Don and All,
The primary purpose of twisted pair wires in a balanced system is the
significant reduction of power line "hum" in the audio, just like the
telephone system. Intercoms may not necessarily use the same 600 ohm
impedance as phone systems, but the principle is the same. The use or
non-use of twisted pair is not significant for coupled RF, even though
induced or coupled RF is also a common mode problem. (Depending upon
system grounding, the use of shielded twisted pair might offer some RFI
relief. However, it sounds as if your lines are fully installed and if
operation is normal, other than the RFI, then the wiring is adequate for
intercom audio performance.) Design of magnetic components and/or
electronics uses the balanced line concept well at audio frequencies. (The
60 Hz "hum" is common mode and easily canceled. The desired audio is
driven differentially across the pair. Transformers or electronics pass
the differential audio and reject the hum.) High level RF also appears as
common mode to the intercom, but at a much higher frequency. It tends to
couple easily and "blow through" or around front end components and then
gets rectified ("detected") by one or more non-linear devices in the
system.
I agree with your assessment about paying a (seemingly) very high price for
a few dollars worth of components. However, the story in so many places is
that RFI is such a relatively rare event, for the majority of consumer
devices, that bean counters do not want to carry even the slightest amount
of financial burden to prevent or reduce it. Witness the ever-present
pressure on the FCC to not adopt European-style susceptibility requirements
for product testing and acceptance. It would be interesting to know if
Nutone will provide any sort of warranty with their pricey filters; if so,
that eases the pain a little bit. I would suggest trying the ferrite beads
to see if they help. Then, perhaps try Nutone's filter, especially if they
will offer a refund for failure to solve the problem. Best wishes.
The group might enjoy this brief anecdote: I work at Rockwell Collins in
Cedar Rapids, IA. The Collins Club has Art's old call, W0CXX, and has two
on-site shacks. Back in January, the club was running a KWM-2 "special
event" at one of the shacks, with the club's KWM-2 driving a 30L-1 linear.
Most of you probably realize that the 30L-1 was designed back when 1 kW was
the legal gallon. By today's standards, it is not "full power", but
respectable for sure. There is a "Brand X" commercial intercom box located
outside of the shack, down the hall a bit, probably about 40 to 50 feet
from the rig. The antennas are another 50 feet over the other way and up
about 50 feet on a tower. The shack is in an industrial high bay building
with steel and concrete panel roof and lots of partial shields. So, there
is reasonable separation (and some dubious value shielding) between the RF
source (antenna and/or rig) and the intercom box. I do not know the
routing of the intercom lines, but I believe they are run in metal conduit.
When I visited the shack during the special event, I heard the call and sig
report for a station being worked well before I entered the shack, as I had
to pass by the intercom. Of course, "Donald Duck" was the operator, but no
doubt about the signal source! I do not know if the intercom has been
worked upon for RFI reduction, but will report anything interesting if I
find an answer.
73, Dale
WA9ENA
---------------------- Forwarded by Dale G
Svetanoff/CedarRapids/Collins/Rockwell on 09/18/2000 01:27 PM
---------------------------
To: dgsvetan@collins.rockwell.com
cc: rfi@contesting.com
Subject: RE: [RFI] NuTone Intercom System
Good input Dale and I will check this out. Don't remember what the speaker
board is composed of, but I'll look tonight.
Did get some information from NuTone after making two phone calls. They
suggested I buy a RF filter from them. After some discussion I learned
that
the freq range of the filters however, split in the middle of our HF bands,
and then that they wanted $40 for each filter. For two L and 1 C component
on a piece of plain circuit board, I felt the pricing was out of line to
fix
a design deficiency. Not knocking NuTone as the system works good, but was
disappointed with this response after spending several hundred bucks on a
system which could have these components built in at the amp for probably
less than a buck. Suppose I could have pushed the issue, but felt I'd seek
information here, than make a decision on what fellow amateurs have found
to be best.
And by the way, have gotten several emails saying that twisted wire should
have be twisted. Installer used the 'recommended' NuTone wire. Will look
at this stuff also, but my recollection in looking at some leftover NuTone
wire was it was not twisted. To late now to fix this for most of the
installation
In any case, I want to thank all of you who have responded on this and I
promise I will compile the information received direct and through the
reflector, and as well what I find as the 'solution' to my problem. Never
cease to wonder at the help available at these sites.
Always remembered what I was taught in basic marketing. Price is what you
pay, value is what you receive and that value is remembered long after
price
is forgotten.
Don W7WLL
-----Original Message-----
From: dgsvetan@collins.rockwell.com
[mailto:dgsvetan@collins.rockwell.com]
Sent: Friday, September 15, 2000 6:24 AM
To: rfi@contesting.com
Cc: Tucker, Donald S
Subject: Re: [RFI] NuTone Intercom System
Don,
I would add just one more comment to Jack's (K6NY) reply about ferrite
beads: check out the remote at your shack end of the system, as well.
I can't speak for what Nutone uses, but I have had industrial experience
with another brand in which the remotes have a small circuit board by the
"call" button. That board contains a diode and a few passive components.
Ferrite beads applied at the main unit cured problems for that particular
system, but I was tempted to install them at the remote because of the
diode. If applying the beads at the amplifier, as Jack recommends, still
leaves you with a problem at the shack remote, it might pay to open the
remote (if it won't void the warranty) and see if there is anything besides
a speaker and volume pot inside.
Good luck.
73, Dale
WA9ENA
Please respond to donald.s.tucker@exgate.tek.com
Sent by: owner-rfi@contesting.com
To: rfi@contesting.com
cc:
Subject: [RFI] NuTone Intercom System
Shack is in a room on second floor of my new two story garage/shop, about
40
feet from house. A Nutone standard intercom system was installed with the
amplifier located in the basement of the house. There are two-way speakers
in several rooms in the house and a line also runs to the garage where one
is located in the shop, another in the game room, and one in my shack. The
amount of RFI is dependent upon power being run and also by the speaker
volume control. To operate I must turn the speaker volume control so they
are all but almost off.
NuTone is not much help. Anyone with intercom experience that can provide
me
with some suggestions? Looked in the ARRL Interference handbook, but did
not
find anything of substance for intercoms.
Don W7WLL
We recommend ferrite split beads to fix this problem. Get 1/2" hole split
beads and put one or more over the bundle of cables coming into the main
amplifier. These cables pick up RF and bring it to the amplifier. The
ferrite will, hopefully, keep it out. If the cable is over 1/2" split the
wire into two or more bundles, each 1/2" or less and put a bead over each.
-- Jack, K6NY, Palomar Engineers
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