When communications performance is not what is expected, the transceiver or, more often the antenna are blamed as prime suspects. However, quite often the humble feed line is to blame.

As in all networks, the system is only as efficient at the weakest link, and if the antenna feed is not transferring sufficient power to the antenna, the required signal strength will not be there. On the other hand, matching the best available cable to the job in hand will let your transceiver and antenna give you the performance they were designed to provide.

RF Coaxial Cable, used for signals above 3 or 4 MHz, consists of an insulated central wire or wires, surrounded by a metal braid or foil. Normally, the central wire is shielded electrically by connecting the outer braid to the common line of the circuit (earth). It is available in differing impedance values with both flexible and semiflexible formats. Shielded Cable, although similar in design to Coaxial Cable is not manufactured to the same high specifications, being designed for audio and video frequencies.

The dielectric material between the conductors largely determines the power capability and loss characteristics. The larger the dielectric thickness and conductor size, the higher the power capability.

So, depending on the type of insulating material, as the cable diameter increases less power is lost. The inner conductor is normally stranded wire or solid copper with the size often given as number of strands/diameter. (eg. 17/0.19, meaning 17 strands, each 0.19mm diameter). A single or, more effectively, a double layer of copper braid, or solid aluminium forms the outer shield and a PVC jacket provides protection from dirt, moisture and chemicals.

When a cable is connected to a device with a different impedance without being matched not all of the power is transmitted across and the signal is weakened. The unabsorbed current flows back along the line causing losses to occur. Radiation from the cable can even have an affect on the designed antenna radiation pattern and cause interference between adjacent systems. Normally the transmitter/receiver determines the impedance to be used. If the antenna is not already matched to this frequency (ie via a balun or transformer), this can be achieved by an ATU, as is normally the case in HF antennas. This is why an HF antenna will require an ATU even if it is loaded to the correct length for frequency.

As a rule, 50 ohm cable is used for transmitting (VHF/UHF, 27MHz and Auto Tune HF) and 75 ohm cable is used for receiving. (The Moonraker Standard 300 ohm TV Halo is matched to 75 ohm via a balun.)

While breakdown and heating (voltage and power) are important considerations, maximum power potential is determined by copper and dielectric losses rather than breakdown voltage, as for every 3dB loss the power radiated is halved. Therefore, applying 1kW to a cable with a 3dB loss will give only 500w out. Thermal insulation provided by the dielectric and outer jacket of the cable ensures that the lost 500w is dissipated in the cable. This means that a RG58/U cable can be used in a 50 ohm system when it has a theoretical pre-loss rms capability which would otherwise melt the conductors.

As the frequency increases, power capability decreases due to increasing copper loss through skin effect and dielectric loss. Running longer than recommended leads in VHF and UHF installations, will result in serious loss of radiated power, unless special low loss cable is used to minimise power loss. If long leads are unavoidable, this needs to be taken into account by increasing transmitter capability and antenna gain and/or using low loss cable.

Cable reference charts normally list nominal dB loss at different VHF/ UHF frequencies per 30.5m (100ft) of line. For example, a 350ft RG213/U cable having a 2.6 dB loss per 100 ft at a particular frequency will have a loss of 9.1dB (3.5x2.8) which leaves only 6.25w out for 50w in. However, a low loss heliax cable (½ in) with a loss of 0.8dB/100ft at this frequency will only have a loss of 2.8dB (3.5x0.8), giving around 25w out from 50w in. (As half the power is lost with every -3dB, the power radiated decreases from the initial 50w to 25w at -3dB, 12.5w at -6dB and 6.25w at -9dB.)

In addition, power capability can be lost over time through the inner dielectric reacting to moisture and chemicals. Naturally, moisture ingress is of concern in marine installations. This is why the inner copper conductors in Moonraker antenna coaxial lines are tinned to help minimise these effects. Both the 50 ohm and the 75 ohm cable have tinned copper inner conductors and tinned copper outer shield, the 75 ohm cable being manufactured especially to Moonraker custom specifications.

Excessive RF operating voltage in a coaxial cable can cause noise generation, dielectric damage and eventual breakdown between the conductors. The voltage in the line can be determined by using the following formula:

So if the input power is 800w and the impedance 50 ohm, the developed voltage will be 200v. However, this assumes that the VSWR is unity 1:1. If it is not you will then need to multiply the developed voltage by the square root of the actual VSWR. Therefore, the same cable with a VSWR of 3:1 will have a maximum potential of 346v, provided the line is at least an electrical ¼ wavelength long (200v x square root of 3 = 346v).

Points to Note:

• Run signal wires as short as possible and locate away from heat sources, power transformers and any high energy areas of the circuit.
• Teflon coated cables are normally used for high temperature applications where PVC is not suitable.

Moonraker coaxial cables are chosen with regard to suitability for environmental extremes and are manufactured to military specifications.