I know this is not exactly the group to post in, but I thought
radio amateurs would have the best ideas for making one's own
antenna. I apologize if this question bothers anybody.

I just spent $800 at an EMC test lab to screen my device for
part 15 compliance. The used a Biconical antenna and a spectrum
analyzer to measure emissions from my device. I was wondering
how inexpensively I could I make my own system for screening?

How hard would it be to make half way decent antenna or antennas that
could listen to a broad spectrum?

Any websites out there have tips on making antennas?

Thanks a lot for your help.
-Mike Dorin

Hi Mike

I would suggest that the problem isnt so much in finding a good antenna
but calibrating the entire system so that you can measure the absolute
signal strength.

That may also involve a screened room and something to dampen the
reflections inside it. Depends on the required accuracy.

But to answer your question, any broadband design would work. It would
be smart though to use something as low gain as possible to avoid
pointing errors. For your purposes it might be possible to use the one
piece of equipment you had measured, take its actual radiated signal and
use that as a relative measure against new equipment.

Lots of websites! My favourite is http://www.cebik.com/radio.html


Cheers Bob VK2YQA

Mike wrote:
I know this is not exactly the group to post in, but I thought
radio amateurs would have the best ideas for making one's own
antenna. I apologize if this question bothers anybody.

I just spent $800 at an EMC test lab to screen my device for
part 15 compliance. The used a Biconical antenna and a spectrum
analyzer to measure emissions from my device. I was wondering
how inexpensively I could I make my own system for screening?

How hard would it be to make half way decent antenna or antennas that
could listen to a broad spectrum?

Any websites out there have tips on making antennas?

Thanks a lot for your help.
-Mike Dorin

On 21 Nov 2004 05:45:35 -0800, (Mike) wrote:
How hard would it be to make half way decent antenna or antennas that
could listen to a broad spectrum?

Hi Mike,

That would be the cheapest part of your quest and it wouldn't come
close to a start. The investment is in the shielded room and its
equipment.

73's
Richard Clark, KB7QHC

I do this at home, but I don't try to make absolute measurements.
You can easily use a dipole, just as the lab will do when they want an
accurate look at a specific spike. You'll need to do this outdoors, away
from emitters. For me that's a practical impossibility, as I'd have to cart
the equipment quite a ways to get to a quiet site.

What I did, is to build a shield enclosure from double-sided PCB material,
soldered together with brass reinforcements on the corners, and finger stock
to seal the lid. I use a small pickup loop to feed my spectrum analyzer, or
IC-R8500 receiver.

Use a small microwave tray to set your pcb up off the copper!

By itself, this will give you unreproducible and very frustrating results.
The next thing you need is some sheets of 3M Mosfoam. The stuff they ship
chips in.
Black, squishy, carbon loaded foam. This stuff absorbs RF quite nicely, and
you get double effect because what dosen't absorb is reflected by the copper
for another pass through the foam..
Line all the walls with this, and you'll be able to make repeatable relative
measurements.

Now, just find your highest spike, and kill it, repeat till it gets
difficult.

You can find some notes on this at www.dvanhorn.org

Mike:

If you're looking for absolute information, like the report the EMC test
lab produced, you could spend from $50,000 for the "basics" for an open
site (outdoors, away from large signal sources, hopefully nearby or where
you live), not counting such things as a turntable for the Device Under
Test (DUT), on to a Million or more for a fully shielded, equipped
"Screen Room". 'Been there, done that, (both kinds). Keeping everything
calibrated so the results are accepted by FCC and others can be somewhat
spendy and time consuming too.

(By the way, I'd have really loved a bill as low as $800 for scanning a
device for Part 15 when I was doing it.. preliminary scans often cost
well over that!).

On the other hand, if you want to "get an idea" of what your device is
doing, or to debug it after you have a scan in hand, you can surely get
by with nearly any broadband antenna for the frequency range of choice
and a fairly sensitive spectrum analyzer. These aren't the hard parts.
The hard part is the time you're going to invest to scan your local
environment, so you can document the "local emitters" so you know whether
the emission is from your device or is ambient noise in your
environment. Of course, you may also need to do some of your scans "off
hours" (like 3am) when the local emitter isn't broadcasting so you can
make sure your device doesn't just happen to emit at the same
frequency... (I spent countless hours doing this as well. If most
people knew what it took to get a new device accepted and into
manufacturing, perhaps they wouldn't balk at the price as much.)

