Do you have a "Rainbow Tuner"? You know, one of those nifty little end-fed half-wave antenna tuners with an SWR bridge that uses multiple LEDs to indicate the antenna match? If you do, you'll know that it's just great as a portable companion for field operations with any of your QRP transceivers ... except that it's only good on 40m or 30m.
Well, Rainbow Tuner designer Joe Everhart, N2CX has determined a way to extend the range of his Tuner by adding a couple of simple components ... now you'll be able to use the Rainbow Tuner on 20m and 80m too!
The NJ-QRP Club has obtained a quantity of the parts (especially the hard-to-find mica trimmer capacitor) and is willing to share that find with QRPers and homebrewers. N2CX is allowing the club to provide the parts as an inexpensive kit ($7 post paid) that is easy to assemble and quite flexible in how it can be mounted to the original Rainbow Tuner. Ordering instructions for the Add-On Kit are located at the bottom of this page.
OK, What is the AOK?
The Rainbow Add-On Kit (or AOK for short) is a set of components used to extend the operating range of the Rainbow Tuner. The AOK is designed to work with the tuner by itself or as installed in the KE6RIE "Rainbow Enclosure".
The Rainbow's tuning range can be easily extended to 20 and 80 meters by changing the effective value of the parallel tuned circuit components. This is accomplished by connecting either an inductor or capacitor to increase or decrease the tuned circuit's resonant frequency. The AOK components can be connected (individually) across the tuner's ANT and GND terminals to effect this increase in range.
A toroidal core and #26 magnet wire are used to make an inductor which will increase the tuning range to cover the amateur 20 meter band. A 400 pF mica compression trimmer is needed to enable 80 meter operation. Note that as with the "stock" Rainbow Tuner, the antenna used must be a half wavelength or multiple thereof to be within tuning range. [Further background and construction information is contained in the AOK article printed in Jan '99 QRP Quarterly.]
Here's a photo of the assembled AOK ... parts provided in the kit include: toroid, magnet wire, mica trimmer cap, perf board, rubber feet, and a copy of the AOK construction article as published in QRP Quarterly for January 1999. (Note: Connection terminals and wires are options left up to the homebrewer.)
In Case you Missed the Rainbow Tuner
The Rainbow Tuner was an award-winning N2CX design and Jersey QRP Club project offered as a kit for several years. The Rainbow Tuner is a simple combined tuner and SWR indicator tuner designed for use with end-fed half wave wire antennas on 30 and 40 meters. The tuner portion is a tapped parallel-tuned resonant circuit that transforms the high impedance of the end-fed half-wave to 50 ohms so that common coaxial feedline can be used. Its tuning range is limited by the capacitance range of the compression mica trimmer mounted on its pc board. The KE6RIE enclosure kit is supplied with a panel mounted variable capacitor with a similar tuning range. The Rainbow Tuner is no longer available but full details (including manual, schematic, application notes and reviews) can be found on the Rainbow web page on this website.
You can view or download N2CX's AOK article published in Jan '99 QRP Quarterly via the PDF file. (Be patient ... it's almost 1MB in size!)
