A Bright Idea for Wrestling With SWR in the Field
|By Richard Fisher, KI6SN
|The LED-based standing wave ratio (SWR) bridge popularized recently by Dan Tayloe, N7VE, and modified by Charles Lofgren, W6JJZ, continues to get quizzical looks and questions, it seems, wherever it is shown or mentioned.
A recent reference in The ARS Sojourner and the Worldradio QRP column brought several letters asking for details about the circuit -- how it is built and how it works.
The short answers are: Easily and great!
What makes this circuit so popular among QRPers is its amazing sensitivity at low power output levels. When put in line with an antenna tuning unit, the bridge can accurately indicate reflected power into the microwatt region.
For low power enthusiasts, that's important. Operators out in the field have used this SWR indicator to help them get the very most out of the system and terrain they're dealing with.
The accompanying SCHEMATIC shows how basic the LED-based resistive bridge is: three 50-ohm resistance legs in the circuit are combined with the antenna system, which electrically constitutes the fourth 50-ohm leg. If the antenna is not 50-ohms, that's where the antenna tuning unit (transmatch, for example) comes to the rescue.
When the antenna is out of tune, the LED glows brightly. When the antenna is tuned to 50-ohms, all four legs are equal, and current nulls across the sensing unit. The LED goes out.
A double-pole, double-throw toggle switch places the unit in or out of line. This is important, because the circuit introduces significant loss, and is therefore only used during tune-up.
Two 100-ohm resistors are put in parallel to form each of the unit's three 50-ohm legs. A toroid with 5-turn primary and 25-turn secondary creates the path to the LED sensing unit. A germanium diode, 1,000-ohm resistor, .01 disc ceramic capacitor and LED available from Radio Shack complete the circuit.
At KI6SN, a garden variety Radio Shack experimenter's printed circuit board was used to mount the parts -- negating the need to use "ugly construction" or to etch a board specifically for the project.
Here's a complete list of parts, with their quantities listed in parenthesis:
(6) 100-ohm 1/2 watt or larger resistors (brown, black, brown) (1) 1,000-ohm resistor (brown, black, red) (1) 1N34A germanium diode (1) .01 disc ceramic capacitor (1) FT-37-61 or FT-50-61 toroid (1) DPDT toggle switch (1) clear lens 20 milliampere LED (Radio Shack part No. 276-307, 276-309 or 276-066) (1) experimenter's prototype printed circuit board (Radio Shack part No. 276-159a)
While the LED glows very brightly when thereís a mismatch, I was afraid that under brilliant sun in the field it might be a bit challenging to "read." So in the units here, I opted to recess the LED bulb inside the enclosure. A peep hole drilled in the side of the chassis allows the operator to see the LED in virtual darkness -allowing for the finest of tuning to put the LED out.
The accompanying PHOTOGRAPH shows the bare-bones unit without enclosure flanked by a basic version in a plastic box from Radio Shack, right, and a deluxe version with switchable LEDs -- one protruding through the front of the chassis, and another recessed inside the enclosure. I can switch between the LEDs depending on the lighting conditions that exist while I'm tuning for lowest SWR. If it's bright outside, I use the recessed LED. If it's dark outside, the LED protruding through the front panel works just fine.
The schematic shows the fundamentals of the circuit. A HAND DRAWN PARTS DIAGRAM shows the component layout on the Radio Shack experimenter's board. (Note: The drawing is viewed from the component side of the pc board.)
Thereís nothing very critical about the parts layout, and ugly construction or etching your own PC board are certainly options to consider if you don't want to use the Radio Shack experimenterís board.
OPERATION: When you've completed construction, connect the LED SWR Bridge between your transceiver and antenna tuning unit (transmatch). Toggle the bridge's switch to put the unit in line. Next adjust your transmatch for greatest band noise. Then key down your transceiver. Chances are, the LED will glow brightly. Carefully adjust the transmatch to progressively make the LED grow dimmer and dimmer. When the LED is complete out, you've got a matched antenna system.
Some operators have reported that their LED SWR Bridge is so sensitive, it refuses to go out on some bands. In that case, just tune for the dimmest LED reading. Chances are good that you're dealing less with an antenna mismatch, and more with some stray RF.
Also, some users have noted that the LED glows occasionally even when the bridge is switched out of line. This is not a problem, however. Consider it an output indicator in that configuration. It's certainly not doing any harm.
In any event, the LED-based resistive SWR bridge is a bright idea for operators heading into the field.
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Richard Fisher, KI6SN, executive editor of The ARS Sojourner, is an avid QRPer, builder and experimenter living in Riverside, CA.