Manual Coax Switches: The Ugly Truth

21Jul17

“Wow! I just got another transceiver. I’m gonna need a coax switch. I’ll get a used switch off the web.”

Bad idea… Used coax (coaxial) switches come in all conditions. A few will be like new. Others will have burned contacts from surges and arcing, corrosion that can’t be easily removed, worn silver on the switch contacts, slop in the switch mechanism and many other ills. Why would you want to connect your high-quality transceiver through a junk switch that could present a high SWR, or worse?

Quality coax switches have high isolation (typically in excess of 50 dB), low RF resistance (insertion loss), low VSWR and firm, crisp switching. Isolation is very important when switching two or more rigs to the same antenna. RF leakage from one switch port to the other can overload and even damage other radios in the system. As long as the ports are well isolated from each other, that problem is controlled.

Insertion loss reduces your received signal and creates heat (resistive loss) in your transmitted signal. Normally, a few tenths of a dB is easily tolerated. As frequency increases, insertion loss usually does, too. A switch with 0.2 dB insertion loss at 3.5 MHz may be well over 1 dB on two meters! It’s important to look at the full specs.

Switch SWR is also important. We all want our antennas to operate at maximum efficiency. That usually means watching our SWR. A good switch will have an SWR of less than 1.2:1. This rating goes back to insertion loss; the lower the SWR the lower the insertion loss will be, and vice versa. Again, SWR normally increases with frequency.

Mechanical design is also very important. You want the switch to feel solid and to have crisp motion when the switch is used. Avoid lightweight switches with sloppy switch mechanisms. Robust mechanical feel is usually a good indicator of internal construction.

Coax switches have limitations on maximum RF frequency and power. DON’T rely on engineering fudge factors here. Exceeding these ratings can cause outright switch failure and damage to your gear!

 

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