This circuit, from the 70's, can be assembled with a more modern FET like the BF245. The circuit is powered by a 9 V battery, as its consumption is very low.
When the voltage exceeds the value given by the zener, this circuit from a 1990 documentation...
This circuit for the 11-meter band, operating at 29.255 MHz was found in a Radio Electronics dated...
Found in a Popular Electronics from the 9s, this circuit has output for RS-232 interfaces and current...