[RFI] Anyone heard of this type of noise?
w8ji at contesting.com
Mon Nov 17 08:58:39 EST 2003
> sounds at 1 mhz. This can be attributed to the poorly shielded TV's now
> produced to keep cost down. Never had trouble like that years ago but it's
> the sign of the times...RF junk is amongst us at every turn.
TV sets have been a problem for years, I had severe problems with birdies
from sweep systems even in the early 60's, when I first became a Ham. In
general TV's seem to be a bit better now than in years past. I actually have
considerably more problem with switching supplies and digital systems than
TV sets, including burglar alarms and power supplies in computers and other
The real problem is direct coupling of noise into connecting external wires
or cables, and the worse cases are when two conductors or cables leaving the
offending device have a noise differential between them. For example a
neighbor's computer almost a mile away was feeding the power line AGAINST
the telco connection in the modem, setting up a nice push-pull excitation of
the two systems. This stuff conducted for miles, and adding a normal filter
or choke to the power line made only a small change (even with fairly high
The cure was to add multiple turn bifilar chokes with an impedance PEAK at
1.8-30MHz (I used several turns through 73 material, 43 would be far too low
for HF but better at VHF) on the power line, UL/CSA rated bypass capacitors,
and run the Telco and power lines through filters in the same box that
shared a common ground with the power line safety ground.
What this does is force all lines to the same RF potential, preventing any
differential excitation of conductors in a single line or between different
types of line.
You can do the same thing with CATV or MATV lines, and the power line on a
TV set. Nearly all of the RFI ingress and RFI output of TV sets is from the
same type of "push-pull" excitation. The actual mechanisim of excitation is
the TV tuner or modem that connects to the coax or telco line is normally at
another end of the chassis and wiring, and the circuit system generating the
trash is connected between that area and the power line. This forms an
efficient two-wire push-pull output for even very low levels of RF ground
potential difference inside the unit. It also works in reverse for RF
ingress into the consumer device.
Years ago a 5kW AM BC station was just a few hundred feet from a large
apartment complex. The CATV system installed triple shield cables and
filters, but never cured the bulk of problems.
I went to each apartment and other points in the system and simply grounded
the safety ground of the power line to the CATV wiring, and acheived
virtually 100% cure without filters! As a matter of fact, in the rewire we
pulled out all the triple shield and installed normal coaxial lines without
any problems (so we could bring the cables in parallel with the power line
If I were going to cure anything like this, I'd buy a common reliable surge
supressor that passes Telco, CATV, and power through. I'd be sure the device
had UL/CSA rated line bypass caps and bypass caps from the telco to the
common ground of the CATV/ power line grounds. In a severe case I might add
a few #73, 75, or 77 mix cores with a dozen turns of suitable bifilar line
in series with the power, cable, or telcro ports. It should or could even be
a large core externally mounted placed between the TV or consumer device and
this "common ground suppressor" with the normal type of wire wound through
A very simple thing like that would cure virtually all ingress and egress
problems by making sure ALL lines entering and leaving a device are at the
same RF potential. It's important to pick a material with an impedance peak
at the problematic frequency range.
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