System Description
| The equipment used both miniature valves and transistors in its circuitry. The valves were of the type popularly referred to as 'pencil valves', on a miniature wire-ended 8-pin base. The transistors were a mix of silicon and genrmanium devices. Within each unit, a transistor invertor supplied the valve HT, and most of the other voltages required. The DC supply to all units was 24V positive earth, intended to be supplied from lead-acid batteries. A tap on the battery at 12V also required by the receiver. |
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| Item | Decca type no. | Comments |
| Master Drive Unit (MDU) | 9216 | Used on master station |
| Low Power Transmitter | 9371 | 10W output; Used on master and slave chain stations. |
| Medium power transmitter | 9362 | 40W output; used on long range and permanent sites |
| Receiver | 9217/III | Used for both user receiver and slave stations. |
| Blower Unit | 9217/BM | Cooling blower. Provides base stand for receiver. |
| Distribution Unit (DU) | 9218 | Provides metering and control of power supplies. |
| Blower and Synchro Supply Unit | 9369 | Provides power for synchros in receiver for remote displays and plotter; incorporates cooling blower. |
| Track Plotter | 9232 | Provides continuous track plot |
Click on the links within this table for more information..
Minor units/accessories:
- Receiver antenna and matching unit
- Transmitter Junction Box (TJB)
- Transmitter Antenna Capacitor Unit
- 24V Batteries
- Mains Battery Charger
- Generator charging set
HiFix Chain
The HiFix system required 3 transmitter stations. One of these, the Master, was located centrally between two slaves, and at the intersection of the baseline from Master to Slave 1, and Master and Slave 2. The sites transmitted in sequence on the same RF frequency, as shown in the diagram below:
Click here to listen to simulated HiFix transmissions!
Master Station
The master station consisted of a Master Drive Unit (MDU) and a Transmitter unit. The MDU contained a highly stable crystal oscillator from which all frequencies used in the chain are ultimately derived. The MDU provided RF drive to the transmitter unit, which contained an RF power amplifier.
Slave Station
The slave station consisted of a receiver and a transmitter. The receiver picked up the transmission from the master station, and kept an oscillator in phase lock with it. It also used the trigger pulse to synchronise a timer. At the appropriate time, the slave's transmitter was keyed and radiated the slave pulse.
User Receiver
The same receiver unit could be used either on board ship or vehicle mobile. It picked up the transmissions from the chain transmitters, and gave a continuos readout of position relative to the transmitter sites on decimal digital counters. As an option, the receiver could be connected to a chart recorder to give a historical trace of location.
HyFix Type B
The system described above is referred to as Type A, or Lane Integration working. This type cannot resolve lane ambiguity and so it is necessary to know where you are are starting from, and then the HiFix receiver will count the lanes as they are crossed, hence 'integration'. Loss of signal (which can happen due to lightning noise, for example) means that you must needs back track to your last known position- a marker bouy for example.
HiFix type B, or Lane Identification working, uses a second set of transmitters working on a different RF frequency, but located at the same Master and Slave sites. This allows the lane to be positively identified. The additional RF frequency needs to be about 10% lower than the main one. This is how it works
Consider an area where, on the main channel of 1900kHz, there are 10 lanes. At the lower frequency, 1710kHz, there will be only 9 lanes (because of the longer wavelength). So when the lane counter on the HF receiver reads 1.0, that on the LF one will read 0.9, and so on. If we subtract the LF reading from the HF reading, we find that the difference is always one tenth of the HF reading. If we move the decimal point one place to the right, we have the full lane reading of the HF pattern. Here is an example: HF lanes: 4.50, LF lanes, 4.05. Difference is 0.45. Moving the DP one place to the right results in 4.5.
In this way we can resolve the ambiguity to 10 lanes- we don't know whether the lane number in the above example is 4.5, 14.5, 24.5 so we have to be able to determine this by other means. However, we are still 10 times better off than with Type A!
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