On Feb 1, 7:00*am, RHF wrote:
On Feb 1, 6:06*am, "mc" wrote:





I don't think a single 0.01-uF, 250-volt capacitor provides sufficient
isolation between the phone line and the radio. *I would use a much smaller
capacitance.

Ringing voltage is typically 100 V, 20 Hz. *Audio is typically 2 V, 2000 Hz.
We want to keep both of these out of the radio, and yet have fairly low
reactance at 2 MHz and up.

This is a rough back-of-the-envelope calculation, of course.

At .01 uF, capacitive reactance is...
* Xc = 1/(2*pi*F*C)
* 800 k ohms at 20 Hz
* 8 k ohms at 2000 Hz
* 8 ohms at 2 MHz

I would go with 0.001 uF (= 1000 pF) or even less, maybe 100 pF.

At 100 pF, capacitive reactance is 100 times higher:
* 80 megohms at 20 Hz (ringing voltage is very well blocked)
* 800 k ohms at 2000 Hz
* 800 ohms at 2 MHz

For a receiver with a relatively high input impedance, 100 pF is fine. *For
one with a lower impedance (to match 50 ohms, for instance), 1000 pF is
better.

Either way I'd use a 600-volt (not 250-volt) capacitor in order to have a
lot more "headroom" in case of lightning, etc.

AND THERE IS A SIMPLER WAY. *What happens if you connect your receiver's
antenna cable to a piece of wire which you wrap around the telephone line,
without a direct electrical connection? *This should give you something like
10 to 100 pF of coupling with absolute electrical safety.

VY 73
N4TMI

N4TMI,

Simpler to first try Wrapping 3~5 Turns of the Telephone's
Wall-to-Phone Cord around the 'portable' AM/FM Shortwave
Radio : If the Signal is stronger with less Noise then you could
proceed with more pleasing and permanent connections.~ RHF

Telephone Extension Cord, Modular Plug-to-Plughttp://keysan.com/ksu0672.htmhttp://www.cablesnmor.com/telephone-cord.htmlhttp://www.allelectronics.com/cgi-bin/category/129/Cables_-_Telephone...
*.
-ps- Telamon be sure to Count* this as another one
of my 'Clueless Posts' -cause- IT IS ! ;-{
Plus Extra Credit for the : Cut, Copy and Paste Stuff. )
* Rec.Radio.Shortwave Newsgroup Readers :
Please I Need Your Votes ! -and- I Want Your Votes !
In this Weeks Race for the RRS "Clueless Troll" Award.
*.- Hide quoted text -

- Show quoted text -


Telamon,

This Message and Thread Needed both
Your Vast Knowledge and Great Expertise.


but alas you were once again 'mia' ~ RHF


-ps- that's 15 for me and 0 for you