Here's my experience with the original (1950s- 1960s) Dow Key relays.... These are single pole / double throw coaxial relays used for a rig that usually has its own VFO. The bigger, newer Dow Key relays I've never tried. BTW, two Dow Keys can be used to make a double pole / double throw ant relay for a rig with an RF input - for a common "linear in/linear out" design.
I will try to stay conservative regarding ratings here: They are great for boat anchor power levels like the Apache, DX-100, 30L-1 linear power level (four 811As) , etc. Let's say it's good for a few hundred watts of carrier max. However, when we start approaching 1300W pep, (~300W carrier AM at 110% modulation) especially on the higher bands, they can have problems under certain stressful conditions.
The biggest risk is a flashover from the TX to the RX T/R RF circuit. I have blown out several receiver front ends over the years using antenna relays not up to the job. Failure can cause a major event down the receiver chain. Relay sequencing can help immensely.
I have blown out two of these Dow Key relays over the years. I admit I was pushing them. One failure was using a single 4-1000A in linear service. (two Dow Keys DPDT) That was about 2.5KW pep out - and BAM! Lost a good TS-930 transceiver and Dow Key at the same time. Kenwood repair service wouldn't touch it - said it was hit by lightning...
If you are going to be exceeding 1300W pep out, might as well be safe and get a bigger relay. The one pictured above in Ed/ K8DI's link, a 240VAC Potter Brumfield 24VDC relay coil DPDT, is a beauty for QRO. The limitation is the 240 VAC voltage rating being 350V peak at 50 ohms. 700V = 5KW pep at 50 ohms, so 3-4KW pep is about it max. I run these in MOST of my rigs, regardless of RF power level. They have never failed yet. The connections are not as short/direct for 20-10M as a Dow Key, but WTF. I also use these Potters for my 240 VAC primary step start and other heavy AC switching.
I consider the risk of a TX to RX flashover between poles as the biggest potential problem. Carefully inspect the TX/RX relay path isolation before using any relay for the ant switching job. These Potter-style relays have ample arm poles and contacts spacing to handle maybe ~4KW pep, so you should be quite safe with a 2500 watt pep output rig.. Remember that it's important to have an ample safety margin in case you try to load up the wrong antenna, an ant leg comes down in a storm - or an antenna connector fails, etc; events causing the antenna swr/voltage to soar.
* Use a relay with
good contact spacing as well as the relay coil being far enough away from the contacts and arms so not to generate an arc path when stressed.
Standardizing the entire shack for 24VDC coil keying is a good idea too. Surplus 24V relays are very common.
** And, for eagles who dare,
vacuum relays for antenna switching are the very best way to go when doing QRO. I use vac relays in my three homebrew plate modulated AM and linear 4-1000A rigs for antenna switching. Never had one fail in over 40 years.
Tom, K1JJ
