March 6, 2012

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Return of the Rainbow Tuner!"
 ... A multiband end-fed half-wave antenna tuner and multispectral SWR bridge able to fit into a mint tin! 

This week we'll be extending our previous week's discussion of SWR measurement, this time detailing the principles and techniques of the the "Rainbow Tuner".  This was an easy and inexpensive kit available some 12 years ago from the NJQRP Club, yet is still extremely popular and sought after today.  The Rainbow Tuner is a resistive-type SWR bridge that displays relative SWR by illuminating combinations of colorful LEDs. The tuner handles up to 5 watts of RF power and includes a tunable parallel resonant circuit ("ATU") using a tapped inductor.  It readily tunes an end-fed half-wave antenna (EFHWA) to cover 7.0-10.16 MHz. When assembled, pc board mounts nicely in an Altoids mint tin.

Some good circuit fundamentals and clever use of circuits is illustrated in this design, and may be easily replicated on the homebrewer's bench for convenient use in the field.

~George N2APB & Joe N2CX

Audio Recording ... (Listen to MP3 recording)

Checkins ...

Text Log from this week's session ...

<20:03:37>"Joe N2CX":
<20:14:55>"George - N2APB": EFHWA = End-Fed Half-Wave Antenna ... A high-Z feedpoint that many other ATUs have difficulty matching
<20:15:22>"George - N2APB": But a simple tapped coil and capacitor can do a pretty good job for us here.
<20:21:03>"Paul - wa0rse": PCBs or artwork available?
<20:21:45>"George - N2APB": PCB is part of the kit that will shortly be available again.  (At least a short run of ~100)
<20:24:40>"Paul - wa0rse": This is such a cool circuit with the single comparator IC!
<20:26:12>"George - N2APB": Yep, pretty clever.  Only implementation of an SWR meter using this technique that I'm aware of.
<20:30:59>"Dave AA3UR": Batteries also available at Home Depot near wireless doorbell buttons
<20:37:22>"George - N2APB": Some extra details are available on the NJQRP website for the Rainbow Tuner ...
<20:41:21>"Carlos - PY1EGG": I have to leave now, 73 to all...
<20:41:32>"Charles WC5MC": 73
<20:47:38>"Paul - wa0rse": Poly-varicon means what?
<20:48:37>"Ray K2ULR": Poly-varicon is a variable capacitor where the plates are insulated by polyethylene sheets
<20:50:46>"George - N2APB": Many of the principles that Joe is discussion concerning different antenna variations, different bands, etc are described online at the "Application Notes" section of the old Rainbow Tuner webpage on the NJQRP site ....
<20:51:56>"Paul - wa0rse": Indeed! A goldmine to ponder there, for sure, George!
<20:52:19>"George - N2APB": And the 80m mods that Joe was mentioning is called the "Rainbow AOK" Add-On Kit, located at ...
<20:55:04>"George - N2APB": One time, Joe made me a simple-yet-nifty EFHWA pair of wires (half-wave radiator for 20m, and a quarter wave length for the cointerpoise) that wrapped around a piece of pcb material for easy transport.  I still use it today when going to the field!
<20:56:51>"George - N2APB": American Morse web page for very cool hogged out enclosures ...
<20:58:10>"Frank N3PUU": Joe brought that to a meeting last year.. that is a sexy box
<20:59:13>"Ray K2ULR": It's out of stock at the moment, however.
<21:01:18>"Russ G3OTH": joe take a look at
<21:03:06>"Russ G3OTH": was there any problem using it
<21:03:52>"Paul - wa0rse": Very nice session, fellas. Thanks! 73's!
<21:04:22>"Russ G3OTH": ok looks like it could be an alternative then
<21:08:41>"Chris - w0anm": Thanks Guys, again!!
<21:10:50>"Russ G3OTH": george you might like to add this link
<21:13:43>"Russ G3OTH": Goodnight and thankyou once again George and Joe for another very nice presentation.

Session notes ...

-Developed in 1996 as part of a design contest for Altoids tin projects


- Simple QRP tuner for half wave end fed antenna

    Includes a multicolor bar-graph SWR indicator showing actual SWR


- Intended for use either at home or in the field




-        Originally sold as a kit by NJQRP club



Block Diagram



End fed half wave antenna characteristics – Hertzian antenna

-        Impedance in the range of kilohms

-        Can be much more efficient than shorter or random length wires

-        Convenient for portable or temporary setups

-        Also resonant at harmonics

-        Many tuners can't handle high impedance


Half wave wires and tuners popular since early days

-        Tuner is basically a parallel resonant circuit with means of matching to low Z

-        Generally tunable with variable capacitor



**************** BREAK **********************



Rainbow Tuner schematic diagram



Tuner circuit description

-        Parallel tuned resonant circuit

-        Tapped for impedance match

-        Tunes at least 40 to 30 meters

-        Likely 20 meters as well


SWR indicator

-        Resistive Wheatstone Bridge

-        Approx 50 ohms

-        6 dB attenuation

-        Lessens tunep QRM

-        Protects rig from high SWR

-        Gives forward and reverse voltages

-        Diode detectors

-        SWR readings

-        Voltage comparator ICs and resistors

-        Measures forward and reverse voltage ratios

-        Multicolor LED bar-graph

-        < 1.5:1

-        1.5 – 2:1

-        2:1 – 3:1

-        3:1 – 5:1

-        > 5:1

-        Auto power on circuit

-        Must have RF switched in and out



**************** BREAK **********************



Rainbow Tuner Mounting and applications


Designed to fit in Altoids mint tin

-        Uses 12V “N” size alkaline battery

-        Duracell MN21

-        Will work up to 3W or so with 9V battery

-        But won't fit in Altoids tin


Originally used compression trimmer for tuning

-        Limited tuning range

-        Cleaned out surplus dealers

-        Changed over to polyvaricon type

-        wider tuning range and can use knob


Need to switch feedline

-        Eliminate 6 dB loss

-        Power auto-switches

-        Tight fit for switch in Altoids tin


Other enclosures

-        Incorporate into case with rig

-        American Morse Equipment case


Use on other bands

-        Polyvaricon can tune 20 meters as well

-        Add fixed capacitor in parallel for 80


Efficiency issues

-        EFHWA can be more efficient than wave or random wire

-        Less ground loss

-        Polyvaricon add loss 








  1. EFHWA by N2CX - <>
  2. EFHA by AA5TB - <>
  3. W8JI comments - <>
  4. NJQRP Rainbow Tuner page - <>
  5. American Morse Equipment AA1 Enclosure - <>




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