Don Bowey wrote:
On 11/16/06 1:16 PM, in article Vy47h.320898$1T2.140009@pd7urf2no,
"Ken Weitzel" wrote:

Bob Urz wrote:
Anybody got a good resource on how to build a phone line filter?
I am near a AM radio tower and its really killing my dial up modem
at times. I tried a DSL filter in line. but i think the AM band is
not getting attenuated enough to do much good.

Its hard to peg characteristics. Would a modem be considered a 200
ohm nominal load on the phone line?

What type of filter for AM radio?

Dual inline inductors?
parallel capacitor with series resistor maybe?

Any ideas or links?

Hi Bob...

Spent my working lifetime in broadcast, so can virtually guarantee
that you don't have to do a thing

Just call the station, ask for the Chief Engineer, and tell him of
your problem.

If your country is anything like Canada, he'll be there in a flash,
and all of your problems will be gone

If you want him there even faster'n a flash, just ask him if
you're supposed to be telling him about the problem, or should
it be the DOT/FCC/whatever country you're in

That's bad advice.

Take care.

Ken


Just exactly why you think the radio station or FCC is going to care
about RF getting into the premises wiring or telephone cable? The
station engineer may make a filter recommendation, but is under no
obligation to do any more than that.

The issue is... What non-linear device is causing the RF to be
rectified so it is audible. It could be a bad joint, a telephone, or
whatever..

Don

Ken, time have changed in that the engineer nowadays has multiple stations
to maintain. when i get a RFI complaint like this my first question is to
ask if Telco has been called. if the problem is before the customer demarc
no filter in the world will help as the interference is audio at this point.

if it clean at the demark and occurs only when the inside wiring is hooked
up then something can be done with filters. sometimes the problem can be
traced to a single plastic phone.

often the telco can test the line without sending anyone out.

all too often after a storm a telemetry line at a transmitter site will go
bad or partly bad. no amount of filtering will be very effective.

the charicteristic impedance of a "copper" line is about 150 ohms. the Z
was brought up to 600 ohms in dedicated circuits by 1:4 transformers.
nowdays it's all digital.

in the last 6 years i have not gotten a call back after referring a
complainant to the telco.