2 meter/70 centimeter Answering Machine for HTs

The plethora of HTs with PL subaudible encode and decode features coupled with the fact that many repeaters are “open” and do not require a PL to access, opens up a new dimension of ham radio. Imagine leaving your HT on all day long and only receiving calls from friends. Come home from a day away and find who called you in minutes.

The Repeater

First, you need to identify a repeater in your area that does not require a PL tone to be used, BUT will retransmit any PL tone sent to it. Example, radio A is tuned to a repeater at 145.150 and is transmitting a PL of 114.8 Hz. This repeater requires no PL. HT B is tuned to 145.150 also, and is waiting to DECODE a tone of 114.8 Hz. Now, transmit using HT A, through the repeater, and see if HT B “opens up,” i.e., decodes the tone of 114.8 Hz. I have found that most repeaters that do not require a PL will retransmit any PL sent to them.

Great, now we can keep our HT on all day long, tone squelched at 114.8 Hz on 145.150 repeater and get calls only from our friends. This is not a new idea. While it may go against the grain of traditional hamming, this setup may be very useful in some cases. I have always enjoyed listening to all traffic on a repeater, but have been in situations where this selective calling is a blessing.

The Hardware

Using a VOX cassette recorder, we could record any activity across our tone squelched radio. The only problem is that any other noise in the room will trip off the recorder. This is not ideal and here’s the solution: using a patchcord from the audio out port of the HT to the VOX recorder’s mic input port, we can eliminate the threat of outside noise tripping our recorder. There is a problem with this, though. First, we can no longer hear the radio’s output and secondly, there ia a huge impedance mismatch. Remember, most audio out ports on HTs are looking for 8 ohms. If we add a high quality communications speaker in parallel with our patchcord, we have solved the issue of an impedance mismatch. The speaker’s 8 ohm impedance wired in parallel with the VOX recorder’s higher impedance will yield a resistance of very close to 8 ohms. This will satisfy the HT’s need for 8 ohms to be present at its audio output port. This speaker also serves the purpose of allowing us to hear what is coming across that repeater.

Finally, you will need to add a resistor in series with this patchcord. The resistor's sole purpose is to make sure any audio laid down on the VOX recorder’s tape is clear and not distorted. The resistor you choose should be a balance between the type of recorder you have and how high you wish to keep the volume on the HT. I keep the volume all the way up. If I am out of the room, I can hear a call coming across the radio. I believe I have found the best resistance for all cases and I will update this article when I am satisfied with the results.

A More Advanced System

At home, I use a somewhat more advanced system. I have created an optical link between my HT and a computer (a Commodore 64). The computer not only logs the time someone calls me, but also activates a blinking LED and a relay that activates a simple paging system, so I am alerted to messages if I am down in the basement or elsewhere on the property. We are working on a similar setup for IBM PC architectures. If you are interested learning more, please email me and I will forward the details.

The Energy Side

Please scan the solar power articles on this page, because the system I have described, including the computer, is all powered by the sun. This system has been operational for 2 years without a hitch. Initially, I needed to watch the water level in the deep cycle batteries, but I have since switched over to sealed gel cells.