RadioBanter - View Single Post - "Can twisted wire replace shielded wire?"

November 28th 05, 02:31 PM posted to rec.radio.shortwave

On 27 Nov 2005 18:23:39 -0800, wrote:

David wrote:
snip
Common mode rejection.

Look it up.
++++++++++++++++++++++++
Just for grins lay a 100' of twisted leads, lamp cable and coax out on
the ground.
Attach 50 Ohm resisitors at the far end of each.
Swtich from one to the other and see which one picks up the most RF.
MW stations will be received very well with both the twisted pair and
lamp cord.
Very close/strong MW stations may be recieved with evenpoor quality
coax.

Next build one of these to go between the feedline and your receiver:
http://www.dxing.info/equipment/coax_leadin_bryant.pdf
Repeat the experiment.

If your results are anything like mine the twisted pair and the lamp
cord will
still have significant reception.

True "common mode" only occurs when there is perfect balance between
the
conductors, with no "realworld" artifacts to effect that balance.

The results I provided are real world, not from some SPICE simulation.

I tried this with both a R2000 and a AOR 7030+. And I got nearly the
exact
same result with the AOR giving slightly better MW rejection with the
coax
and the in line filters.

I preffer to use real world experiments whenever I can.

I was surprised at how little difference the quality of the coax, or
even the
Z mattered. I expected no measurable difference between 50 and 75 Ohm
coax.
But even 93 ohm, low capacitance, coax acted the same. And the type of
shield
made no difference. By type of shield I am reffereing to: single braid,
double braid,
braid + foil and even some real cheap foil and drain only.

I even did a repeat of this test at a ham friends QTH where he was
putting up a
repeater and had about 200' of hardline stretched out on the ground.
Worked pretty
well, but I had to add the chokes in the PDF link I gave.

Now Triax did make a big difference. But only if you only grounded it
at one end,
the receiver end being about 20dB better. And Triax did not benifit
from the common
mode supression. It amde no difference.

Terry

Try it with a true balanced input.