While browsing around at the Dayton Hamvention, I found an incredibly cheap LC antenna tuner -- the MFJ-16010. For only $15, I picked up this little tuner in hopes of using it when operating QRP portable. Only problem? No SWR meter!
Inspired by the NorCal QRP Club's Tuner Upper, which uses an LED as an SWR indicator, I began looking for a way in which I can incorporate this into my newly-acquired little tuner.
As it turns out, this can be accomplished with a very simple Wheatstone Bridge circuit. The transceiver input acts as the signal source, while the antenna output acts as the 'fourth leg' of the bridge. An LED indicator circuit, coupled using a toroid transformer, indicates the mismatch -- the brighter the LED, the worse the match. The goal is to get that LED to go out. The circuit is switched in or out by the use of a DPDT switch.
Here's a schematic of the circuit so elegantly drawn by W6JJZ -- a modification of N7VE's design. The input comes from the variable inductor, the output goes to the transceiver SO-239 connector.
For good outdoor visibility (and because they're just so damn pretty), I choose to use a high-brightness blue LED. All other parts are as specified in the schematic.
Here are some inside shots of the modification:
...and the final product:
Last update: 2006-06-01