Topband: Topband QRN

[Tom Rauch]

Hi Brad and all,

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.

As I recall 73 material has about 30-80 ohms per linear inch of ferrite in
parallel with the conductor on 160 meters. It will also work well as an
absorber on much higher frequencies, certainly well beyond HF bands. 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.

Multiple turns through ONE core is much more effective than adding multiple
cores with only one pass through. Impedance goes up about by the square of
the turns increase. 4 turns through a 1" deep window has about the same
impedance as 16" long stack of cores with one pass through!!! Two passes
through are like having a 4" stack of cores.

You also do NOT need to use "fancy" winding techniques. It makes no
measurable difference at all below a few hundred MHz if you do that fancy
"winding flip" the Handbooks show. It will move stuff around a bit high in
the VHF area, but is not even significant up there!

All in all, it is **much** more effective to "connect" the TV power line to
the TV antenna lead through a coupling system like I described earlier than
to just slap a few beads at the problem. The reason is the TV set is
probably driving power lines as one leg of an "antenna" compared to the
Antenna Input of the TV.

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,
although every product could be different in actual results.  For this mode,
it is very difficult to get enough series impedance to eliminate problems
even with multiple turns, and almost impossible with a "single pass" winding
through a bead. A typical W2DU balun, with a string of beads a foot long,
only has several hundred ohms of Z on 160. On the other hand "shorting" the
radiation by RF connecting the power cord and antenna lead (with suitable
.01uF  VDE/CE/UL rated capacitors) puts a few ohms load across the
unintentional noise generator, and greatly dampens any radiation.

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.

73 Tom