Seeing recent discussion by online experts insisting that power relays are not suitable to RF prompts an interesting and relevant application of a good antenna analyser.
Above is a sweep of an A/B changeover relay intended for HF application at up to 100W and lowish VSWR. The sweep is actually from 1 to 61MHz to be confident that there is not poor behaviour just outside of the HF range that might present on another implementation of the same design.
In fact, the InsertionVSWR at 30MHz is just 1.05, better than lots of HF antenna switches, and InsertionVSWR remains quite good to 54 MHz (top of the 6m band)
Now we hear about power relays and inductance, low voltage withstand, yada yada yada… but have these experts made measurements to support their position, have they opened their minds to cost effective solutions?
Above is the changeover relay that was measured. It is an ordinary power relay rated for 5A per contact set at 50Hz (it will be lower at RF) and there are two parallel contact sets, and withstand between contacts of 1400V.
Key to it’s low InsertionVSWR is the compensation provided by the short coax stub connected to the common coax jack as explained at High frequency compensation of T/R relay.
This is where the analyser comes in, finding the optimal length of stub can be done by cutting a stub to longer than needed, and trimming it down whilst sweeping the InsertionVSWR until the response suits the application.
- Duffy, O. May 2012. High frequency compensation of T/R relay. http://owenduffy.net/transmissionline/concept/RelayComp/index.htm.
- Duffy, O. Dec 2012. Another Morse beacon keyer – A/B RF switching. http://owenduffy.net/module/smbk/AB.htm.