The vertical antenna shown is a ¼-wave vertical radiator suspended from a tree with only two sloping radials instead of the conventional three or four. This is a practical field-expedient configuration often used by portable and emergency operators where space or support is limited.
Unlike a classic ground-plane that relies on multiple radials to form a symmetric RF ground, this design uses two opposing radials to provide the necessary return current path and impedance transformation. A vertical quarter-wave radiator forms one half of a dipole. The missing half is replaced by a current image in the radial system.
With only two radials:
- The vertical antenna becomes electrically similar to a bent dipole.
- Current flows up the vertical and returns through both radials.
- The radiation pattern remains mostly omnidirectional, but with slight bidirectional enhancement in the plane of the radials.
- Feed impedance typically rises to 60–75 Ω, which still matches well with 50-ohm coax when radials are sloped 30–45°.
The fundamental length formula remains:
L in ft=234/F
Mechanical Layout
Top Support:
The vertical wire is tied to a tree branch using an end insulator. The tree acts only as a mechanical support, not an electrical element.
Feed Point:
The coax center conductor connects to the vertical wire. The coax shield connects to a small hub from which two radials extend symmetrically.
Radials:
Two quarter-wave radials slope downward and away from the trunk, ideally in opposite directions to balance currents.
End Insulators:
Each radial terminates in an insulator tied to rope, preventing detuning from nearby branches and soil.
Current Choke:
A 1:1 choke (coax coil or ferrite) at the feed point suppresses common-mode current on the feedline, which becomes more important with only two radials.
Although three or four radials give better symmetry, two radials:
- Provide a complete RF return path.
- Maintain vertical polarization.
- Produce low-angle radiation suitable for DX.
- Are easier to deploy in trees or on portable masts.
Field measurements typically show only 0.5–1 dB loss compared to a full three-radial ground plane when installed clear of ground and objects.
Construction Tips
- Use copper-clad steel or stranded copper wire for strength.
- Keep both radials exactly the same electrical length.
- Maintain at least 30° separation angle between radials.
- Avoid the tree trunk touching the feed point or radials.
- Final trim for minimum SWR after installation, as nearby foliage introduces capacitive loading.
Related Posts
- 40m Vertical Antenna: Compact Base-Loaded Design for Powerful HF Operation
- GM4JMU Shortened Dipole Antenna: Compact Performance for HF Bands
- Aperiodic Multiband Wire Antenna: A Simple and Effective Design