This calculator is used to find the gain of an
axial-mode non-tapered Helix Antenna and its physical dimensions. It uses
either the classic Kraus model or the Emerson model.
The model can be selected by the user. The differences in the models is
that the Kraus model is an analytic one that assumes the current
distribution is constant along the helix. This leads to a prediction of
ever increasing gain as the helix length increases. The gain from this
model increases by 3 dB for every doubling of the length for a fixed pitch
angle.
On the other hand, the Emerson model which is mainly based on NEC
computer simulation accounts for the current distribution in the helix.
The current decreases along the helix which results in a flattening of the
gain as the length is increased. Note that the Emerson model is only
valid for a helix between 2 and 7 wavelengths long.
Both models use a square ground plane 0.6 by 0.6 Wavelengths in dimension. However, values between 0.6 and 1.2
Wavelengths can be used.
The Emerson model also predicts a much lower gain than the Kraus model
(4 to 5 dB).
It also results in a greater length than the Kraus model.
Each model has certain constraints as used in the calculator.
The calculator uses a circumference of 1.0 wavelengths and a pitch
angle of 12.5 degrees for the Kraus model as used in the ARRL Satellite
Experimenters Handbook The Emerson model uses a fixed spacing
between turns of 0.24 wavelengths while the radius varies with the total length
of the helix (number of turns).
