PHONE RFI ON 160/75 METERS
I quickly found when I went to 1000 watts on 160 and 75 meters that my telephones were susceptible to fairly significant RFI problems. It was bad enough that the kitchen phone with the built-in digital message recorder would ring all by itself whenever I keyed down on either 160 meters or 75 meters. The other phones would squeal and buzz and the phone in the station room would not even recognize its own keypad tones when attempting to dial a number. Obviously the lines were loading up nicely with RF at the higher power. There was no intereference running at 100 watts output. The antenna is a halfwave on 160 meters with open wire feed to allow tuning on 160, 75, 60 and 40 meters. The antenna is fed through a T-Match network to 18 feet of RG-8 leading to an outdoor Amidon 4:1 balun (HBHT-200 10KW) mounted on a 10 foot steel mast clamped to two 8 foot ground rods in the backyard and coupled to several 100+ foot ground wires that wind their way through the soil in the woods. 85 feet of open wire line leads up to the feedpoint. The station is well grounded so this was definitely a proximity problem. The antenna is up at 70 feet at the feedpoint, but the ends hang down in inverted vee fashion to about the 40 foot level. The house is within 30 feet of the antenna at some points.
So, I tried a few remedies:
1. I checked all the phone line connections, and rewired a few spots that appeared to be flakey. No improvement was noted.
2. I ran .01uf (1KV) caps from the red and green leads to ground at the entry point block. No improvement was noted.
3. I confirmed that the black and yellow twisted pair were connected to ground, and ran .01 (1KV) caps from red and green to ground at every junction box in the house. No improvement was noted.
4. Wrapping the lines 10 turns through a number of split ferrite cores did not make the slightest difference.
5. So, reading the reviews at EHAM I trotted off and picked up a Radio Shack Telephone RFI Noise Filter (CAT # 279-151) and placed it inline with the spontaneously ringing phone. No improvement was noted on 160 meters, but the interference was lighter on 75 meters.
6. Being curious, I opened up the Radio Shack filter and found that it had what looked like a resistive pad with two unmarked capacitors installed. A closer looked revealed that the resistor-like devices were actually small chokes. Obviously the component values were not designed to deal with RF below the 75 meter band.
7. Out of frustration, I did what every phone tech has told me never to do. I put a .01uf (1KV) cap across the red and green lines at the entry point block. Almost all the RF was gone from the lines! The spontaneously ringing phone stopped ringing, but I could still hear myself on voice peaks on all the phones. I put another .01uf (1KV cap) across the kitchen phone connection block and all the phones on the first floor began to behave normally. Putting a .01uf (1KV) cap at each phone cleaned all of them up perfectly. All the phones functioned normally, but I was still curious about finding an easy filter solution.
8. While wandering the aisles of Home Depot, I spotted the G.E. "DSL Phone Line Filter" (Model # TL26249). As I recalled, DSL utilizes RF between 10kc and 1 mhz. The upper end of that is not too far from 160 meters! So I grabbed one, (they cost a little less than 8 bucks). When I got it home and opened it up, it was built exactly as I would build a filter to take RF out of a phone line. A heavy inductance inline with red and another inline with green. A cap was placed across red and green at the output of the filter. I left my already installed .01uf (1KV) caps at the entry block from red and green to ground, but removed all the .01 uf caps that were across the red and green lines throughout the house. Then I installed one of these filters inline with each of my phones (5 total). All my RF problems were gone! These filters are cheap and they choke 160/75 and 40 meter RF completely! I don't know how they handle RF at higher frequencies, but they are awesome for clearing up low frequency RF on your phone lines!
The Radio Shack filter costs just under $15 and does not cover 160 meters. The G.E. filter costs under $8 and will block 160 meter RF.
Note: During the winter of 2008 I switched from ATT phone service to Cox Communications cable phone service. The first thing that I noticed was that even with all my filters in place, I had considerable phone interference on 160 meters once again! The clue to the solution was that the level of interference was exactly the same on every phone in the house. This suggests a common mode source problem with the new equipment Cox installed at the service entry. This was readily resolved by grabbing a block of ferrite and running about 10 turns of telephone cable through it and then mounting it at the service entry. The problem went away completely. Apparently common mode RF was flowing right over my service entry filters and entering the Cox installed phone modem. The choke keeps the RF out and the problem is gone.
Path: Service entry block at left > through ferrite block > .01uf bypass to ground > DSL Filter (seen at top) > distribution terminals at right
Ferrite block with 10
turns of
phone wire to eliminate common mode interference