end effect in antennas

Papabravo

Joined Feb 24, 2006
19,252
can anyone explain what is end effect in antennas ?
how it effects the antenna performance ?
An antenna is an arrangement of conductors designed to radiate RF energy. The impedance of an arrangement of conductors can be calculated or measured at any point along those conductors. When you get to the end of a conductor there is an impedance discontinuity. At an impedance discontinuity there are two things that happen to waves, as dictated by the wave equation: transmission and reflection. Some of the RF power goes through the discontinuity and some of it is reflected back to the source.

Impedance discontinuities happen at other places besides the end of a conductor. The other thing to keep in mind is that, "The RF gotta' go somewhere".
 

KL7AJ

Joined Nov 4, 2008
2,229
can anyone explain what is end effect in antennas ?
how it effects the antenna performance ?

The end effect of an antenna is an effective capacitance which makes a dipole antenna effectively slightly longer. A half-wave resonant dipole is going to be slightly less (about 5%) than a half wavelength in free space. If the wire were infinitely thin, there would be no end effect, but the two halves of a dipole act like small capacitors across the feedpoint.

The general rule for a thin-wire dipole is: L (feet)= 468/Freq (Mhz) This formula takes into account the end effect. Without end effect it would be 492/freq.

Eric
 

Papabravo

Joined Feb 24, 2006
19,252
You said:
tnx for ur reply sir
but i have a doubt regarding its relation (end effect ) with antenna length.i got from a book that ' beacuse of end effect the effective length of antenna is less than actual length ' is this beacuse of reflected power causing standing waves ?? plz exlpain sir.

I don't answer private messages, I only answer in the open forum:

The end effect changes the complex impedance of the antenna/feedline system so that the resonant frequency occurs at a shorter physical length than the one half wavelength predicted by theory without end effect. I believe the end effect is effectively modeled by an additional lumped capacitance.
 

Papabravo

Joined Feb 24, 2006
19,252
ARRL Antenna Handbook is an excellent practical resource. They tend to be short on the theoretical underpinnings of the construction projects.
 
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