V antenna construction and working

A V-antenna is a travelling/standing-wave wire antenna made of two long straight wires (legs) arranged in the shape of a "V", fed at the apex by a balanced line. It is a simple directive HF antenna for point-to-point links.

Construction

 Structure (top view)           Non-resonant type
                                    R/2
         leg 1 (l = several λ) ____/\/\_
        /                         |
 feeder<  apex angle 2θₘ  ===> beam  (bisector)
        \                         |
         leg 2 ________________/\/\_
                                    R/2

• Two straight wires of length l (2λ to 8λ), horizontal, on poles above ground.
• Apex angle = 2θₘ, where θₘ is the angle of the main lobe of one long wire: cosθₘ ≈ 1 − 0.371λ/l.
• Fed at the apex by a 600 Ω open-wire balanced line, 180° out of phase.
• Far ends left open (resonant V) or terminated with resistors (non-resonant V).

Working principle

1. Each long wire radiates a conical main lobe at angle θₘ to the wire.
2. Since the legs are fed in antiphase and are mirror images, choosing the apex angle equal to 2θₘ makes one main lobe of each leg lie along the bisector; there they add in phase.
3. Lobes in other directions are out of phase and partly cancel, giving a directive beam.
4. In the resonant (unterminated) V, the wave reflects from the open ends, forming standing waves; radiation goes both ways along the bisector (bidirectional).
5. In the non-resonant (terminated) V, matched resistors absorb the forward wave, so only a travelling wave exists and the beam is unidirectional.

Radiation pattern

 Resonant V (bidirectional)   Terminated V (unidirectional)
    <===  ◁V▷  ===>              ◁V  ====>
   back lobe = front lobe        weak back lobe

Characteristics

Advantages: simple, cheap, needs only 3 poles, wide band. Disadvantages: large area, side lobes, fixed beam, power loss in terminating resistors. Applications: HF point-to-point communication, amateur radio.