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Section 4
Feeders and Antenna

Plugs and Sockets

4G1 Recall that the plugs and sockets for RF should be of the correct type and that the braid of coaxial cable must be correctly connected to minimise RF signals getting into or out of the cable.

Identify BNC, N, SMA and PL259 plugs as shown

 

The two connectors shown above are those which you must be able to recognize. The PL259 is the connector most often associated with HF and VHF coaxial feeder and the BNC with UHF coaxial feeder (but it is also used at VHF). The inner part of the coaxial cable it connected to the centre of the connector and the braid is connected to the outer part of the connector with no short to the centre connector. When the connector is used to connect the feeder to the antenna, the inner part connects with the centre where the signal is coming from and the outer part is connected to the chassis of the Transceiver which is usually at ground potential.

There are two other connectors you need to know abot the N type which is a larger version on the BNC and the SMA which is a smaller version of the PL259

The point to understand here is that the connectors are of different quality and whilst this might not matter at all at HF it does at UHF and to a lesser extent at VHF. At UHF the PL259 would be said to be a "lossy" connector, as it absorbs some of the power trying to reach the aerial. The better constructed, and usually constant impedance of the, BNC is therefore to be preferred at the higher frequencies.

In each of these connectors the centre and the screen must be properly connected at both ends of the cable and there must NOT be a short circuit between the centre and the braid (screen) else the cable will not function properly.

You will want to carry out a resistance check on any cables that you make up to ensure that there is not a dead short between outer and inner conductors (This is assuming no connection to the antenna as you could have an inductive connection which given you a false reading- in this case you need to know that the outer is connected and you may have no other way than by piercing the outer so that the meter probe can be put in contact with the braid and the other probe placed on the outer of the connector, make the continuity reading and when satisfied all is well tape up the small hole in the cable outer).



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