Dave and Tom,
Dave, I have to agree with Tom's separately posted comments about the line
filter.
Manufacturers of facility filters (those that are generally wall-mounted
and used in conjunction with shielded "screen" rooms) offer a variety of
filters for power, telecom, and signal control applications. Again, going
back to my days at Lindgren, I can tell you that every so often I would get
a call from a perplexed customer who could not understand why the telephone
he just placed into his shielded enclosure would not work. There were two
reasons for most of the problems: 1) Either the customer had a spare power
line (or signal control) filter laying around and figured it would work for
telephone use, or 2), the customer had a telephone that was part of a
proprietary digital-based system.
Power and general purpose signal control filters do not have controlled
line-to-line impedance (necessary to maintain hum-free phone line
operation) and they may have a passband that is too limited and which
severely limits the voice frequency response (supposed to be 300 to 3000
Hz, nominal) of the phone system. That limited passband, along with higher
by-pass shunt capacitance is what kills the proprietary digital phone
systems. (In this case, "digital" does NOT refer to DTMF digits; those are
fully within the voice frequency passband. Some proprietary systems use a
medium speed data stream to support dialing and/or system functions, such
as voice mail, speed dialing, etc. Conventional filters, even telephone
line filters, kill the data stream and render those phones as useless.
So, use power filters to knock down power line RF. Yes, in some cases, you
might find a power line filter that allows a phone to work thru it. That
situation tells me that this is probably not a very good power line filter
because it does not have a lot of shunt cap. Lots of by-pass Xc + lots of
series XL = more attenuation. Filter designers have the difficult task of
balancing the parameters so that what you want gets thru and what you don't
want gets stopped. My hat is off to those proficient in that art.
73, Dale
WA9ENA
David Jordan
<wa3gin@erols.com To:
dgsvetan@rockwellcollins.com
> cc: rfi@contesting.com
Sent by: Subject: Re: [RFI] NOISE
GADGETS
rfi-bounces@conte
sting.com
11/10/2004 12:52
PM
1st, go the the NIC and determine if the RFI is coming in from the
outside phone plant. If yes see note (A) below.
(A) A common surplus AC brute force line filter will do a splendid job
of removing RF from your house phone lines. Place the line side towards
the NIC and the load side towards the phone instrument/s. Ground the
metal case.
2nd, if RFI is not originating from the outside plant, take precautions
as noted in (A) and add additional filters at the telephone jack/s
which feed the phone instrument which is experiencing the RFI.
Have Fun,
dave
wa3gin
dgsvetan@rockwellcollins.com wrote:
>
>My last job back in the Chicago area was at Lindgren RF Enclosures (now
>ETS-Lindgren), located in Glendale Heights. The company's building was
>located in an industrial park that was directly adjacent to 50 kW AM radio
>station WMAQ, at 670 kHz. I estimated that our building was about 1500 to
>2000 feet from the base of 'MAQ's tower. I took some rough spectrum
>analyzer measurements, and it appeared that the steel and concrete
>structure of the building was worth about 40 to 50 dB attentuation,
>relative to the 670 kHz signal just outside the walls. Amazingly, there
>were no phone system problems within the building. Now, also adjacent to
>the WMAQ tower area (around the corner from the industrial park) was an
>apartment complex, built with frame construction. Over time, a few
>Lindgren employees did live in that complex. Some of them came to me with
>complaints that they could not use their home telephone because of WMAQ's
>signal. Luckily for them, I had a stash of ferrites that I had gotten
from
>Lindgren's Elmag filter division (now, no longer in business) that were
>used in certain facility filters. These ferrites were about 2 inches long
>by 5/8 inch diameter with a roughly 1/4 inch hole thru the center. I
>suggested that they take the ferrite home, unwire the phone from the line
>connection, pass the line thru the ferrite as many times as possible, and
>then reconnect. I also advised them to use as short of a cord as possible
>between the phone body and the handset. As far as I know, it worked every
>time. Problem: I never did get the specs on that ferrite!
>
>Best wishes with your projects.
>
>73, Dale
>WA9ENA
>
>
>
>
>
>
>
>
> "Jim Brown"
> <jim@audiosystems To: "RFI List"
<rfi@contesting.com>
> group.com> cc:
> Sent by: Subject: Re: [RFI] NOISE
GADGETS
> rfi-bounces@conte
> sting.com
>
>
> 11/09/2004 06:27
> PM
> Please respond to
> Jim Brown
>
>
>
>
>
>
>On Tue, 9 Nov 2004 16:10:38 -0700, Howard Lester wrote:
>
>
>
>>Thank you!
>>
>>One 'quick' question: just how beefy do the speaker cables need to be to
>>
>>
>get
>
>
>>good, solid bass? (I will go to the site to read the articles.) I am not
>>
>>
>yet
>
>
>>aware of, say, 14 gauge twisted pair wire - shame on me, I guess.
>>
>>
>
>West Penn, Gepco, and Belden all make stranded twisted pair cable in the
>#12 - #14
>gauge range. You can also "roll your own" out of stranded THHN from Home
>Depot by
>sticking it in a twist drill. And you can "double up" on pairs of
>instrumentation cable.
>
>
>
>>I have known for a long time about twisted pair helping to reject RFI.
>>
>>
>Tell
>
>
>>that to the homebuilders; they seem to refuse to use it for telephone
>>wiring, even though it is allegedly "code."
>>
>>
>
>Don't know about the "code" part, but CAT5 and similar cables make
>EXCELLENT
>telephone wiring. For one thing, they have very low capacitance, so they
>will help a bit
>with data transmission.
>
>
>
>>I had a ham friend who offered
>>to do the wiring of my new home with correct telephone cable. Even though
>>she worked for the phone company, my builder refused to allow her on the
>>premises. If I wanted twisted pair, I would have had to pay a premium. I
>>said screw it -- it's only my house, and I'm the only ham around. No one
>>around (also with 'bad' wiring) has complained about my 100 watt station.
>>
>>
>
>One of the last instances of RFI that I'm chasing with my KW station (and
>antennas quite
>close to the house) is into my wife's telephone. I've got a bunch of
>ferrites on the wiring,
>but it's still getting in. I'm starting to wonder if maybe it isn't
>rectifying in some of the
>ancient telco termination gear in my basement. I haven't shifted all my
>telco stuff to
>CAT5 yet, but I probably should.
>
>Jim
>
>
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>
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>
>
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