Topband: TX/ RX Antenna Switching

[Tom W8JI]

> If something goes wrong in the RX antenna switch, you don't have to
> worry about the rig transmitting into its own receiver.  That can't
> happen.  The only thing that can happen is that you could damage the
> antenna or its preamp.  The reed relay is just like the one in your
> radio, so it shouldn't be unreliable.  I've never heard of one of
> those failing.

Reed relays, and on rare occasions other relays or relay systems, come in 
two variations.

Some use a single contact bar that moves between  NO and NC positions. These 
are OK, because they can never do two connections at once if something 
fails. The contact carrier is either one way or the other. These are 
potentially good as transfer relays.

Some use dual reeds or dual contact carriers that are mechanically 
independent. If something goes wrong, they can make two connections at the 
same time. They also can have sequence issues where they make two 
connections at once. Some vacuum reed relays have two glass tubes inside the 
coil with independent NC and NO reeds. These relays are questionable for 
anything. They can lock in two contact positions at once, tying all three 
terminals of a DPST together. As a matter of fact, they actually do this for 
a millisecond or so when transferring. These are bad relays for many 
applications.

By the way, some popular radios transmit RF even after telling the external 
relay line to release, and some transmit at the very same time as they tell 
an external relay to transfer. Most radios are good, but not all radios are 
good at timing. Some hot switch on the leading edge, and some on the tailing 
edge.

73 Tom