[Lowfer] HDTV RFI, request recommendations for reduction of noise

[Clint Turner]

There are several devices intended for removing common-mode currents on 
coaxial cables:

- Analog TV "Hum-bucking" coils.  These are the things that break up 60 
Hz common-mode currents on TV tie lines to prevent hum bars from 
appearing.  Typically, they are potted toroids in die-cast aluminum 
boxes with a pair of BNC or UHF connectors (depending on the vintage) 
and have several hundred millihenries of inductance.  They do have a bit 
of capacitance so that they may be less transparent at HF, but they do 
work for certain at AC line frequencies and up to several hundred kHz.

- CAT-5 adapters for analog video.  These can be found cheaply and they 
are typically miniature coax or twisted/parallel wire on a small, high 
permeability core, already packaged.  These are just common-mode chokes 
and with analog video, the sync and low-frequency video is actually sent 
unbalanced, but as the frequency goes up it becomes "balanced" due to 
the choke.  Last time I checked one of these the "crossover" frequency 
(e.g. where the impedance of the choke was 3-10 times that of the video 
line impedance) was in the several hundred kHz range.  These might also 
work for breaking up RF circulating currents, but I don't know how high 
they will go without more imbalance/loss.  The preferred units use 
miniature coax, but most I've seen recently use twisted wire.

The first of these (for AC) are pretty heavy and can be a bit pricey, 
but they will definitely do the job at VLF/LF.  If the coax to which 
they are connected is put through several large chunks of ferrite (e.g. 
flyback core, or the toroids that KB4OER mentioned - or even a CAT-5 
adapter) then you have a very wide-frequency common-mode blocking device 
that would work from AC to high HF.

The CAT-5 adapters are all over the map in terms of what they contain 
and their overall quality and the best thing to do is to tack an 
inductance meter between one input and the corresponding output lead:  
I've seen them from a few hundred uH to several 10's of mH.  In 
practice, one would then take the thing apart and replace the 
input/output connectors with BNC or whatever you use.

Again, my favorite is the good old flyback core - plentiful if you can 
intercept old CRT TVs/monitors on their way to the recycler, but getting 
the core out without breaking it can be a trick, though. Usually, the 
top half comes out without much difficulty, but I usually have to 
(carefully) heat the bottom half with a heat gun and then push it out 
with a *WOODEN* dowel as the glue bond breaks.  If the ferrite breaks, 
it can be reassembled using super glue with practically no loss in 
efficacy since the break is usually very clean and the cyanoacrylate 
adhesive is very thin and the gap is very small.

Alternatively, the yoke from the CRT can be used and are much easier to 
extricate and they already come with metal clips that will hold the 
halves together and one can get several turns of fairly large coax 
through the center.  Typically, for coax and control lines that 
terminate at the same place, I parallel them and put them all through 
the same yoke core - both to save space, but also since they will all 
likely have the same sort of induced common-mode currents on them 
anyway, having gone to/come from the same place.

73,

Clint
KA7OEI/CT