By G8MNY (Updated Nov 19)
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Wanting to find which lead on a devices like TV & STB set up, has the most RFI current in it, I made a simple current probe from a clip in ferrite choke. It can be used with a Rx or spectrum analyser. And for Tx RF work too, finding if balanced line is balanced, coax line is really RF free etc.

THE CURRENT PICK UP
/~~~~\
QRM cable Hinge / /~~\ \ Locking
================= ùÃÄ´ ÃÄ´û Catch
:::::-¿screen Rx_________\ \__/ /
10 ())))_³____ Coax~~~~~~~~\\±±±±/
turnsÀÄÄÄÄÄÄÄÄÄÄ50ê Coax Heat Glue^ Coil

The clip on ferrite is about 2cm dia with a single locking catch, the catch guard I cut off making it easy to unclip. I removed 1 of the ferrite halves from the hinged plastic casing by levering a plastic protrusion out of the way. Then using some very fine enamelled wire I wound 10 turns on it, made a hole in the case, passed the wire ends through it, & then put back the ferrite against the hidden spring inside the case until locked in place. (depends on make)

I glued a thin coax to the case & connected the coil, after DC & RF testing, I soldered some aluminium foil (or copper foil if you have no aluminium solder) to the coax outer & heat glue a narrow strip of it through the centre hole shielding the coil 1/2 a turn from the QRM cable, do not short the search coil with a full turn of screening! Then I put a layer of thin tape (parcel tape) over this to protect the coil.

I have seen other design use a cloths peg to house the 2 ferrite cores.

10 turns was used to give some calibration, & not put on too much wire that the frequency response will be affected. With 50ê load only 0.5ê is effectively inserted in QRM line.

0.5ê 1T
1A============
:::::10T
())))_____100mA
³_________50 ohm = 0.5W = 5V = 74dBmV = 134dBuV


WHERE TO TEST
The clip probe will show the highest current point along a cable & the worst cable in a set up (where a ferrite choke core will be most effective!)

Here is a typical TV set up...

SCART SCART
ÚÄÄÄÄÄÄÄÄÄÄÄ¿ RGB ÚÄÄÄÄÄÄÄÄÄ¿ RGB ÚÄÄÄÄÄÄÄÄÄÄ¿ \|/UHF
³ TV ÃðÄÄÄÄÄÄÄð´ D.V.D. ÃðÄÄÄÄÄð´ S.T.B. ÃðÄÄÄÄÄÙAerial
³ DISPLAY ÃðÄ¿ ÀÄÄÄÄÄÄÄÂÄÙ ÀÄÂÄÄÄÄÄÄÂÄÙ
ÀÄÄÄÄÄÄÄÄÂÄÄÙ ³ ÚÄÄÄÄÄÄÄÄ¿ ÀðÄMains ³UHF DC³ ÚÄÄÄÄ¿
MainsÄÄÄÄðÙ ÀÄð´ V.C.R. ÃðÄÄÄÄÄÄÄÄÄÄÄÄðÙ ÀðÄÄÄÄÄÄÄð´Plugà Mains
ÀÄÄÂÄÄÄÄÄÙ ³PSU ³
MainsÄÄÄÄðÙ ðtest points ~~~~

The clip probe will show which of these 14 places may the best place for a choke. And you can even check an added choke is having some effect..

QRM Test
==========ððð=ð====
SOURCE choke³
ÀÄÄRx

As single clip on choke is not very effective (5dB!). More of them or a few turns on one is much better. You will also soon know if you have an AF "steel tape core" & not an RF ferrite core as it will have little effect at RF!

SPECTRUM
This is typical..

CRT TV SMPSU PC
³ ³º| ³ ³ ³
³± ³ººº| ³ ³ ³ ³ ³
³±± ³ººººº| ³ ³ ³ ³ ³ ³±±±==.._ݳ______±_ ³ººººººº|:. ³ ³ ³ ³ ³
15kHz 4,4 6 38MHz ÀÐÐÐÐÐÐÐÐÐÐÐÐÐÐÁÄÄÄÄ ÀÄÄÁÄÄÁÄÄÁÄÄÄÁÄ
lines 45kHz 50Hz FM hummy lines Clock lines

Now as most things have SMPSU & PC architecture in them, so combinations occur.

Even a USB keyboards show up like this...
³
³ ººº ºººº
³ ºººººººººººººº
³ ºººººººººººººººº
ÀÄÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÐÄÄ
5 10 15 20 25 MHz

TIPS
Some devices may put out a lot of RF current on some leads & not on others!

Not all these currents lead to radiation that affects ham bands!

It often depends on the wiring & wire aerials at that location. E.g. a few uA of current into a 1/4 wave long TV aerial lead, may cause havoc, but into a small loop of wires around the TV none at all.

UHF \³/ 10m
1/4 WAVE TV ³ SMALL
VERTICAL ANT ³ 37uW EQUIPMENT 0pW
AERIAL ³ ERP on O R LOOP ANT ERP on
³ 40m __ 40m but
³ _³ ³ high
QRMÄðÄÄÄÙZ=37 ohm QRMÄðÄÄÙ on 6m
1mA 1mA


See also Tech Bul "Reducing Electronic RF QRM", "Stopping HF Tx/Rx SMPSU QRM" & "Analyser SWIRES RESEARCH SA87".


Why don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


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