MONITORING CHORDLESS TELEPHONE CALLS by THE RESEARCHER Written for P-80 Systems 2-19-87 Chordless telephone calls can be monitored on any radio scanner that covers the VHF low band (30 - 50 MHz). During recent experiments, I was surprised at just how far these low power devices can be heard. With a proper antenna these signals will trip your squelch about 1/2 mile away. They become noisey but readable at a distance of .2 to .3 mile. At .1 to .2 mile reception is loud and clear. One tenth mile is the length of a typical city block. Using the simple whip antenna that came with my scanner, I can hear calls within a radius of about one block. By connecting an external ground plane antenna, I can increase that coverage out to about 3 to 4 blocks. Chordless telephones have been assigned frequencies on ten channels. Each channel consists of two frequencies. One for the base unit and another for the handset. My experiments have shown that the base unit puts out the stronger signal. It also carries both sides of the conversation while the handset is weaker and carries only the conversation of the person using it. Earlier this week I decided to find out what I could hear by going mobile. I bought a magnetic mount scanner antenna at Radio Shack and stuck it on the top of my car. I used a cigarette lighter plug to provide power for the scanner. I programmed the scanner with the ten base frequencies and went driving through residential neighborhoods. Everywhere I went I found chordless phones active. I quickly became adept at tracking them down and getting close enough to clearly monitor the conversations. The next day I removed the center loaded whip from the base of the antenna by loosening the set screw which holds it in place and replaced it with an adjustable whip. The improvement in reception was dramatic. With the antenna adjusted to any length between 2 1/4 and 5 feet the reception distance was approximately double that of the original center loaded antenna. According to theory, the optimum length for an antenna of this type when used at 46 MHz would be about 5 feet. In my experiments, I could detect no loss in performance until the length was reduced to less than 2 1/4 feet. For convenient reference, I am listing the cordless telephone frequencies here. All frequencies are in MHz. Keep in mind that the base frequencies are the ones to monitor. Channel Base Handset ------- ---- ------- 1 46.61 49.670 2 46.63 49.845 3 46.67 49.860 4 46.71 49.770 5 46.73 49.875 6 46.77 49.830 7 46.83 49.890 8 46.87 49.930 9 46.93 49.990 10 46.97 49.970