Half wave antennas and the Rainbow


From: Joe Everhart (n2cx@voicenet.com)
Date: Fri Nov 29 1996 - 11:01:08 EST


There has been some discussion on qrp-l in the past about 1/2 wave end fed
wires as simple antennas for portable applications. I'd like to add a few
ideas to that discussion, introducing you to the idea and showing its
application with the Rainbow Bridge/Tuner.

As a single band antenna, it is very simple! For example on 40 meters, all
you need is a 67 or so foot length of wire for the antenna and a counterpoise
of some sort. Since efficiency is not the issue (the end fed wire is high
impedance), the counterpoise simply helps to establish some sort of ground
return. My favorite is just a quarter wave length of wire (33-34 feet for 40
meters) laying on the ground.

It's really the same antenna as a center fed dipole except that it is fed at
the end. Being end fed it can be easier to put up than a dipole and, if used
properly, can be just as efficient. Since you need access to the end, it is
easy to erect as an inverted L or as an inverted vee. Just keep the center as
high as possible, because this is where the radiation takes place.

As a portable antenna, this configuration is very handy. Coils of small
diameter wire can very easily be slipped into a sandwich size zip-lock bag and
carried with your favorite trail friendly radio. While a lightweight dipole
may weigh a good fraction of a pound, eliminating the feedline cuts out most
of that, so you end up with a couple of ounce antenna! And as a convenient
support, the 20 foot pole used for the Saint Louis Vertical would be more than

One disadvantage is that you need a tuner.

But that's not a real problem, because a tuner for the half wave antenna
needn't be more complicated than a basic tuned circuit! A crude ASCII sketch
shows the details:

                                     -----------> 1/2 wave antenna
                              | |
                              | |
                              ( | /
                          L ( ----- C
                              ( /
                              ( -----
                     <--------( / |
             Short ( |
             Coax | |
              to -----o-----
              Rig |
                     <--------------o-----------> Counterpoise

The coil L is merely a common inductor, using a compact T-50 or T-68 toroid
core. Variable capacitor C can be as minimal as a compression mica trimmer.

The inductor acts a matching transformer, tapped to convert the high impedance
(500 ohms to several thousand ohms) down to 50 ohms which most QRP rigs like
to see. The capacitor resonates with the inductor to make the whole thing
resonant at the operating frequency.

How small and lightweight is the tuner? - VERY!

The half-wave antenna tuner is the heart of the Rainbow Bridge/Tuner, one of
the finalists in the Norcal Design Competition. And the entire tuner and
bar-graph LED display SWR bridge fit into an Altoids tin tipping the scales at
just under 6 ounces!

Yes, this was just a teaser! Stay tuned for the up-coming formal announcement
of the Rainbow Bridge and Tuner to be offered by NJQRP as a kit. It will be a
perfect companion to the brother Design Competition finalist 38 Special for
30 meters *and* to the already-popular 40-9er.


Joe E., N2CX

work: jeverhart@cayman.vf.mmc.com
home: n2cx@voicenet.com

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