi All,

I have a spec analyser and I looking for suggestions
on antennas and comb generators within an afordable budget..

I have seen some scanner antenna's that are log periodic that have a wide
bandwidth.

The main goal is to get some idea if my product is
radiatating somewhere near the EMC limits or not.

Thanks in advance.

Joseph
 
I

Ingo Cyliax

Jan 1, 1970
0
Hi All,

I have a spec analyser and I looking for suggestions
on antennas and comb generators within an afordable budget..

I have seen some scanner antenna's that are log periodic that have a wide
bandwidth.

The main goal is to get some idea if my product is
radiatating somewhere near the EMC limits or not.

Thanks in advance.

One problem is finding an environment (chamber/freespace) that is
free from other sources of RF so you can be certain if it's your
equipment that is emitting or not. I.e. you may have to build a
Farraday cage.

Later, -ingo
 
R

Robert Baer

Jan 1, 1970
0
Joe.G said:
Hi All,

I have a spec analyser and I looking for suggestions
on antennas and comb generators within an afordable budget..

I have seen some scanner antenna's that are log periodic that have a wide
bandwidth.

The main goal is to get some idea if my product is
radiatating somewhere near the EMC limits or not.

Thanks in advance.

Joseph
The antennas need a calibration curve (sensitivity VS frequency) to
be useful.
 
P

Pooh Bear

Jan 1, 1970
0
Ingo said:
One problem is finding an environment (chamber/freespace) that is
free from other sources of RF so you can be certain if it's your
equipment that is emitting or not. I.e. you may have to build a
Farraday cage.

The simple answer is to listen to the output of the receiver and switch the
EUT off and on - that'll determine where the emission is coming from.


Graham
 
A

Andy

Jan 1, 1970
0
Joe.G said:
Hi All,

I have a spec analyser and I looking for suggestions
on antennas and comb generators within an afordable budget..

I have seen some scanner antenna's that are log periodic that have a wide
bandwidth.

The main goal is to get some idea if my product is
radiatating somewhere near the EMC limits or not.

Thanks in advance.

Joseph

Andy writes:


For frequencies that can be built with a patch antenna, the
following procedure can be done:

1) Connect a signal generator to a patch antenna (for each frequency
to be calibrated, which points UPWARD to the cosmos.

2) Suspend a dippole antenna 20 or 30 wavelengths above the patch.(
Not
a problem above , say, 500 mhz).... (minimal reflections this way)

3) The antenna should be a SMALL dipole, much shorter than a quarter
wave at the lowest freq.

4) Develop a calibration curve for this antenna.


5) When making any measurements , then, at these frequencies, you
must allow
for reflections, depending on your measurement facility. This
procedure
is well established in literature for "open field antenna
measurements".


To simplify this summary:

It is not possible to do this in a simple manner. Antenna field
measurements are extremely complicated by reflections, absorption, and
inaccuracy in the transmitted field. If you buy a CALIBRATED field
strength meter, and attempt to make measurements, you may find up to
20-30 db field strength difference base on 2 or 3 feet of field
strength
meter movement...... this is a bear....

If you have a calibrated large room lined with 400 Mhz ECCOSORB, you
would have not asked ( big grin).

At lower frequencies ... well, you just do the best you can. The
near field curvature will kill you and reflections may bury you....

If you just want to build a ball park value, just put a signal
generator
into a dipole, and mount the pickup antenn 3-4 wavelengths away, and
raise
and lower it a half wavelength or so to see the range in values. This
ain't bad for ball park figures.....


Sorry,.... but that is why EMC engineering is such a definite
specialty.....

Andy , retired NARTE Licensed EMC engineer
 
J

John Larkin

Jan 1, 1970
0
Hi All,

I have a spec analyser and I looking for suggestions
on antennas and comb generators within an afordable budget..

I have seen some scanner antenna's that are log periodic that have a wide
bandwidth.

The main goal is to get some idea if my product is
radiatating somewhere near the EMC limits or not.

Thanks in advance.

Joseph

These guys have some neat looking stuff:

http://www.electrosmog.com/



John
 
P

Paul Mathews

Jan 1, 1970
0
Joe.G said:
Hi All,

I have a spec analyser and I looking for suggestions
on antennas and comb generators within an afordable budget..

