THE HELICAL ANTENNA
I had the opportunity to build a couple of these for a screen set. The actors wore a hidden cordless microphone transmitting between
400 and 700 Mhz. The Helical antenna was connected to a receiver where the voices could be heard while the actors moved around.
The helical is a uni-directional antenna. In other words it favors one direction.
It is very broad banded. It can cover a large spectrum of a band maintaining a good match.
It has circular polarization. This allows it to communicate equally well regardless of the polarization from the other station. This
makes it work well for satellite communication where polarization is constantly changing.
The construction is very forgiving. Sizing of the parts of the antenna are not critical.
There is much information on building a helical. Here are some basic dimensions.
1. The reflective disc is a circular sheet of aluminum. A diameter of .8 to 1.1 wavelength. On 2 meters this is less than 40".
2. The coil is perpendicular to the disc. It has a circumference of .75 to 1.33 wavelength. Or .23 to .42 wavelength diameter. The
spacing of the windings is between .21 and .29 wavelength. At least three turns of coil should be used. The coil material of this unit
(PICTURE), is 1/4" aluminum rod.
3. One end of the coil is connected to the center of a connector. In the one built below it was a BNC. The other end of the coil is
in free space.
4. The disc is the shield side of the feed line. The connector is mounted in the center of the disc.
5. The input impedance measures a bit high. It calculates to 140 X the diameter of the coil. This gives a range of 110 to 180 ohms.
Therefore, a simple matching device is needed at the input of the antenna. The antenna below was for reception only, therefore no
matching device was used.
This is one of the few antennas where the appearance looks like what it actually does. It is not hard to build and the dimensions
are quite forgiving.
HELICAL ANTENNA
73,
Ralph WD0EJA
11-18