Topband: Polarized current and interference

[Tom Rauch]

> > "If you are using a nonbattery radio, make sure that the
electrical
> > socket is polarized. If you can flip the plug over and
it still fits,
> > you don't have a polarized socket."
> >
[http://www.cleveland.com/living/plaindealer/index.ssf?/base
/living/109351285615492.xml]
> >
> Hi, Hermod-
>
> This is obvious nonsense. Here is another 'civilian'
posing as an
> expert in radio. Some correct statements are also
intermixed in the
> article with this misinformation. I'd love to see how his
sources
> support this belief regarding minimizing RFI to receivers.
They must
> have never encountered the topic of AC power line RF
filters.

I think he lifted that from other articles.

In old AC-DC radios, the loop antenna or antenna input coils
were tied to the "ground" side of the power line through the
AVC system's path back to the chassis. That path usually had
a very high value decoupling resistance. A few of those
radios (mostly shortwave models) had antenna posts that
coupled an insulated link wound around the input coil or
loop antenna to an external antenna screw. Others had
capacitor isolated coupling.

If the plug was inserted so the chassis was "hot" with
120VAC and an external antenna and/or ground was used, the
60Hz power line would pump the AVC at a 60Hz rate through a
high impedance path back to the grid of the mixer/oscillator
tube. Just microamperes of current would  superimpose a hum
on the audio.

As strange as it seems, in certain line operated radios used
with external antennas or grounds, reversing the plug
removes or adds "hum" to audio. An AC/DC Sears radio I owned
did that when connected to an outside antenna. It would also
tingle me when in the "hum" mode. Maybe that's why the band
AC-DC released an album titled "High Voltage"?  They might
have had Sears AC-DC radios as kids.

73 Tom