W8WWV - Using Cordless Phones with Radios

Greg Ordy


On another page, I mentioned that I like to operate while wearing a headset (headphone/microphone). This style of operating becomes a nuisance when it's time to get up and move away from the radio. Even crossing the room to pick up a book may be awkward, depending upon the length of the headset cord.

It seems like what you want is a cordless headset that will let you operate around the radio without being tied to it.

One of the locals, W8GB, Greg, also headed down this road a few years ago, and he started working on a solution when he began using 900 MHz cordless headphones with his radio. These are the headphones sold for use with stereo systems. This gave him one side of the link, and he could head downstairs for a stop in the kitchen while not missing the witty banter on the roundtable. I happened to have an FM wireless transmitter/receiver that I used with a camcorder, and we combined that with his headphones to make a full duplex system. While it worked, it was a little awkward. You had headphones on your head, a little box with the transmitter in your shirt pocket, and a clip-on lapel microphone. The idea was right, but the implementation was wrong. The system also was a little costly, totaling around $150 (USD).

At the same time, cordless phones were everywhere. These full-duplex radios seemed conceptually ideal for use with an amateur radio, except that they needed a radio interface, and the entire phone dialing portion was not needed. The price was right, however, and dropping every day.

The problem with a normal cordless phone, as I see it, is that by the time you get to the telephone line, the two sides of the conversation have been combined onto one line. An amateur radio has a separate audio output and microphone input. This is an important distinction. Of course within the cordless phone base unit, there will be a separate receiver and transmitter that will have the desired signals. The problem is that the remainder of the cordless phone circuit combines the two onto a single telephone line. Perhaps an interface like the classic phone patch might work, but now things were getting complicated.

A really motivated person could probably take apart the base unit of a cordless phone and isolate the transmitter input and receiver output. With appropriate level conversion, these signals could be connected to the radio output and input. I really wasn't that motivated, however, so this remained an unsolved problem.

The IBM-3330

I was up at my local Sam's Club store, buying 50 pounds of this, and 50 pounds of that. I happened to be walking through the electronics section when my eye caught a glimpse of something a little unusual. It was a cordless phone sold by IBM. I don't think of them as a cordless phone company, especially as the market saturates with low cost units.  I picked up the box, and on the side, it advertised: Computer Compatible. Works with most computer soundcards and audio interfaces. Better yet, on the back panel was a picture showing separate audio in and audio out cables going to a computer.

Finally, somebody built exactly what I was looking for. The IBM-3330 cordless phone, available for $80 (USD) at Sam's Club has a computer interface in addition to a telephone interface. In fact, the base unit has a button labeled Computer. When you press the button, a light goes on, and the cordless headset signals are brought out as separate audio out and in. Press the button again, and the headset is connected to the telephone line. Otherwise, it is a modern cordless phone, operating at 2.4 GHz, and featuring number memories, speakerphone, channel scanning, security codes, and a base/remote intercom.

Even better, the cordless handset part of the phone was implemented as a headset with microphone. This is as opposed to a normal telephone handset.

I already had a cordless phone at home, but I could not resist buying this one (for the radios!).

What IBM was doing was addressing the growing number of people who talk over the Internet using services such as Yahoo Messenger. Until now, they have been using microphone and headsets that were directly wired to the computer. With this product, the user can have a cordless phone for both the normal telephone line and the virtual telephone line implemented on the computer.

The following picture shows the cordless phone, as well as the two interface cables that I made up for my ICOM 706MKIIG and ICOM 756PRO radios. Please click on the picture for a larger view.

IBM Cordless Phone with Cables
IBM-3330 Cordless Phone with ICOM Interface Cables

In reading the box more carefully, I found that IBM does not really make the phone. It is made by TT Systems. They obtained a license to sell it under the IBM name. I'm glad they did that, however, since it was the IBM name that caught my attention.

The ICOM Connection

On other pages, I have commented about ICOM radios using electret condenser microphone elements. Computer soundcards also use electret condenser elements. This is a great situation for ICOM radio owners since they can use all of the products designed for computer use. At most, cables are needed to interface between the computer microphone and the radio.

Since this cordless phone advertised that it would directly connect to a computer soundcard, I was hoping that this implied that it's audio output level was set to mimic an electret condenser microphone element. That appears to be true. If your radio accepts a dynamic microphone element, you might need to reduce the signal level before entering your radio. Otherwise, you might overdrive your transmitter.

I have two different ICOM radios. Each has a different interface. I built up interface cables for both, and they are described in the next two sections.


On the ICOM 756PRO, the only logical interface is the front panel 8-pin microphone connector. That is because it is the only interface that will work with the VOX feature. VOX is essential because there is no other way to key the transmitter. The rear panel audio input signal does not trigger the VOX circuit.

Fortunately, ICOM brought the receiver audio out on the 8-pin microphone connector as well.

My 756PRO interface is made from an 8-pin microphone connector and two, shielded two-conductor cables, terminated with 1/8" (3.5 mm) stereo plugs. All of these parts are available from Radio Shack. In fact, I purchased 6 foot jumpers with molded connectors on the ends. I cut the cables near one end. I seem to build up so many little gizmos with 1/8" connectors that I can always use the ends, no matter how short or long. If I can use a molded end, I prefer that over soldering on my own plugs. They (molded ends) may actually cost less, and I bet that they can take a little more abuse.

One cable connects to the microphone input and the microphone ground. The other cable connects to the audio out and cabinet ground. That's it.

I wrapped some red tape around the radio microphone input cable, since that appears to be the color code that's used to indicate a microphone (in the computer soundcard world).

I mentioned that I used stereo (two-conductor) cables and plugs. The means that the 1/8" plugs have a tip, ring, and sleeve, as opposed to just tip and sleeve (mono). The connectors used with computers are usually stereo connectors. Since I didn't know how the cordless phone jacks were wired, I proceeded with caution, using stereo plugs, but only using the tip terminal. In other words, I do not use the ring terminal.

My 756PRO interface cable is shown in the picture. It's in the lower right corner. I used short lengths of yellow heat shrink tuning to keep the two cables together.


My ICOM 706 interface is even simpler. It's just two shielded 1/8" stereo plug jumper cables. This assumes, however, that you have one of my audio breakout boxes. This interface is described on another page.

The picture shows the jumper cables (lower left), as well as my 706MKIIG breakout box (to the right of the phone).


At this point (August, 2001), I have only used the headset a few times, but it does do exactly what I wanted. I often ragchew with a gang of folks on 3.805 MHz in the late evening. I can put on the headset and roam around the house, and even yard, all while taking part in the conversation.

With both radios, the audio and microphone levels are normal, and I can switch between the cordless phone and the normal microphone/headphone without having to readjust levels.

Issues and Nits

In experimenting with the phone and the radios, several issues and nits have emerged.

Although I was hoping that more cordless phones with computer interfaces (SoundBlaster) would become available, the truth is that they have not. It seems as if talking for free over the Internet has not really become popular enough to drive the demand for these units. One alternative which I became aware of in early 2003 is the Phonebridge. This is interface between the computer (or radIo) and the telephone (cordless in this case). In order to duplicate my situation you would need the Phonebridge and a cordless phone. While this does boost the cost, at least it is available. Hopefully the Phonebridge will be around for some time into the future. Thanks to Gordon, K8KIS, for bringing this product to my attention.

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Last update: Tuesday, February 25, 2003 08:48:22 PM
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