Jim and Howard,
A couple of things I'd like to mention relative to twisted pairs:
1. Twisted pairs are most effective at minimizing common mode effects at
low frequencies. Ma Bell (remember her?) used 600 ohm twisted pairs for
the station wiring for a very long time. 'Real" telephone wire IS made of
twisted pairs. The twisting is why you can hear the person on the other
end, and not 60Hz hum. However, the twisting will not prevent the coupling
of RF into the telephone device. The RF arrives equally on all wires
within the cable jacket and couples very nicely thru the stray cap of any
hybrid transformers or active coupling devices used to interface the
telephone line. Ferrites applied to the phone line, as close to the
telephone instrument itself as possible, will reduce or eliminate RFI to
telephones IF it introduces enough loss/series impedance at the RF
frequency. Again, from numerous prior posts, don't expect the Radio Shack
ferrites to help you keep lots of HF RF out of your phone.
2. Howard - what sort of wire do you have for your in-house phones?
Unless the builder ran "speaker wire", I would guess that he really did run
twisted pair. This is especially true if you do not have a lot of hum on
your phones. Note that the twists for telephone wire are not given in
twists per inch, but in twists per foot. Therefore, you have to expose a
foot or two of the wire (remove outer cable jacket) to see if the conductor
pairs are twisted. What colors are the wires? Station wire cable will
have RED, GREEN, YELLOW, and BLACK conductors, with the pairing being R/G
and B/Y. Actual multiple pair telephone cable will have a BLUE/BLU-WHT
pair as pair #1, followed by ORG/ORG-WHT as pair #2. It sounds like your
builder probably had some sort of deal with the electrician and/or telecom
provider in not letting you or your friend do wiring. Luckily, my wife
and I were able to put in sweat equity on our house a few years ago by
letting the electrician do all the 120/240 stuff and us doing all of the
telecom stuff (telephone, intercom, Cat 5E for LAN, A-V distribution, etc).
3. Jim, yes, it is possible that you might have some of the old carbon
pile "protectors" on your phone lines. I believe that if corrosion sets in
on those things, you might get RFI effects from the galvanic detector
action within the corroded areas. A good visual inspection might be in
order. On the other hand, I am convinced that, with few exceptions, most
of the available consumer telephones today are junk as far as RF
susceptibility. If you look back in the archives of the reflector over the
past two or three years, there were some postings about a particular model
of Radio Snack phone that was pretty close to bullet-proof. The best
phones for RF rejection are the original 500-type sets from WECo and ITT
Kellogg. If you have them, or find them, keep 'em and use 'em!
That said, let me relate the following:
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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