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Transceiver Coaxial Cable Selector Switch

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If you are going to connect two or more transceivers to a single antenna using an antenna selector switch, be warned, you need to consider how much RF you are dumping into the unused transceiver. There are antenna selector switches that cannot be safely used as a transceiver selector switch. Never assume you can use them interchangeably.

Not all antenna selector switches are up to the task of using multiple transceivers on the switch. They are typically designed for using a single transceiver on multiple antennas.

Two factors need to be considered when selecting a coaxial switch: 1. Output Isolation 2. Grounded Unused Connector

The first factor is the Output Isolation of the signal, which is to say how much signal from your transmission is bleeding onto the unused connection. The better the isolation the less signal will be getting into the unused transceiver. This mostly has to do with the spacing of the internal wiring.

The second factor is the grounding of the unused connector. Most low-end antenna switches do not ground the unused connection. Grounding the unused connection helps to bleed off stray signal to ground rather than into the unused transceiver.

The MFJ-2702 is an inexpensive antenna selector switch that should not be used to connect two transceivers to a single antenna. Although it has suitable output isolation at 70dB or more for the HF band, it does not ground the unused output connector. The MFJ-2703 does more than simply add an additional connector. It also has additional internal parts to ground the unused connectors, which helps mitigate RF from entering into the unused transceiver.

When you are using an antenna switch box as it is intended, to select from two or more antennas for a single transceiver, then these factors are less important in that they are relative to signal loss. When you are using an antenna switch box to connect two or more transceivers to a single antenna, then you take a chance at damaging an expensive transceiver. Don’t use a cheap antenna switch box unless you are certain that it can be used in this manner either by design or by carefully considering its output isolation as well as whether or not it grounds the unused connector. Some coaxial cable switch boxes are advertised as both a transceiver and antenna selector. You should still verify that the unused connectors are grounded rather than open.

Consider the Workman model CX-3 as an example of an antenna selector switch which is barely suitable for its intended purpose. With poor RF isolation of 42dB at HF frequencies and completely lacking a center conductor ground at the unused connectors it isn’t even an ideal antenna switch. If you planned to use it to connect multiple transceivers to a single antenna you risk damaging those transceivers because power from the selected transceiver is going to bleed into the unused transceivers. This would not be a problem at QRP power levels, however, at 100 watts it would be a bad idea. Although immediate damage may not necessarily occur, over time it would inevitably lead to failure of your radios.

Finally, when you are using a coaxial cable switch to connect two or more transceivers to an antenna, be care to observe the switch position before you transmit. Human error can still result in damage to your expensive transceiver. Make sure you never transmit when the switch is not set to the transceiver you are using.

 

-w0dbw