Moving the feed point changes impedance on the fundamental and harmonic bands. The selected split is a compromise and does not guarantee a perfect match everywhere.
Antennas · F1SYH
Windom and OCF dipole
An OCF dipole is fed away from its centre to obtain usable impedance on several bands. Modern versions generally use a transformer and coaxial cable.
HF
OCF
Multibande
Operating principle
Main characteristics
Typical gain
Variable selon bande
Impedance
Environ 200 à 300 Ω au point choisi
Polarization
Horizontale ou mixte
Difficulty
Intermédiaire
Dimensions and design
A common division is near 33/67 percent, although other ratios are used. Dimensions should be validated for the desired bands.
Construction and tuning
Step 1
Define the fundamental band.
Step 2
Choose the wire division ratio.
Step 3
Install a suitable 4:1 or 6:1 transformer.
Step 4
Add a common-mode choke.
Step 5
Measure all bands in the final position.
Advantages
- Several bands with one wire.
- Feed point closer to one end.
- No traps.
- Relatively simple construction.
Limitations
- Common-mode current is frequent.
- The Windom name is often used imprecisely.
- SWR varies across bands.
- Transformer sees complex impedances.
History and origin
Loren G. Windom, W8GZ, published a single-wire off-centre-fed antenna in 1929. Modern coax-fed OCF antennas differ from that historical design.
Safety
Use an effective choke to reduce feed-line radiation.