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Author Topic: RFI on landline phone  (Read 1061 times)
KE2KB
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Posts: 633




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« on: October 30, 2017, 01:12:12 PM »

Hi;
I was informed that my (garbled) SSB signals on 40m were being heard on the wired phone in the house.
This is an old model AT&T phone (with memory dial) but has no external power connection.
This phone, along with a Panasonic cordless phone base and an internal fax modem are connected to the same wall jack.

Our phone service is Verizon FIOS with phone, Internet, and TV.
FIOS uses the old copper wiring in the house to connect their ONT (which is located on the side of the house near the electric service) to the phones in the house.
The copper phone wiring in the house is anything but clean. I found that there are two telco blocks, each with 4 terminals and a ground. The grounds are connected to a BX cable run.

Long story short, I ran a test today and found that my signal can only be heard on the phone when the wired handset is picked up. If the cordless phone is picked up, there is no RFI on it. But once the wired handset is picked up, all phones on the line can hear my signal.
I conclude that the RF is getting directly into the wired phone. So I am thinking that adding a bypass cap or perhaps a choke in-line with that phone will solve the issue.

Are there any general recommendations for this - capacitor/choke value, etc? or is it trial and error?

Another thing I would like to do is rewire the phone line from the first terminal block where the FIOS connects, to the phone jack where the three devices are connected. If I do this, should I use shielded cable? I would think that if I do use shielded cable, I should ground the drain/shield wire at the source (terminal block #1) end. Perhaps I need to move the ground connection off of the BX cable and run it directly (using #6 copper wire) to the electric service ground, which could be the #6 wire that runs between the electric service panel and the copper water line (at the meter), or maybe I need to run the conductor all the way back to the electric service ground rod? I'm not sure what code is for this.

Thanks for your suggestions
Frank
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KE2KB
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« Reply #1 on: November 01, 2017, 08:41:51 AM »

Well, I guess no one uses copper wire anymore.
FWIW; I'm  going to rewire the line with CAT5 or CAT6 twpr.
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WB4SPT
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« Reply #2 on: November 01, 2017, 10:30:34 AM »

Well, I guess no one uses copper wire anymore.
FWIW; I'm  going to rewire the line with CAT5 or CAT6 twpr.

TP wire might help.  The corded phone has electronics as well as the FIOS  box. Ferrite ,paired wiringwill defeat it.
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KL7CW
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« Reply #3 on: November 02, 2017, 08:47:44 AM »

I would just try a simple plug in RFI phone filter (cost probably less than $5) right at the RJ-11 jack near the phone, or even an old DSL filter.  Might help, otherwise wind about 10 or 15 turns of the phone cord around a ferrite core right near the phone.  I agree that updating your cable to CAT cable would be a good idea.  Threshold the RFI each time you do a fix.  That way you will know if you have made progress.
       Rick   KL7CW
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AA4HA
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« Reply #4 on: November 02, 2017, 10:53:12 AM »

Copper based telephone systems have pretty good noise immunity when they are on the outside part of the DEMARC (demarcation point). What happens in your home can be anyone's guess as most house wiring is not even close to being good as much of the wiring may be two or four conductor (but not twisted pair), runs all over the place or has old drops that go in to rooms that are open at the jacks or with the wire ends just closed off inside of walls.

Usually if you have a good, two wire twisted-pair that does not go all over the place and is attached to a decent quality telephone handset (oh, yea, some phones are absolute sh&te in their construction) it is relatively immune to RFI.

You can try looping the phone cord four or five times through a ferrite core (the snap close type cores allow you to do this without chopping off the connector end) as your first step; Do this near each phone and also at the FIOS box.

The next step would be to put in a new wire run using a decent quality two (or four) wire twisted pair cable. If you use shielded cable that is a nice addition and you can attach the shield to the chassis screw at the FIOS box (but nowhere else, you want the shield to be a drain, not a parallel conductor).

Pay attention to cable spacing (not near your coax or antenna) and try to keep runs as short as possible. Do not daisy-chain phone connections but give each jack its own direct run back to the FIOS box.

--------
The same problems appear on data connections and instrumentation lines. I have about 35 years of experience in working on those issues as well as a bunch of years in designing modems for FCC part 68 (telephone modem) compliance testing.
« Last Edit: November 02, 2017, 10:55:17 AM by AA4HA » Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
Free space loss (dB) = 32.4 + 20 × log10d + 20 × log10 f
WB4SPT
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« Reply #5 on: November 04, 2017, 07:29:58 AM »

Whats the rfi spec for part 68?
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K5LXP
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« Reply #6 on: November 04, 2017, 10:07:14 AM »

This is an old model AT&T phone (with memory dial) but has no external power connection.

Just because it's "old" and doesn't have a power source from an AC outlet doesn't mean it doesn't have active electronics, as evidenced by the memories. 

Seems a simple A/B would be to try a different phone to see if it makes any difference.

Mark K5LXP
Albuquerque, NM
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N5EG
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« Reply #7 on: November 04, 2017, 10:33:12 AM »

It's been mentioned on Eham before, and I've used for about 15 years a DSL filter
on the problem phone.  While normally only needed when using DSL on the phone
lines to isolate the phone from the data signals, the filter itself is actually a low
pass filter, it attenuates differential-mode signals quite a bit.  Assuming the phone
is not connected to the AC power line somehow, common-mode interference is not
too likely and thus a torroid choke type filter probably won't do much.

Put the DSL filter near the phone.
Many folks have a bunch thrown into a box somewhere from days past.

-- Tom, N5EG
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KE2KB
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Posts: 633




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« Reply #8 on: November 04, 2017, 05:02:50 PM »

Thanks guys; You gave me some good advice.
I wish I had held onto those DSL filters, but I no longer have any of them.
I do have the two telco filters that were installed in the line about 35 years ago for the same reason. I have no idea as to why the telco installer put them where he did; more than 20ft from the phone jack.

I don't know if I will be able to run my new shielded-tp cable all the way back to the FIOS box. Opening their box might cause issues next time we call for service. I guess I should check with them first.
In any case, I can start my new cable at a point about 4ft from where the FIOS cable enters the house. I think that if I connect the drain wire from my cable, I will run it back to the electric service, and not to the BX cable run that now serves as ground for the phone line.

I think I have a few of those clamp-on ferrites around, and I have some toroidal ferrites of the type T130-2 which I used for my 9:1 un-un when I put up my first wire antenna. But I would really like to buy a "kit" of ferrites (both toriods and snap-on) that would be applicable to HF freqs for future use.
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