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TUCoPS :: Radio :: ham032.txt

Monitoring cordless phones




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


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