A 5/8-wave radiator is not naturally matched to 50 Ω. A loading coil or matching network is normally installed at the base.
Antennas · F1SYH
5/8-wave vertical
A 5/8-wave vertical uses a radiator longer than a quarter-wave to flatten the vertical pattern and increase low-angle radiation.
VHF
UHF
5/8 λ
Operating principle
Main characteristics
Typical gain
≈ 3 dBi selon plan de sol
Impedance
Adaptation nécessaire
Polarization
Verticale
Difficulty
Intermédiaire
Dimensions and design
Electrical length is near 0.625 wavelength, but physical length depends on diameter, matching coil and surroundings.
Construction and tuning
Step 1
Calculate the initial radiator length.
Step 2
Build the matching network.
Step 3
Install an effective ground plane.
Step 4
Measure impedance at the base.
Step 5
Tune length and matching separately.
Advantages
- Lower vertical lobe.
- More gain than a quarter-wave.
- Omnidirectional.
- Common for mobile VHF.
Limitations
- Matching is required.
- Mechanically longer.
- Gain depends on the ground plane.
- Coil losses are possible.
History and origin
The 5/8-wave vertical became popular for mobile VHF communication, where its low-angle pattern improves terrestrial range.
Safety
Its physical length requires strong mechanical mounting.