Application Note #7
Subject: Antennas and the Rainbow Tuner
Now that the Rainbow Tuner kits are out in the world, I've been getting frequent messages of the form "How can I use my Rainbow Tuner with my G5RV/Hamstick/Windom, etc.?"
Unfortunately, the answer to the question is "You probably can't". You see the Rainbow Tuner is a special purpose tuner designed to be used only with a half-wave end fed wire antenna. General purpose tuners that can be used to match almost any antenna have to be much more complicated (and expensive) than the simple one in the Rainbow.
The intended application is described in the QRPp and QRP Quarterly articles presenting the Rainbow and in the descriptive material on the NJQRP web page (http://www.njqrp.club) Unfortunately, in their excitement to build a cute little kit, some folks have misunderstood that information.
The half wave antenna that the Rainbow is intended for is a very simple wire approximately one-half wave long that is simple and easy to use for portable hamming. An article in the Spring '97 issue of 72, the NEQRP newsletter, will have more info on EFHWA's (End-Fed Half-Wave Antennas). At a recent NJQRP meeting, we cut a 46 foot piece of hookup wire to length, strung it up to a convenient tree as a "sloper" and made a QRP contact between NJ and Chicago all within 15 minutes using the Rainbow Tuner.
The Rainbow's tuner circuitry is set up to transform the half-wave's high impedance down to 50 ohms and has a limited tuning range since the antenna is nearly resonant. It simply doesn't have the capability to cope with unpredictable impedances.
The Rainbow manual *does* include a section describing in general terms how you might go about reconfiguring the tuner for other antennas, but that is a matter for the experimenter.
On the other hand, the SWR bridge part of the Rainbow *is* useful with any HF antenna or tuner fed with 50 ohm coaxial cable. It operates automatically and is self-calibrating to give accurate SWR readings with HF QRP rigs in the range of about 200 mW to 5 W. And it is small enough to be built right into the case of even a rather small QRP rig.
If you have any questions about the Rainbow's capabilities or uses, please feel free to contact me.
Joe E., N2CX
from Southern New Jersey, y'all