By Richard Fisher, KI6SN
QRP Columnist, Worldradio
Reprinted with permission
from the July 1997 Worldradio


Forget about the pot o' gold. An efficient antenna is the real prize at the end of the New Jersey QRP Club's Rainbow.

Designed by Joe Everhart, N2CX, of Brooklawn, NJ, and a team of radio amateurs from NJ-QRP, the Rainbow antenna tuner/SWR indicator is a handsome kit developed specifically for 30- or 40-meter QRP operation into end-fed half-wave antennas.

The Rainbow was co-winner in NorCal QRP Club's 1996 Dayton Design Competition, coordinated by veteran QRPer Doug Hendricks, KI6DS. Everhart credits Hendricks with providing NJ-QRP the encouragement to offer the Rainbow as a kit.

It's another example of the outstanding contributions regional QRP clubs are making to the low power community worldwide.

The project derives its name from four tiny LEDs (light emitting diodes) -- red, orange, yellow and green -- that collectively show the way to a matched antenna. Ironically, a full display of color is not a sign of peace and harmony where this Rainbow is concerned. Four lit LEDs means your SWR is somewhere in the 3:1 to 5:1 range. Not good. And if all four are aglow with the red at particularly high intensity, you're above 5:1, and headed for meltdown.

The goal is to reduce the display so only the green LED is illuminated. A simultaneous show of orange, yellow and green means you're in the 2:1 to 3:1 SWR range. Just yellow and green indicate 1.5:1 to 2:1. If it's green only, however, you're in business -- the SWR is less than 1.5:1.

During tune-up, the operator adjusts the Rainbow's mica trimmer capacitor and searches for the optimum inductor tap, working toward a good match. The unit will do just fine in power ranges from 200 milliwatts to 5 watts.

The tuner itself is a simple parallel tuned resonant circuit featuring a multi-tapped toroid output. It was designed specifically for use with half-wave end-fed wires in the 7 to 10.15 MHz range, converting the antenna's high impedance to 50-ohms through the inductor. The Rainbow's small screwdriver-adjusted mica trimmer then tunes out unwanted reactance.

A clever absorptive-resistive sensing circuit is employed for the unit's SWR bridge. It's only used during tune-up, however, as it presents a 6-dB loss if left in line during on-air QSOs. A simple DPDT switch toggles the circuit in or out. And if you've inadvertently left the bridge in line, the LEDs will blink in rhythm with your CW as a reminder to switch the bridge out.

As is commonly found in absorptive-resistive bridges, the Rainbow's SWR circuit consists chiefly of a handful of resistors and diodes, forming voltage dividers and performing rectification. It's an LM339N comparator, however, that crunches the math. This 14-pin chip, working in concert with some precision resistors, simultaneously compares forward and reflected voltages and then tells which LED to light and when, indicating SWR.

A power source between 9- and 12-volts DC is needed to power the bridge circuitry. Since the sensing unit and LEDs are powered only during brief tune-ups, a tiny 12-volt alkaline cell available from Radio Shack, works great, and will last a long, long time. There's no external ON/OFF power switch needed because the Rainbow only springs into action when it senses the applied transmitter power.

Don't be fooled by the seeming simplicity of the Rainbow. There's some pretty sophisticated electronics happening here. Consider this from the Rainbow's manual:

"The rectified forward RF sample voltage from the bridge is used to turn on (a VN10KM field effect transistor) to energize the rest of the circuitry. A positive voltage here turns on (the VN10KM and a 2N3906), applying the battery voltage to (the LM339N) . . . and to the green LED. It is also fed to the negative inputs of (the LM339N) through a resistive voltage divider.

"When the positive input of a comparator section is more positive than the negative input, the output is an open circuit, so the corresponding LED does not light. When the positive input is less than the negative input, the output goes low, turning on the LED connected to that comparator section." Is this cool stuff, or what?

For all intents and purposes, the switch placing the SWR bridge in or out of line serves as the power switch by default.

The Rainbow tuner, SWR bridge and battery are all mounted on a spacious silkscreened printed circuit board just 3-inches long and 2-inches wide. The only off-board parts needed are an enclosure, antenna binding posts, a DPDT switch, transmitter input jack, battery and battery holder -- all available from Radio Shack. You'll also need a short piece of small coaxial cable to form the switchable jumper between the SWR bridge and tuner. I used a 4-inch length of RG-174, and soldered it out of the way on the bottom of the board.

