| The Hertz antenna has been around
since about 1880 and there are an unlimited number of variations on that theme.
However, each of those is constrained to be a resonant antenna. The wires may be
straight or bent into various shapes, but they must be resonant or resonated
with an antenna tuner. Resonance is necessary to allow maximum current flow.
Hertz antennas are based on the concept that current flow on a wire causes the
development of a magnetic (H) field, and a changing magnetic field creates an
electric (E) field. Because one field creates the other, they have a time phase
difference of 90 degrees. The speed at which the two fields travel is different.
Therefore, the two become in time phase at a nominal distance of about 1/3
wavelength from the wire. This is called the far field. When they have the
proper phase, amplitude, and physical relationship radiation is created.
Compare that to the EH Antenna
where the E and H fields are in time phase at the antenna. Because the two
fields are very efficiently integrated, the radiation resistance is higher than
a wire antenna. A major factor is that the elements of a EH Antenna have a much
larger diameter than a wire, and the two elements therefore have high capacity
between the elements. This large capacity and high radiation resistance combine
to provide very wide bandwidth and high efficiency. All of this in an antenna
that may be less than 1% of a wavelength, compared to wire antennas that are 50%
of a wavelength.
Keep in mind that the cylinder
length to diameter ratio sets the antenna radiation pattern. We typically use a
ratio of 6 for all Ham bands above 40 Meters and a ratio of 12 for 40 meters and
below. When the antenna is mounted as a vertical, the larger ratio allows high
angle radiation. The bandwidth of the antenna is determined by the
diameter.
You will find information
to allow you to copy antennas for various Ham bands, as well as technical
information that will allow you to roll your own. We hope you enjoy this
information and the use of the antennas you build. There are an untold number
already on the air and many more added every day. If you choose to buy rather
than build, check the links for companies that manufacture and sell EH Antennas,
then check their distributors.
Ted
W5QJR
To broaden your
understanding of the EH Antenna for HAM's, please visit our documents
Library. |
