With so many people signing online, their old phone wiring may only come to a single central location like the kitchen. This has made a rapidly growing market for Phonex and their competitors. They do have a problem, particularly in large homes with long runs to the panel, where shortwave broadcasts are picked up! Because of this problem I got given a set, pretty nice as they cost almost $100. I started to use them in our house so we could have a corded phone in the living room for the computer. Well, they certainly wouldn’t get close to supporting the 28.8k speed I’m used to connecting at. Also, at night we had the same problem with hearing shortwave transmissions on the 3 MHz downlink base->extension. So I decided to do something fun with them – put them on the air!
For the base unit, I desoldered the single turn transformer wire coupling into the power line, and connected it to a 5 meter length of wire, hanging down from an upstairs window. For the extension unit, I also disconnected the powerline coupler and connected it to a 3 meter length of wire. These wire lengths were chosen for convenience, not for tuning. The radiation efficiency was clearly a fraction of 0.1%
I was surprised at how well it worked. Yes powerlines are not designed for HF, but you are on a common conductor pair, not radiating. Also, they had only been coupled by a single turn transformer. Who knows how well the impedance was matched, it may also have been low efficiency.
Using a long extension cord on an outdoor outlet (remember the powerline was no longer used for an antenna) the maximum usable range was perhaps 50-75 meters. That wouldn’t bother anyone, but I was hungry for more range. I wanted to take this thing to 1 kilometer.
Antenna design for maximum phone jack extender range
The problem is the dual frequency nature of the system. The base transmitted at 3 MHz and the extension transmitted at 8 MHz. The VSWR of a dipole resonant at one frequency would be pretty high on the other frequency. I felt any of a fan dipole, trap dipole, or broadband dipole would be appropriate.
Putting out a constant signal on HF is not nice. I decided to go for a short test during the daytime where no one could be bothered.
I decided I would go for a wildly impractical test–connecting the base unit to a broadband 80-10 m dipole. Even though it wasn’t actually usable this way because I had no phone line to the radio shack, it would be a heck of a long range test.
Panasonic RF-2600 receiver
I used my battery powered Panasonic RF-2600 for this test (one amazing radio–given to me in partial payment for helping a guy pack up to move). It takes 6 D-cell batteries and thank goodness for the shoulder strap! It has a 95 cm telescoping, rotatable whip antenna. The Panasonic RF-2600 reception (AM/SSB/CW/FM) is incredible–the guy told me the previous owner used it in Africa to listen to American ball games on battery power!
You can probably guess where this story is going–I set out walking in the field listening on the 3 MHz side. I kept walking…and walking…and walking! I got to 1 km and the signal was plenty strong, quarter-scale on the signal meter. At this point I decided output power must be at least of the 100 milliwatt range used by the Part 15 AM MEDFER transmitters (500-1700 kHz). There was no way I could conscionably put this on the air, especially at night. I wouldn’t want to tie my main dipole up, anyway. I decided to see if I could use the system in our outbuildings with short antennas.
I put the base unit in the house on the short wire antenna, and the extension on the other short wire antenna as in experiment #1. Daytime coverage in the 50-100 m distance is fine, you just have to keep the antennas away from metal. I thought I had come upon a daytime winner, but sometimes especially in winter we work outside at night, so that’s the real test. Well, I don’t have to tell you it was a real letdown. Since the units were susceptible to interference while on conductive house wiring, they were majorly susceptible using actual antennas on-air!
Using the old trick of calling your own number, get the busy signal, hang up and it will ring and you can both pickup and talk as an intercom, we could not connect in the evening due to strong legal on-air users. Thus the experiment had to end.
I restored the units to their original legal condition, and gave them to someone living in a small aluminum-sided house. It must provide enough shielding on HF, because he hasn’t complained. He doesn’t have a computer so just the voice alone is enough for him.