Parallel conductors and their connections produce an electrical length greater than the physical length. The antenna behaves as a folded system with different impedances on each band.
Antennas · F1SYH
Cobra antenna
The Cobra is a multiband wire antenna whose two legs are made from three-conductor line connected in a specific configuration.
HF
Multibande
Ligne trois conducteurs
Operating principle
Main characteristics
Typical gain
Variable selon la bande
Impedance
Variable, coupleur nécessaire
Polarization
Principalement horizontale
Difficulty
Intermédiaire
Dimensions and design
Several versions exist, commonly between about 22 and 44 metres overall. Dimensions depend on the required bands and conductor line.
Construction and tuning
Step 1
Choose identical three-conductor line.
Step 2
Follow the end-connection diagram accurately.
Step 3
Build two mechanically symmetrical legs.
Step 4
Feed with balanced line.
Step 5
Use a balanced tuner or suitable balun.
Step 6
Measure all bands before final trimming.
Advantages
- Several bands in reduced physical length.
- No traps or concentrated loading coils.
- Can use common multiconductor cable.
- Good efficiency with low-resistance wire.
Limitations
- A tuner is normally required.
- Hard to predict without modelling.
- Thin conductors may introduce losses.
- Connection errors are easy to make.
History and origin
The Cobra became popular as a compact multiband amateur antenna using readily available multiconductor cable.
Safety
Ends and some junctions may carry high RF voltage.