KB9II's QRP Antenna Bridge
I thought it would be a good idea to put in a few schematics for accessories that I've found useful here. One of these is an antenna bridge for QRP applications. I know of a few "thru-line" or wattmeter styles, but most SWR meters can't really go down to the low powers in QRP work. Anyway, WB9SKE/W9FY had an old RadioShack SWR meter, so I pulled a "circuit-ectomy" on it and put in the bridge circuit. I added a DPDT switch to allow "straight-through" operation to give me 6 dB more power to the antenna. You could just pull the bridge out of line, too. Notice that I didn't use a Wheatstone Bridge with three 50 ohm resistors, but instead I used a "resistive splitter" to equalize the SWR mismatch at open or short loads at 1.6:1 at the input. With that load, the rig is always happy during antenna tuneup, and the bridge can be switched out when finished or left in-line if the antenna isn't a very good SWR. The higher wattage resistors were made by paralleling larger values, like two at 100 ohms, or two at 33 ohms.
