Goal: To change the length of a wire doublet via remote in-line switches (i.e. without traps).
Potential constraints - rf arc over
Probably the biggest issue... can be handled at low power fairly easily. At high power much harder.
- contact current ratings
Probably not a huge deal for any but the shortest antennas loaded very efficiently.
- wind resistance
Similar issues to traps, maybe a bit better if you can keep the voltages down, like in low power situations.
There have been a couple of cool designs in QST that do this... one with logic circuits and relays, the other with pneumatic switches. But I think they're 'cool' mainly from a creativity standpoint. Traps are a very good solution for a lot of reasons. I've done antennas with relay switching; I have one I'm using right now:http://www.n3ox.net/projects/2017moxon
and it has occasional problems on RX. It's relatively manageable, especially after I sprayed relays with contact cleaner but it's not super reliable. And my relays don't have to stand off the kind of voltage you see at the end
of a resonant section.
I've built a lot of antennas with switches in them, even going pretty fancy (http://n3ox.net/projects/stepperswitch/
) and over time I've come to prefer antennas that don't require any switching. Just eliminates a lot of points of failure. The new rotary vertical switch is very reliable so far but it has some power handling problems on 160m. Arcs from the switch contact to the switch shaft above about 450W. If I need a single-vertical low band solution at another QTH I am probably going to try to build a super trap vertical with very high quality traps.
Any time you're relying on switch contacts at a high voltage point you may be in for a tough time. And you have to keep a controller circuit alive. No problem if it's a manual unit that's just a bunch of mechanical switches; more of a problem if you want to computer control it.