I have seen some scanner antenna's that are log periodic that have a wide
bandwidth.

The main goal is to get some idea if my product is
radiatating somewhere near the EMC limits or not.

Thanks in advance.

Joseph

It's not at all easy to calibrate an open area test site, and shielded
enclosures are much more difficult to calibrate, since standing waves
are inevitable unless you have the big bucks to make them
semi-anechoic. However, relative measurements can be very useful,
especially after you have some data from a calibrated measurement site.
If you're located favorably (relatively quiet background spectrum), you
might be able to get some useful relative measurements without really
getting calibrated. Highly directional antennas can help (aim them
away from strong background sources and toward the EUT), and you'll
need a 20dB preamp with most spectrum analyzers. If you decide to
build a Faraday enclosure, there are lots of ways to make them
inexpensively. How you make the joints is more important that what
metallic material you use. Some people use drywall coated with foil.
We bought an old steel ocean cargo container and put some effort into
making all its joints electrically connected.
Paul Mathews
 
J

Joe.G

Jan 1, 1970
0
What if a get a Comb generator calibtated at a OATs test site.

Then I can do relative measurements regardless of antenna cable loss etc.

Meansure the comb freq / level and compare to the OATs test site results.

Provided the RF comb frequencies are up near the limits.

Then I can know weather to worry about passing the limits by a large safe
margin etc.

Joseph
 
K

Kryten

Jan 1, 1970
0
Some colleagues gave me a good tip.

They can do EMC testing with proper kit, but before you submit stuff for
expensive testing you can detect most of the gross RF leaks just by waving a
cheap AM radio around it.

Passing that test saves a wasted day of expensive EMC testing.
 
P

Paul Mathews

Jan 1, 1970
0
Comb generator approach is OK to the extent that you approximate an
open area test site. Faraday enclosures can have quite strong
resonances between comb peaks. Noise generators are the preferred
signal source, since standing waves aren't such a problem.
Paul Mathews
 
J

Joe.G

Jan 1, 1970
0
Hi Paul,
Are the suich things as wide band noise generators 10kHz to 1GHz for EMC
testing.

I hadn't seen andy around.

Any pointers to wide band noise genators would be appreciated (kits /
project and pro gear).

Regards
Joseph
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Joe.G <[email protected]>
wrote (in said:
Are the suich things as wide band noise generators 10kHz to 1GHz for
EMC testing.

I hadn't seen andy around.

Any pointers to wide band noise genators would be appreciated (kits /
project and pro gear).

ARRL or RSGB Handbook (if possible, both). Look for 'noise diodes'.
 
P

Paul Mathews

Jan 1, 1970
0
Expect to pay between $50 and $500 for a good broadband noise source on
ebay. Quite often, the seller doesn't have specs, and it's buyer
beware, but there are excellent values to be had. An ebay search on
'noise source' or 'noise generator' will turn up some possibilities:



Paul Mathews
 
T

Terry Given

Jan 1, 1970
0
Ingo said:
One problem is finding an environment (chamber/freespace) that is
free from other sources of RF so you can be certain if it's your
equipment that is emitting or not. I.e. you may have to build a
Farraday cage.

Later, -ingo

A 20' container works nicely. Reefers are better, they are a lot warmer
in winter.

Cheers
Terry
 
T

Terry Given

Jan 1, 1970
0
Kryten said:
Some colleagues gave me a good tip.

They can do EMC testing with proper kit, but before you submit stuff for
expensive testing you can detect most of the gross RF leaks just by waving a
cheap AM radio around it.

Passing that test saves a wasted day of expensive EMC testing.

Bruce Carsten's EMI sniffer probe is good there, too. Use with a tunable
receiver, can be very cheap.

I have a mate who's company has an EMC test setup (alas no anechoic
chamber) and thus have 4 hacks at it:

1) design for minimum EMI
2) sniffer probe tests.
3) use their test-setup, only $100/hr
4) then pay big bucks for the real deal.

Cheers
Terry
 
P

Paul Burke

Jan 1, 1970
0
Terry said:
A 20' container works nicely. Reefers are better, they are a lot warmer
in winter.

I find hot toddies warmer in winter, as the reefer doesn't really give
out much heat.
 
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