Yagi / Linear Definition:

Named after one of the inventors, the "Yagi" can produce the most gain for the least material.
Most of the halfwave elements are parasitic except for the feed dipole. They are easy to feed and
match and produce very high gain (up to a least 20 dbi for a single) and can be stacked easily for
3 dB more each time the number of antennas is doubled. The limitation is fairly bandwidth. Typically,
10% bandwidth is the practical limit. Yagi antennas are used up to 3 GHz.
Cross Polarized can be produce right and left hand circularity as well as selectable horizontal or vertical polarity (polarity diversity).

Log Periodic Definition:
These designs are the Kings of wide bandwidth. Log Antennas are generally a series of dipoles tuned and spaced in a logarithmic fashion across a given bandwidth.
All the dipoles are fed and feed impedance varies from 50 to several hundred Ohms depending on the design. They easily cover 10:1 bandwidth (100 to 1000 MHz) or even wider.
They are generally directive but with modest gain (5 to 10 dBi).
Gain can be increased by spreading the two feeder structures into a “V” shape. They can be designed to create circular polarity. They can also be stacked for more gain. M2, Inc. manufactures all designs as noted above.

Yagi / Crossed or Circular Definition:
Two sets of yagi elements are placed on one support boom. When fed 90� out of phase, Circular Polarity is produced. This produces a circular field in and in front of the antenna. This requires the antenna to be either rear mounted or supported by a non conductive (fiberglass) mast to keep any conductor from cutting the circular field. The feedline(s) must also be run off the rear of the antenna for the same reason. Switching from right circular to left circular quite easy. The most typical application is for satellite communication where signals passing through the earth�s ionosphere can radically change the polarity of the signals.

Helical / Helix Definition:

Typically wound in a one wavelength per turn spiral, the Helical/Helix produces a wave front of fixed circular polarization. The sense of the Circular Polarization matches the clockwise or counter
clockwise direction of the spiral. The Circular Polarization field cannot be reversed. The Helical/Helix typically produces 3 dB less gain than a Yagi for a given 2:1 is possible with this
design (200 to 400 MHz).
They are typically rear mounted in front of a 1/2 wave length or larger planar reflector. No balun is
required and the 50 Ohm feed point is easily accessible at the back side of the reflector surface.
These antennas can be placed in an array configuration for more gain but matching over a 2:1
bandwidth is sometimes a drawback.

Phased Array Definition:
Placing directive antennas in an array of like antennas and spaced so the individual antenna fields just overlap. This can be done with
Yagis, Helicals/Helix, Log Periodics and other directive antenna types like Patches, Cubical Quads and even Dipoles over a reflective
plane. Matching is easy for Yagis and narrow band antennas by combining, in phase and matching using 1/4 wave, 2, 4, 6 and 8 port
power combiners. Arrays of Helicals/Helix and Log Periodic broadband antennas requires special matching techniques.