> I just took apart one of Radio shack's $19.95 telephone RFI
> filters and was appalled to find that it contains nothing but
> a small toroid with about 25 turns of the telephone signal
> leads wound on it. Now I know a dollar doesn't go very far
> these days, but really!
Actually, it contains the toroid, the wire, the small PC board and
connectors on it, the male and female phone line connectors, the case and
the Radio Shack markup.
> Does anyone know if the K-Com (which costs the same and looks
> very much the same) is the same electrically?
K-Com sells their filters to Radio Shack. The units are identical. If you
have a nearby K-Com distributor (most ham stores sell the filters), you can
save about $5.00. With S&H, ordering from K-Com is about the same price as
Radio Shack.
> For $20, they could at least put in a couple of by-passes.
MOST telephone RFI is caused by common-mode pickup on the lines. In most
cases, adding caps across the lines does not cure telephone RFI. Many
people also run modems on their phone lines, and faster modems can run into
problems if there is significant capacitance across the line.
Now, one can make an effective common mode choke with an F-140-43 ferrite
core and some telephone wiring. These are a bit bulky to use with some
handheld phones, though. Neighbors are usually more amenable to the small
case of the Radio Shack/K-Com filters. There is a real advantage to having
your neighbor buy a commerical filter as opposed to giving them a homebrew
filter; it helps the neighbor better understand that telephone-I is not your
regulatory responsibility. Plus, if anything EVER goes wrong with the phone
the filter is on . . .
See http://www.arrl.org/tis/ and click on the TIS Pages and the RFI/EMI link
for info on telephone and other RFI problems.
73 from ARRL HQ,
Ed Hare, W1RFI
ARRL Laboratory Supervisor
225 Main St.
Newington, CT 06111
860-594-0318
Internet: w1rfi@arrl.org
ARRL Web: http://www.arrl.org/.
ARRL Technical Information Service: http://www.arrl.org/tis.
-----Original Message-----
From: Pete Smith [mailto:n4zr@contesting.com]
Sent: Saturday, March 25, 2000 3:07 PM
To: rfi@contesting.com
Subject: [RFI] Phone RFI filter
I just took apart one of Radio shack's $19.95 telephone RFI filters and was
appalled to find that it contains nothing but a small toroid with about 25
turns of the telephone signal leads wound on it. Now I know a dollar
doesn't go very far these days, but really!
Does anyone know if the K-Com (which costs the same and looks very much the
same) is the same electrically?
For $20, they could at least put in a couple of by-passes.
73, Pete Smith N4ZR
n4zr@contesting.com
Don't forget to update your entry on the World Contest Station Database, at
http://206.102.70.3/search.htm
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