With any radio installation: Amateur, CB, PMR, or just ICE receiver there are 3 basic and separate circuits which the installer needs to consider:
1. The DC circuit, providing the power to the rig. Ideally the wire should be as heavy as is practical to carry the maximum required current with the minimum voltage drop. Once again, ideally both positive and negative are taken straight back to the battery with the appropriately rated fuse in the positive line as close to the battery as possible. The rig need not be connected to the earth of the vehicle, but no harm will come if it is. Often the power supply is through the accessory socket. Power should only be provided to the rig using the proper cable in the CD circuit and not through the outer braid of the aerial coaxial cable.
2. The audio circuits. Sends the sound to the loudspeakers, or in the case of some LR vehicles send outputs and inputs to and from the external equipment such as amplifiers, CD players, mp3 players and microphones. Generally these wires should not be allowed to come into contact with earth. Where microphones are in use, certain conditions can occur where a ‘hum’ can be caused by a fault condition called an ‘earth loop’.
3. RF or aerial circuits. The cable connection and the aerial itself. Ideally the cable should be one single, continuous length of coaxial cable. If for ease of installation the aerial cable really has to be cut then it must be rejoined using a proper pair of coax connectors. Crimped, soldered or taped joints on coax cable just will not work properly. Generally, for ICE systems the impedance of the cable is 75 ohms and for communications systems it’s 50 ohms. The cable is connected to the rig using a suitable coax plug; in most cases there is a plug to match the cable to the socket on the rig.
Often, the most contentious part of the aerial circuit is the ‘radiating element’ itself. Ideally the centre conductor of the aerial cable will connect directly to the aerial itself and the braided outer conductor to the vehicle body at the base of the aerial after removing the paint and cleaning down to bright metal; this connection is sometimes referred to as the ‘ground plane’ connection. There is no need to provide additional wiring to connect this ground plane to the vehicle or battery; for instance when I was doing such installations on fire appliances, the cab roof was often found to be GRP or fibreglass. By lining the inside of the roof with kitchen aluminium foil and making the ground plane connection to it, perfectly adequate performance was achieved.
Some aerial configurations do not need a direct connection to a ground plane, such as a ‘mag-mount’ for VHF, the ground plane connection occurs by capacitive coupling and any mismatch is usually able to be tuned out by cutting.
Once all the connections have been properly made and checked, then the task of tuning the aerial (if required) can be undertaken.
