G8MNY > TECHNI 14.11.21 11:20l 121 Lines 4868 Bytes #999 (0) @ WW
BID : 52988_GB7CIP
Subj: Clip on QRM Probe
Sent: 211114/0935Z @:GB7CIP.#32.GBR.EURO #:52988 [Caterham Surrey GBR]
To : TECH@WW
By G8MNY (Updated Nov 19)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
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.
³_________50ê = 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...
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´PlugÃ 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..
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!
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±±±==.._Ý³______±_ 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 ººº ºººº
5 10 15 20 25 MHz
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=37ê QRMÄðÄÄÙ on 6m
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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