A little info about Antenna Switches

So, what do you do on a rainy Saturday? Go bother another ham at his shack! I bothered two of them! I stopped by Doug K4ROK’s shack and changed his microphone cables to new ones then drove to Dave’s W4EJ and saw his new K4 set up. What a blast it is! In the process of checking his work bench out, I spotted some older rotary antenna switches connected to his radios. Why would Dave have these old antenna switches connected to his new K4? It turns out they are about 90-100 dBm separation between connectors. It virtually totally isolates the different antenna inputs from each other. I asked about what he did with the older Alpha-Delta switches he used to have and he had removed them, placing them in a box. Looking in the box, there were two types of 4 port antenna switches present, one Alpha-Delta-4 and one MFJ-1704.

I asked what was the difference between the two, other than price, as they have the same form factor. Dave decided to show me how to test the separation between the channels.

Step one: turn on the spectrum analyzer, connect one end of the two leads to the scope and the other ends together, shorting out the two leads. Turn the calibration to Zero.

Disconnect the two leads from each other and then connect them to the input and output of the switch.

Turn the switch selector to the connected input, then turn it to one of the other open ports to see the difference in the signal (should drop as there is no signal). This was also true when the open ports were open or shorted out with shunts on them all. Theoretically if there was total isolation, when the selector switch is connected to the input wire, there should be a zero reading. When the selector switch is turned to an open input, there should be no signal (only background noise of the machine).

Here are the results:

For the MFJ-1704 switch there was a 53dBm separation of the two ports, more than adequate to keep two antenna inputs from mixing in the switch causing interference.

The Alpha Delta-4 switch had a separation that put the signal below the noise floor of the instrument (unmeasurable) or less than 75dBm on this set up. Now this is SEPARATION! As you can see, you may have to pay for quality to get the separation you need to get a clean signal into your Elecraft K4, a radio that can hear and detect the difference. If I had a poorer quality radio, it may not make that much of difference as the radio cannot detect and reach the noise floor of the switch anyway, a version of the radio being a little deaf so that the interference from the switch on the signal is not detected.

The older rotary style antenna switch that Dave was using had “N” type connectors on it. The center connector was connected to one of the 6 output connectors by only connecting the center pin of the N connector to the output and not touching anything else achieving the high degree of separation. Makes you think about the quality of the equipment you purchase may limit your station’s ability to communicate. Each piece of the radio frequency path can have significant effects. Every decision about which switches, which connectors to use (N vs VHF), the quality of the cables, the radio and antenna all have significant effects on the ability of your station to communicate effectively.

Happy hunting for high quality parts at a bargain! 73 Peter.

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