AFFECT OF END-EFFECT ON OCFD

Or, Why is the OCFD NOT an exact Harmonic antenna?

 

We all know that the simple dipole antenna (mono-band) is cut shorter than a true physical length of a half wavelengeth dipole.

It is typically about 5% shorter than a true physical half wavelength.

 

Why?  

 

As a result of capacitive loading of the unterminated wire ends against ground, known as "End-Effect", the dipole is cut about 3% shorter than a true physical half wavelength would be.  

 

In addition, the Velocity Factor of the wire used in the antenna requires shortening the wires by 2%.

 

As a result, the dipole is actually 5% shorter than the true physical length...... 2% velocity factor plus 3% due to end effect.

 

Now the critical point to understand:  

  • In an OCFD antenna, the Velocity Factor (shortening of about 2%) applies to ALL bands, fundamental and harmonic.
  • But the 3% "End Effect Shortening" only applies to the half-wavelength with unterminated ends.  
  • On the fundamental frequency, there is only one half wavelength and it has unterminated ends.  It has 3% shortening due to End Effect (plus 2% VF, total 5% shortening).
  • On the Harmonic Bands (i.e. 2nd harmonic, 3rd Harmonic, etc.) the affect of End Effect only applies to one of the multiple half-wavelengths.  
    • EXAMPLE: An 80m Half-Wavelength Dipole
      • Is made of two unterminated quarter wavelength ends which have 3% End-Effect shortening plus 2% VF shortening. 
      • That same dipole on 40 meters is made up of 4 quarter- wavelengths; two quarter-wavelengths on each side of the feedpoint. Or, we can view it as two half-wave dipoles.
        • The two quarter wavelengths in the center have their ends terminated (with wire or feedpoint) so they do not incur End-Effect. Therefore the inner dipole has 2% VF shortening but no End-Effect shortening.
        • However, we have made it the same length as the outer half wavelength (which was also shortened by 3% due to End-Effect) - so the inner dipole is 3% too short, and resonant 3% higher than the outer dipole. 
  • This makes the overall antenna (on its second harmonic) about 1.5% too short.  This raises its 40m resonance higher than twice the fundamental frequency.
  • It's 3rd harmonic is 2% too short.
  • It's 4th harmonic is 2.25% too short.
  • etc. 

 

The higher we go up in harmonic frequency, the less the end effect plays a role in the equation.

 

Therefore the resonant frequency of the OCFD continues to increase higher and higher above the theoretical harmonic frequecy, as we move up the bands.  Eventually it falls outside of the ham band.

 

The only way to have resonance inside of all of the higher harmonic bands is to place the fundamental frequency below the actual ham band.

 

An 80m OCFD with a correctly designed balun (i.e., my DJ0IP Hybrid Balun) normally has negligible Common Mode Current on the feedline, and as such, must be cut so that the frequency of minimum SWR is in the range of about 3.440 to 3.460 MHz.

 

DOWNLOAD:

 (In this drawing,  S=Skewed and NS=Not Skewed)

Windom Not Harmonic
This document depicts why the Windom (OCFD) is not a true harmonic antenna.
NOTE: S = Skewed and NS = Not Skewed
Windom Not Harmonic.pdf
PDF-Dokument [383.4 KB]