27 Feb 2013 – VHF Tracking – Stan Frey GM8ZQY – GM3HAM – Lothians Radio Society

Stan Frey GM8ZQY

At the LRS meeting on 27th Feb 2013 Stan Frey GM8ZQY talked about VHF Tracking. His comprehensive presentation covered tracking technologies and applications to both commercial (vehicle tracking), and non-commercial uses, starting with tracking the 18 species of bats in the UK, fitted with tiny transmitters operating on 173 MHz (licence-exempt).

HF DF or “Huff-Duff” was used in WWII to track radio transmissions from German submarines from sites throughout the UK including Anstruther in Fife and Bower in Caithness. The Germans thought that 1-minute transmissions would be short enough to prevent DFing but they were wrong! The DF technique used amplitude comparisons between four vertical antennas arranged in a small square, as patented by Frank Adcock. The Adcock – Watson-Watt DF system used N-S and E-W pairs of antennas plus a “sense” antenna to remove the 180 degree ambiguity. The signals were input to a cathode ray tube compass-rose display which showed an angled line from the centre which indicated the direction of the signal.

Coastguard DF used Adcock antennas on VHF e.g. Marine Ch. 16. Stan showed a picture of a marine DF receiver used from 1980 onwards, especially in the RNLI lifeboat service, for tracking ships. It also received the main distress beacon frequency of 121.5 MHz used for marine and aviation distress beacons. Nowadays Global Positioning Satellites (GPS) are used along with beacons on 406 MHz. Measurement of Doppler-shift from GPS satellites allows location to be determined in +/- 2km squares.

Doppler shifting of frequencies is caused by the movement of the object. Rotation of the antenna would be too fast and thus a “pseudo-doppler” effect is used e.g. for the location of missing / stolen vehicles. An encoder feeding a small transmitter on the vehicle allows transmission of a unique ID (identifier). A DF receiver fitted with a decoder outputs a “location indicator”, used by the police to locate the vehicle.

The Lo/Jack company, launched in Boston MA in 1986 uses a patented Doppler VHF stolen vehicle recovery system. It is now in use in 31 countries worldwide. The Tracker Network (UK) Ltd. was set up in the UK in 1993 and tracks vehicles ranging in value from £5000 to £5 million. Over 20,000 vehicles have been recovered at a rate of 2-3 per day, resulting in many arrests.

VHF signals are penetrative into buildings, containers etc, allowing multiple similar suspect containers to be investigated for stolen vehicles, drugs and firearms.

The GPS GSM Jammers, introduced about 2007, have no effect on the VHF signal but block the GPS / GSM signals.

The original Tracker receiver was linked to a VHF High-Band transmitter running on the vehicle 12V battery supply. The antenna required crude tuning but has been replaced by an automatically-tuned antenna wire. Police vehicles use four vertical antennas in a square on the roof while police helicopters have four avionics antennas fitted underneath them.

Masts: Stan showed photographs of the Craigkelly mast in Fife and the mast at Emley Moor in Yorkshire. Both masts are used by Tracker for UK radio network base stations.

Vehicle transmitters: Self powered versions are now available having a battery life of 5 years, while the vehicle powered versions average a current drain of only 800 microamps, when the vehicle is parked, by using a timed receiver.

Plextek (Cambridge) designed F1 racing Telemetry units operating on 3.0 to 4.25GHz with a transmitter power of 1-2W, sending 140 parameters plus voice. Plextek also designed stolen vehicle recovery transceivers. Over 5 million of these units have been sold worldwide.

Hundreds of Repeaters provide detection of stolen vehicles across the UK, even when concealed in a shipping container in a port.

Antenna arrays: the original 1990s design with four antennas on the detection vehicle required four holes to be drilled in the roof. These have been superseded by mounting them on an aluminium plate positioned under the light bar on a police vehicle.

Amateur DF receivers and kits – see list at bottom, provided by Stan.