Topband: TX/ RX Antenna Switching

[Tom W8JI]

> Getting away from what we are discussing now (limiters) and back to the 
> original question... it really is just a few bucks for a relay for a rig 
> (without separate T and R jacks) at the 100W level, to add separate T and 
> R based on the amp key line.
>

........but the line has to be sequenced properly. That can be a problem in 
many systems. To work safely and properly....

1.) The switch has to be wire controlled from the radio (it cannot use RF 
keying). Some radios actually output RF after the relay send line goes high, 
although in most cases direct key and a relay works with a fast relay.

2.) The relay has to be fast. Some radios, if they have any TX line time 
lead, only have a few milliseconds. The relay has to transfer before the RF 
gets there.

3.) The relay has to be failsafe and reliable.

Two very common design flaws are:

a.) manufacturers always want to have an RF keyed system to keep customers 
happy (plug and play), but a COR system will never work correctly. RF must 
be there to transfer the relay, so the relay hot switches every single time 
a TX period starts. Carrier operated relay systems can work if we can 
tolerate hot switching, which always causes broadband clicks or pops and 
wears away at the relay (and PA transistors).

b.) people often put diodes across relay coils. That slows relay release 
considerably over a non-clamped coil.

c.) people sometimes use relays that can weld or fail into two connection 
modes at once (reed relays in particular).

> It's ironic that for most of half a century, the ARRL handbook showed 
> designs for T/R switches, and now those with entry level radios have to 
> build the opposite of the T/R switch to have seperate T/R antennas :-).

It would be far better to go inside the radio and modify the radio, but if 
the radio has enough TX line time lead, an external relay can work. A good 
design would connect the system from radio to the TX antenna (or amplifier) 
with any failure in wiring or power supply, and never allow the radio to 
transmit into the receiving antenna under any condition.

This means there are all levels of ways to do this, and sometimes subtle 
errors (like a poor choice of relay configuration, or even the logic used) 
can cause issues.

I think the diode issue for protection is important as general information 
for anyone wanting reasonable performance. Wideband RX antennas accumulate a 
great deal of signal level.

Average power from every signal in the passband adds, but instaneous peak 
voltage sums from all random signals. This means the peak voltage can become 
very high on broadband systems, ruling out use of simple back-to-back diode 
clamps as protectors. Multiple signals in receiving systems create a very 
high ratio of average to peak power (and instantaneous peak voltage).

I think the safest thing is to modify the radio, if possible.

73 Tom