If you have access to a remote, open site (the ideal is somewhere that is
accessible without 4 wheel drive, in a small box canyon or valley, nearly
or completely ringed by mountains, and as far away from civilization as
possible, to limit the number of nearby emitters... Of course, in this
day and age, these are pretty hard to find, and I've known people to take
an old airplane hanger and screen it with appropriate sized copper mesh,
bonded together to make a very large faraday cage... (One place actually
used copper window screen, soldering the edges together. Must have cost
them a fortune, but it was pretty quiet, considering it was smack dab in
the middle of one of the most RF polluted places on earth, (Silicon
Valley) ).

Half way decent antennas that have pretty wide range aren't that tough to
find. Think log periodic and you have a good start... Of course, they
won't be calibrated like the ones the EMC lab has, but you can get a
really good idea of what's going on, and later, you can "borrow" the
antennas and use them for Ham use (temporarily, of course). At one open
site, we used three different, log periodics of overlapping ranges, and
all worked pretty well. Of course, to get into the HF region, we had to
put the antenna quite high from the ground so we could go to vertical
polarization, which also meant the DUT had to be on a platform at equal
height. Think about making an extremely well sealed, wood tower about 20
feet tall, with a turntable for rotating the DUT, all without any metal
fasteners! Even the turntable was made without metal (billiard balls make
really good non-conductive bearings if you do it right) with the rotator
motor down at ground level. All in all, it would have been a lot more
fun if there hadn't been a schedule deadline looming... Oh yeah, you
should also be able to change the height of the receiving/sampling
antenna so you can look for emission off of the horizontal plane of the
turntable...

As I remember it, Create had a log periodic that covered something like
50 Mhz to 1.3 Ghz, and EMC test systems had one that does something like
80 Mhz to 2 Ghz... A quick search on the web shows that KMA makes one
that covers 26 Mhz to 1.3 Ghz, and another that covers 13 Mhz to 30 Mhz,
so it can still be done.

Good Luck
--Rick AH7H

Mike wrote:

I know this is not exactly the group to post in, but I thought
radio amateurs would have the best ideas for making one's own
antenna. I apologize if this question bothers anybody.

I just spent $800 at an EMC test lab to screen my device for
part 15 compliance. The used a Biconical antenna and a spectrum
analyzer to measure emissions from my device. I was wondering
how inexpensively I could I make my own system for screening?

How hard would it be to make half way decent antenna or antennas that
could listen to a broad spectrum?

Any websites out there have tips on making antennas?

Thanks a lot for your help.
-Mike Dorin

Dave VanHorn wrote:


Now, just find your highest spike, and kill it, repeat till it gets
difficult.


For hunting for the exact spots where the DUT is leaking
crud, try using a split ferrite toroid. See at my page:
http://www.geocities.com/wa2ise/radios/ham.htm

"Mike" wrote in message
om...
I know this is not exactly the group to post in, but I thought
radio amateurs would have the best ideas for making one's own
antenna. I apologize if this question bothers anybody.

I just spent $800 at an EMC test lab to screen my device for
part 15 compliance. The used a Biconical antenna and a spectrum
analyzer to measure emissions from my device. I was wondering
how inexpensively I could I make my own system for screening?

How hard would it be to make half way decent antenna or antennas that
could listen to a broad spectrum?

Any websites out there have tips on making antennas?

Thanks a lot for your help.
-Mike Dorin


$800 is about 1/2 day of test time at a typical EMC lab. If you got a
conducted emission test, a radiated emission pre-scan in a shielded
enclosure, a diagnosis of the failure, amelioration of the problem, and a
radiated emission test on an OATS, all for $800, I suggest you buy a lottery
ticket tomorrow, cause you're on a roll!

Ed
wb6wsn

On Mon, 22 Nov 2004 00:42:57 -0800, "Ed Price"
wrote:


"Mike" wrote in message
. com...
I know this is not exactly the group to post in, but I thought
radio amateurs would have the best ideas for making one's own
antenna. I apologize if this question bothers anybody.

I just spent $800 at an EMC test lab to screen my device for
part 15 compliance. The used a Biconical antenna and a spectrum
analyzer to measure emissions from my device. I was wondering
how inexpensively I could I make my own system for screening?

How hard would it be to make half way decent antenna or antennas that
could listen to a broad spectrum?

Any websites out there have tips on making antennas?

Thanks a lot for your help.
-Mike Dorin


$800 is about 1/2 day of test time at a typical EMC lab. If you got a
conducted emission test, a radiated emission pre-scan in a shielded
enclosure, a diagnosis of the failure, amelioration of the problem, and a
radiated emission test on an OATS, all for $800, I suggest you buy a lottery
ticket tomorrow, cause you're on a roll!

Ed
wb6wsn

Our last test run cost us US$2200. That did not include the
preliminary scans, nor the consulting time to correct the problem
uncovered in them. Just so we could ship three units into Canada.

Bob McConnell
N2SPP