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.
Capacitive loading of the unterminated end against ground.
As a result the dipole is cut about 3% shorter that a true physical half wavelength would be.
BUT WAIT: Velocity Factor. ???
Yes, due to the velocity factor of normal antenna wire being only about 0.98% (2% shorter), it turns out 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, the Velocity Factor (shortening of about 2%) applies to ALL bands, fundamental and harmonic.
- but the 3% "End Effect Shortening" only applies to halfwavelengths 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
- EXAMPLE: The half-wavelength made of the outer two quarter wavelength ends is unterminated and has 3% End Effect shortening, but the half-wavelength made up of the two quarter-wavelengths in the center has its ends terminated (with wire or feedpoint) so it does not incur End Effect. Therefore the inner dipole has only 2% VF shortening but no End Effect shortening... but we have made it the same length as the outer half wavelength - so it is 3% to short and resonant 3% higher than the outer dipole.
- This makes the overall antenna (on its second harmonic) about 1.5% too short.
- It's 3rd harmonic is 2% too short.
- It's 4th harmonic is 2.25% too short.
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 as we move up the bands, eventually outside of the ham bands.
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.
This document depicts why the Windom (OCFD) is not a true harmonic antenna.
Windom Not Harmonic.pdf
PDF-Dokument [383.4 KB]