LED-O-METER

The Led-O-Meter gives a visual indication of RF. There are many ways to use it. As a simple tune up indicator (diode probe) on a circuit board when tweaking transmitter stages. Or as a field strength meter when placed near or wrapped around an antenna. I've even used it to tune up my FT817 portable ATU's whereby I have inserted the small pigtail into the output socket and tuned for maximum smoke as the old saying goes. You could build it into an old pen case or tube to make a hand probe or similar.

It should also be possible to use this as a high SWR indicator. Transmit into a dummy load and adjust a variable resistor or trimmer capacitor whilst touching "point x" at the TX antenna output socket so that the LED extinguishes. Now when you have a high SWR the reflected voltage that may ADD to the voltage across the Drain and Source of a FET (or Base and Collector of a BJT) and cause device breakdown will be detected. Note, detection is NOT protection.

If required, you can adjust it's sensitivity by inserting a high value resistor at "point X", circa 100K to 1Meg, which would be frequency insensitive, or a lowish value capacitor. Low values = higher impedance to RF. Anyone remember the days when old receivers had something called an antenna trimmer ? It was nothing more that a low to medium value variable capacitor in series with the antenna. As the capacitance as lowered so would the input signals to the front end. Not sure why they used a capacitor rather than a resistor though.