Connector types

I’ve talked about the importance of matching the impedance of the transmitter to the aerial: an impedance mis-match causes energy to be lost and can reflect energy back into the transmitter amplifier, potentially damaging it.  Impedance mis-match can occur anywhere there is a connection – such as the lead from the transmitter to the aerial – and Radio Frequency (RF) connectors are designed to give a continuous impedance such that the connector is as invisible to the RF as possible.  There are, inconveniently, many different types of connector, but it seems that the PL-259 (sometimes called M-type) plug is commonly used for HF radio. This is the ‘male’ plug that mates with the ‘female’ SO-239 socket.  This connection is not self-evident.  BTW usage of these terms male and female predate today’s ‘genderist’ sensitivities by possibly a century, so I hope no-one is offended.

Coaxial Cables

The connectors are designed to be used with 50-ohm co-axial cable rather than the 75-ohm cable used by domestic TVs.   Cables vary in terms of the thickness of the conductors and the dielectric (the insulation between the conductors), and you need to choose one that’s compatible with the connectors, otherwise you’ll have difficulty in fitting the plug perhaps causing a reflection loss or even a ‘noisy’ connection that creates crackling or even interrupts the connection.

Cable losses

There is a baffling variety of co-ax but I’ve chosen an RG174 AU which says it complies with a military standard.  It has an attenuation of 17 dB per 100 metres at 50 MHz, which would be high if you needed a long run. [A loss of 10 dB means that the power is reduced to one-tenth of its starting value and a loss of 20 dB means that the power is reduced to one-hundredth of its starting value, a massive drop.]  In fact, the cable is only 3 mm diameter, which is rather thin, so I hope it will be alright.

I probably should have gone for RG58 which is 5 mm in diameter and has a lower attenuation (11 dB per 100 m at 50 MHz). The supplier will fit the plugs, so I’ve gone for PL259 male plugs on both ends, as the equipment has SO239 sockets which mate with them.