Topics:
| 9.1 | Selective Calling |
| 9.2 | Modems |
| 9.3 | Facsimile |
| 9.4 | Electrocardiographs |
| 9.5 | Morse Code |
| 9.6 | Linear Amplifier |
| 9.7 | Aerial Tuning Units |
| 9.8 | Remote Controlled Coaxial Switch |
| 9.9 | Telephone Handset |
| 9.10 | Boom Microphone |
In addition to the basic facility of transmitting and receiving speech a radio network of HF SSB transceivers can be used as the medium for transferring other forms of information. These can include teletyped messages which are typed out on a teleprinter by the operator for transmission and which are received and printed out automatically. Computers can be linked by radio and enabled to "talk to each other" and pictures can be sent over radio links. All these facilities are usually provided by additional "add on" equipment which simply plugs in to the basic transceiver. Note that SSB transmitter power rating is in watts pep, the equivalent continuous power output will be less than half that figure. So when continuous tones are being transmitted e.g. using a modem, the transmitter output must be kept within the specified limit.
Selective calling systems in various forms have been in use on radio networks for many years. The facility provided can be likened to that of a telephone in as much as you are able to cause a bell or buzzer to ring at a selected station by transmitting its individual calling code. For your HQ station to be able to call each station in the network individually you will require a selective calling unit with a different calling code for each station. This unit will also be capable of receiving the code and activating its own bell. All the other stations will require a unit to receive only their particular code and also to be able to transmit the code. The extent of the calling facility can be varied according to your needs e.g. the HQ station could call all stations at one time or each station could have the same ability as the HQ station etc.
Units are available which can be plugged into an auxiliary socket on the transceiver through which all the connections are made. Other units are battery powered, give an audible output and have to be held in front of the microphone whilst the transmit button is pressed. To be able to receive a call the transceiver must of necessity be witched on and have the aerial connected. The transceiver can be left switched on all day with the loudspeaker off to await a call. Care must be taken to disconnect the aerial etc. when lightening is near. Calling units which use a series of tones are best suited to AM and FM transceivers but can be used on SSB provided that the transceiver is tuned within the limits stated e.g. + or - 500 Hz. Other units transmit a tone which is pulse width modulated and suitable for AM, FM, and SSB transceivers. The calling signal is transmitted for some 5 to 10 seconds and the units give very few false alarms.
The word MODEM is derived from MOdulator/DEModulator. It will produce the necessary modulation, usually 2 tones to enable data to be sent over an audio channel i.e. a telephone line or a radio channel. The unit also demodulates incoming tones and converts them into a data format. There are various pairs of tones and different durations of tones used in the many signalling systems. Therefore it is necessary to ensure that the modem at each end of the link and the transceivers are capable of handling the particular signalling system. Where a modem is used with a SSB transceiver link a tuning aid is necessary to enable the clarifier to be adjusted to set the tones at the correct pitch.
A facsimile or fax machine converts a static black and white picture into electrical signals which can be transmitted over the audio channel of a radio link. The machine can also receive fax signals and convert them into a picture. The picture is transmitted by frequency modulating an audio sub-carrier e.g. a nominal 1900 Hz which transmits white when at 2300 Hz and varying scales of grey through to black at 1500 Hz.
Low frequency wave forms such as those from some medical sensors e.g. electrocardiographs (ECG), electronic stethoscopes etc. can also be transmitted over audio channels. The same method is used as in facsimile transmissions see para.9.3. ECG waveforms have been transmitted from Africa to America on frequencies allocated to amateur radio operators. This transmission method also enables you to have "a stethoscope a 1,000 miles long".
The international morse code covers all the letters of the alphabet and all numbers. This code was well used in the early days of wireless communication. Today its use has greatly diminished. When a signal is weak or when there is a lot of interference you can hear and distinguish the dots and dashes of the morse code when you are unable to understand the speech. The operators at both ends of the link will have to be able to send and receive the morse code at a sensible speed. The minimum speed for a useful link would be some 8 words per minute. A morse key and possibly a tone oscillator will be necessary.
The transceivers mentioned so far have produced power outputs up to 150 watts pep. A linear amplifier can be connected between your transceiver aerial socket and the aerial itself to amplify the 150 watts pep to some 500 to 1,000 watts pep of transmitted power. A linear amplifier producing 1,000 watts pep will require from its mains or battery supply an average input power of some 500 watts. However if this power is obtained from a 12 volt source e.g. battery, then an average of 40 amps, peaking to over 100 amps would be necessary. Therefore it is best to only use such amplifiers from a 230 volts or 110 volts supply.
A linear amplifier enables you to transmit more power and to be heard more clearly by the distant station. However you could also cause interference to other users of the same or adjacent channels as well as other receivers within a few hundred metres of your aerial. Linear amplifiers can be left permanently connected in the aerial circuit and only switched on when another station is having difficulty receiving you. When the linear amplifier is switched off the transceiver output is automatically connected straight to the aerial therefore it will still be necessary to disconnect the aerial after using the equipment.
The use of these units have been described in para. 2.6.3. for tuning long wire aerials to a transceiver. They can also be used to tune a dipole or other aerial for use at a frequency for which it was not designed.
Where a transceiver needs to use more than one aerial either a manual or a remotely controlled selector switch can be used. A remote switch will be connected to the transceiver by a multicore cable so that when a channel is selected the appropriate aerial is automatically selected by the remote switch.
A handset which looks similar to those in use on a normal telephone can be connected to the transceiver. The handset will have an additional button or lever switch to provide a "press to transmit " facility.
If it is necessary for the person speaking on the radio to look around whilst speaking e.g. whilst driving a vehicle, then a boom microphone which can be fitted to a pair of earphones should be positioned in front of his mouth.