Topband: Topband QRN

[Brad Rehm]

Hi Tom, et al.!

Some good comments about the problem.  Let me add a few more:

"My old cure here was a power plug with a built-in **suitable for line
bypass** .01uF cap between each conductor and the safety ground pin.
(The
USA has a three wire system with a neutral, a safety ground, and a
"hot"
wire). The CATV line is then grounded to the safety ground. This
reduces
TVI, and interference back to the radio station. I found this much
more
effective than anything else."

You bet!  Steve would have to build a fixture to hold the capacitors
to the back of the plug, though.  TV sets usually have 2-wire power
leads, so the "fixture" at the wall plug would have to decouple the RF
to the safety ground.  He also has to be sure to use X2- or Y2-type
capacitors that are UL approved for across-the-line applications.
They're also called "Suppression Capacitors."  I know you mentioned
this, but I want to stress the point.

Here's a thought about using capacitors in this way: We assume that
the caps route the RF to a low-impedance load (the safety ground
system) and that the efficiency of the driver (the TV set) will go
down because its impedance is relatively high.  The mismatch results
in reduced radiation from the AC wiring in the house.  This works if
the impedance of the Safety Ground wiring is low.  If it isn't, it
could and probably will radiate as effectively as the hot and neutral
wires did before the capacitors were added.

This could happen if the RF conducted on the power cord is
common-mode.  Both line and neutral can "drive" Safety Ground.  And
this is the reason Steve should be prepared to try ferrites too.  They
dissipate the RF instead of re-directing it.

"One problem with adding a core is the core flux density could be
high, reducing
effective impedance, unless you have equal and opposing currents in
the winding."

The 60 Hz AC currents are opposing in the power cord, so they wouldn't
affect the core.  The strongest conducted RF emissions--the kind we're
concerned about here--are usually in the hundreds of microampere
range.  This is well below what it would take to saturate a 2 1/2" OD
core.

"Having done EMI certification, I know certification tests don't
really watch
the antenna-to-powerline radiation mode nearly as much as they work to
prevent  differential (push-pull) mode radiation down the power
line...."

Yes.  The problem, so far as hams are concerned, is that Radiated
Emissions measurements are all made above 30 MHz.  Labs are only
required to measure Conducted Emissions from 150 kHz to 30 MHz, and
the ANSI C63.4 CE test setup is designed to prevent measuring
displacement currents between the power and antenna leads.

Moral: If you're a ham, take a transceiver with you when you buy your
next TV, VCR, or DVD player.  Listen for emissions.  Then, turn the
transmitter on and see how it affects the gadget.  (There's usually
some reciprocity between emissions and immunity in electronic
devices.)  A 5-Watt 2 meter HT works fine for this.  It's interesting
to be able to shut down all but one or two VCRs on a shelf of ten at
Sears.  I always buy one of the ones that my HT can't get to.  They're
often the least expensive, and I have very little trouble with the
ones I've selected in this way.

"Some TV's (ie plasma display TV's) radiate so much from the case that
nothing you do to input and output leads stops their QRM."

I've tried the HT trick on some of these.  You can imagine how quickly
that gets the salesman's attention.  High susceptibility is usually
related to high emissions at some frequency or another.

73,
Brad, KV5V