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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]

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.

QRM cable                         Hinge  / /~~\ \  Locking
=================                      Ĵ    Ĵ Catch
     :::::-screen            Rx_________\ \__/ /
 10  ())))_____             Coax~~~~~~~~\\/
turns50 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 50 load only 0.5 is effectively
inserted in QRM line.

    0.5 1T
    _________50    = 0.5W = 5V = 74dBmV = 134dBuV

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
 Ŀ  RGB    Ŀ  RGB  Ŀ     \|/UHF
     TV       D.V.D.   S.T.B.  Aerial
   DISPLAY  Ŀ                    
     Ŀ  Mains    UHF DC         Ŀ
Mains      V.C.R.       Plug Mains
            Mains             test 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
  SOURCE   choke

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
==.._ݳ_______       |:.                         
 15kHz 4,4 6    38MHz            
 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...


   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 \/ 10m
 1/4 WAVE   TV                         SMALL
 VERTICAL  ANT    37uW               EQUIPMENT   0pW
  AERIAL          ERP on      O R     LOOP ANT   ERP on
                  40m                      __    40m but
                                         _     high
        QRMZ=37                 QRM   on 6m
           1mA                            1mA

See also Tech Bul "Reducing Electronic RF QRM", "Stopping HF Tx/Rx SMPSU QRM" &

Why don't U send an interesting bul?

73 de John G8MNY @ GB7CIP

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