Tuesday, February 15, 2011

End Fed Zepp Antenna


A popular antenna often used to save space is the End Fed Zepp. The End Fed Zepp gets its name from the fact that it was used as an end fed wire trailing out from the rear of Zeppelin airships. It consists of a 1/2 wavelength horizontal radiator wire connected to one conductor of a length of parallel 300 ohm or 450 ohm twin feeder, often quoted as being 1/4  wavelength long.

Zepp Antenna by K4EFW

K4EFW notes: "...A half-wave resonant antenna can be fed from its end. When fed this way, it is also known as an end-fed zepp. An end-fed zepp will work on its fundamental frequency and on odd and even harmonic frequencies. The end of a half-wave antenna has very high impedance, and an antenna fed this way is said to be voltage fed. Feeding a half-wave resonant dipole in the center means it is current fed. The normal way of feeding the end-fed antenna is with ladder-line. One side of the ladder-line is connected to one end of the antenna and the other side of the ladder-line is connected to nothing. To secure the unconnected side of the ladder-line, it is connected to a short wire running between two insulators.

Since the antenna is connected at its high impedance point, no current flows into an antenna, but there will be a large current in the center of this antenna. No current flows from the open side of the feed-line because it is at a zero current point.  The end-fed zepp can be matched by cutting the ladder-line to a quarter wavelength with the bottom end of the ladder-line shorted. A certain distance above the short is a 50-ohm feet-point and it can be fed directly with coax. You will have to find the 50-ohm point by trial and error. This method of feed makes it a single band antenna".

1 comment:

  1. Thank you for the posting on the end fed zep antenna