Re: A.M. radio interference

On Sun, 29 Apr 2007 10:19:13 -0400, axolotl <kevin@xxxxxxxxxxxxx>
wrote:

Don Foreman wrote:

Can't argue with shielded twisted pair, and that may indeed solve the
problem.

I don't see the logic of discouraging use of an LPF but encouraging
use of ferrite beads. Either you attenuate BCB freqs or you don't.

I agree that slowing down the edges a little might help a lot. Getting
rid of the energy is easier than trying to keep it contained.

Ferrite beads are most assuredly filters but have the advantage that
they don't store all the energy that goes through them. Attenuator cores
are designed to have a high Rx at higher frequencies, getting rid of the
offending energy as heat. They filter without reflections. Different
materials have different attenuation/frequency curves.

R/X of most ferrites doesn't start to become appreciable below 10 MHz,
much less 500 KHz. An impedance-matched RLC filter, Butterworth or
Chebychev or whatever, can be designed with low corner freqs, will
also dissipate higher-freq energy as heat and will also filter
without reflections. A second-order Chebychev with corner of 150 KHz
would have nearly 40 dB of attenuation at 1500 KHz, which is KSTP's
freq.


Down at MF, you need to use a directional antenna to increase the S/N
coming into the radio. I'm not sure what kind of effective directional
MF antenna could be delivered by UPS in a box. Perhaps one of the old
marine DF sets with the ferrite rod loop and monopole stick.

An n-turn loop wound on crossed sticks or around a plastic 5-gallon
pail works pretty well. It won't have a sharp peak but it will have
a sharp null. Larger is better; using crossed 8-foot 2 x 2's as a
form would make a dandy BCB loop.

After I put beads (actually, wrapping the lines around an attenuator
toroid will be more effective) on the data lines, and put filters on the
power supply lines of the data box, PC, and radio, I would try remoting
the antenna. Consult the ARRL manual, build your favorite high
directional gain antenna, and bring a balanced line back to the radio,
coupling to the radio with a few turns around the ferrite rod inside.

Or use a car radio. Car radios at least used to have excellent AM
sections with tuned RF stages and, of course, an antenna jack.

Some guys really like baseball.



Kevin Gallimore




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