Not Enough SpotsThanks to the current low sun spot cycle, the 40 meter band arguably provides the best HF radio propagation. The problem with this band, is that a half wave dipole wire antenna is 20 meters long. If you string that from a mast or a tree, you may need 30 to 40 meters of space for the wire and ropes. Most modern backyards are not that large.
One solution is to upgrade to a Country Manor House, but for most radio amateurs, that would be too much lawn to mow. On the other hand, fitting a 40 meter band dipole into the attic of a regular sized home or cottage, certainly seems impossible, but it is actually quite easy: Curl the dipole up into a helix!
Clue Up On HelicesThe following NEC card stack defines a dipole made from two helices, wound around a 5 meter length of "Two by Four" lumber, for a 75% overall size reduction, which should fit into the attic of most homes.
"Two by Four" lumber is actually 1.5 by 3.5 inches, or 38 by 89 mm in size. With each arm of the dipole a 2.5 meter helix, wrapping it with ten meters of wire results in 39 turns per arm, for a spacing of 64 mm. Hammer some 1/2 inch panel nails into the lumber to help you keep the windings even, use a couple hundred thumb tacks, or glue the wire down with a glue gun. Keep it simple - using glue should be less painful than thumb tacks.
An alternative is 4 inch PVC drain pipe. It costs more, but may look nicer. Note that the outside diameter of 4 inch pipe is 4.5 inches (114.3 mm). The radius is therefore 57 mm in both x and y axes.
Kraus Helical Antenna Designer
The helical antenna work published by Kraus in 1948, shows that a thin helix radiates in normal mode, while a fat helix radiates in axial mode. This space efficient dipole is made from two thin helix antennas tied back to back.
The NEC2 GH card defines the helix on the z-axis, which appears vertical in the simulator, but in practice one would install this antenna horizontally. The GM card is used to rotate and copy the helix around the y-axis to make a curled up dipole. Because this is a thin helix, the radiation is normal to the z-axis, the same as with a straight wire dipole.
You can model the antenna with CocoaNEC on a Mac, or with xnec2++ or xnec2c on Linux or BSD as described in another post.
Here is the Clue by Four Antenna Model:
CM Clue By Four Attic Antenna
CM Fit a 40 meter band, 7 MHz, HF dipole antenna into your attic
CM Copyright reserved, Herman Oosthuysen, 2017
CM Copyleft defined by the FSF GPL v2
CM 40 meter helical dipole, for 5 meter, 2 by 4 lumber
CM A 2 by 4 is actually 1 1/2 by 3 1/2 inch, or 38 x 89 mm
CM 1/10th wavelength is 4 m
CM 1/1000th wavelength is 40 mm
CM Max Segments is 10,000 / 40 mm = 250
CM Radius is 19 x 44.5 mm
CM Circumference is 38 x 2 + 89 x 2 = 254 mm
CM Turns is 10,000 mm / 254 mm = 39.37 turns
CM Length is 2500 mm
CM Spacing is 2500 mm / 39 turns = 64 mm
GH 1 250 6.40E-02 2.50E+00 1.90E-02 4.45E-02 1.90E-02 4.45E-02 1.00E-03
GM 1 1 0 180 0 0 0 0 0
FR 0 0 1 0 7.00E+00 0 0 0 0 0
EX 0 0 1 0 1.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00
RP 0 91 120 1000 0 0 2 3 5000
The antenna pattern of the twin helix is a regular dipole doughnut with a gain of about 1.5 dBi and an impedance of about 90 Ohm:
The Clue By Four Helical Dipole Antenna
Don't fret too much about the design of a wire antenna. Use NEC to show that it works, then build it and add a simple little antenna tuner to it, as described at the bottom of this page http://www.aeronetworks.ca/2017/08/hf-radio-bitx40.html.
Now you can enjoy some DXing, without risking life and limb climbing trees, or erecting masts, while the antenna is protected against the elements, safe and snug inside the roof of your home.
Slinky SpringsSlinky Spring antennas have been around since forever, but steel wire will not work as well as copper wire (for transmit) and a slinky will rust and degrade rather quickly. The main thing with a DIY helical antenna is to check the design with NEC before you build it - vary the length of wire, the number of turns and do a frequency sweep to get a feel for it.
The feeling I got, was that most designs on the wild wild web use too much wire and try to compress the overall length too much. If it doesn't work well in simulation, then it won't work well in practise either!
You can use the same trick for the 80 or 20 meter bands, to fit all your HF wire antennas into whatever space you have available and wind your dipoles around a piece of lumber, or a plastic drain pipe. The result should be a good alternative to the classic flagpole, or downspout antennas.
Groundless RhetoricYou can also use a helix for a 1/4 wave vertical flagpole antenna, but there seems to be a radial wire fetish or something in ham circles. Every discussion on verticals, have long stories about how to lay radial grounds in a lawn, using hundreds of meters of costly wire.
If you simulate a vertical in NEC, then you need a ground plane, or radial wires for the purpose of the simulation, but in reality you have the earth, which is a pretty good earth, really!
Unless you are living in a sandy desert (like me) you should not need radial wires. Hammer a 1.5 meter lightning rod ground spike into the earth and be done with it. Instead of digging trenches, go fishing or watch a ball game. If the ground is dry, change the timer dial on your lawn sprinkler system and if you do live in a desert like me, a chicken wire mesh is way cheaper than copper wire radials...
Tuners and TrapsSomeone asked whether one could make a multi-band antenna this way.
A simple way to make a 40/80 meter antenna would be to wind a second set of identical helixes and attach them through 10 uH inductors to lengthen the dipole for 80 m operation. A ~10 uH inductor is simply ten tight turns around the two by four http://hamwaves.com/antennas/inductance.html.
In any case, you need a tuner on this type of antenna, since it is rather inductive (the shorter you make it, the more inductive it becomes) and it needs to be hooked to your radio through a L-match antenna tuner as shown at the bottom of this post http://www.aeronetworks.ca/2017/08/hf-radio-bitx40.html.