Helical antennas give a circular polarized wave. They are one of the easiest to design. Find a tube with a circumference equal to one wavelength, and wrap wire in a helix spaced a quarter wavelength. The greater the number of turns the greater the directivity or antenna gain. Receiving and transmitting antennas must be wound in the same direction, since the wave is polarized.
Equations:
\( G = 10.8 + 10 \log_{10} \left( \left( \frac{C}{\lambda} \right)^2 N \left( \frac{S}{\lambda} \right) \right) \) (Note 1)
\( Z = \frac{150}{\sqrt{C/\lambda}} \) Ohm
\( D = \frac{\lambda}{\pi} \)
\( S = \frac{C}{4} \)
\( L = N \sqrt{\lambda^2 + S^2} \), where \( S \) and \( \lambda \) are in cm.
\( \text{HPBW} = \frac{52}{(C/\lambda) \sqrt{N (S/\lambda)}} \), Half power beam width.
\( \text{BWFN} = \frac{115}{(C/\lambda) \sqrt{N (S/\lambda)}} \), Beam width first nulls.
\( A_e = \frac{D \lambda^2}{4 \pi} \)
Note 1: This is taken from Kraus - "Antennas for All Applications". It is commonly believed to be too optimistic by about 3dB-4dB.