The Rainbow's PC board, parts and beautifully illustrated 20-page instruction and operation manual are top quality. There are step-by-step building instructions, a full schematic, graphics showing board layout, tuning capacitor modification details, bridge switching connections, LED illumination legend, installation ideas, antenna configurations and tuner circuit modifications. There are also sections on testing, troubleshooting and the technical details of Rainbow circuitry.

For its inaugural kit project, the New Jersey QRP Club has done a superb job.

The Rainbow at KI6SN went together in just a couple of hours without a hitch. The board sits comfortably in a roomy enclosure 1.25-inches high, 2.75-inches wide, and 5.5-inches deep. The transmitter input jack and IN/OUT SWR bridge switch are on the front panel. Antenna binding posts are on the back. There's nothing to stop more ambitious builders from putting the Rainbow into even smaller quarters with the LEDs panel-mounted, using an off-board panel mounted tuning capacitor, or employing a rotary switch to change toroid taps. The possibilities are almost endless.

Modifications to the housing of the mica trimmer capacitor and having to wind a tapped toroid make the Rainbow a truly homebrew effort. Putting it together, however, is not beyond the capabilities of about anyone who has some building experience.

The completion of my Rainbow was timed to coincide with the NorCal QRP Club-sponsored "QRP to the Field" contest 26-27 April. Such competitions are great proving ground for new gear.

The night before the contest I carefully cut a length of insulated No. 22 stranded copper wire to a length of 66-feet, 6-inches -- about a half wavelength at 7.040 MHz. The antenna configuration also calls for a quarter wave counterpoise, measuring 33-feet, 3 inches.

The Rainbow manual offers several ideas for configuring end-fed antennas, including the inverted L, vertical and elevated horizontal. A 35-foot pine tree at my operating site, however, made the decision an easy one for me: a half-wave sloper.

One end of the 66-foot wire was hoisted to the top of the pine, while the other end sloped to the antenna post on the back of the Rainbow tuner. Meanwhile, the 33-foot counterpoise was connected to the Rainbow's ground post and stretched along the ground directly beneath the 66-foot antenna.

And with that, the antenna work was done.

Connecting a NorCal Sierra transceiver to the Rainbow's tuner's input, it was time to get down to business. The DPDT switch was toggled to put the SWR sensing circuit in line. At key-down, all four LEDs glowed brightly. Adjusting the mica trimmer managed to darken the red and orange, but I ran out of capacitance to put out the yellow.

A small shorting stub is used to select taps on the toroid. I moved the stub to the next tap, reset the trimmer to about mid-range and keyed-down again. This time the green, orange and yellow glowed brightly, with the red at just a flicker. Again adjusting the mica trimmer, the red went out, then the orange, and finally the yellow. Just the green remained lit. Viola, less then 1.5:1 SWR.

For kicks, I continued turning the trimmer adjustment clockwise and the yellow came back on briefly. Backing off put the yellow out again, with green the sole survivor. Sharp tuning, indeed. But a very good match. No ambiguity here.

The whole tune-up procedure took about three minutes. And that's good. The contest was on, and it was time to see what the Rainbow could do.

Band conditions on 40 meters were not good. Static and wave absorption were high, signals were down, and the Sierra's 900 milliwatts was going to need all the help the Rainbow and end-fed antenna/counterpoise could give it.

In all honesty, my expectations weren't very high. But 39 QSOs and six western states later, I was a believer. The Rainbow performed marvelously.

The log shows that signal reports ranged from 339 to 599, and I worked darned near everything I could hear -- including QRP contest stations in New Mexico, Idaho, Washington, Nevada, Arizona and Northern California from my Southern California perch. Not bad for several hours of battery-powered, daytime milliwatt operation.

The Rainbow is sure to open the doors of backwoods QRP to many operators seeking simple, lightweight and efficient antennas for backpacking operation. It's well suited, too, for the apartment dwelling amateur who requires low-profile configurations. Just 66-feet of magnet wire and a good ground can be pretty inconspicuous.

And given its peformance in "QRP to the Field," I wouldn't hesitate using the Rainbow with an end-fed wire permanently at a home location.

The Rainbow tuner/SWR indicator is available from the New Jersey QRP Club for $25 (plus $3 shipping in the U.S.; $5 DX). To order, write: George Heron, N2APB, 45 Fieldstone Trail, Sparta, NJ 07871 (e-mail: Make checks payable to George Heron.

If you've been looking for a little splash of color to spruce up your QRP layout, NJ-QRP's Rainbow may be just what you're looking for. It's a great performer in an inexpensive package. A bright idea, indeed.


Last Modified Aug 1, 1997 - George Heron, N2APB (
Last Modified Aug 1, 1997 - George Heron, N2APB (