V antenna advantages and both types

A V-antenna is a directive HF antenna made of two long wires (legs) arranged as a horizontal "V" and fed at the apex by a balanced transmission line.

Construction

              leg 1 (l = 2λ to 8λ)
         /‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
  600 Ω <   apex angle 2θₘ     ===> beam
 feeder  \___________________________
              leg 2

• Two straight wires of length l (several λ), supported by poles above ground.
• Apex angle = 2θₘ, θₘ being the angle of the main lobe of a single long wire: cosθₘ ≈ 1 − 0.371λ/l.
• Legs fed in antiphase by a 600 Ω open-wire line; far ends open or terminated.

Working principle

Each leg radiates a conical main lobe at θₘ to its axis. When the legs are set at ±θₘ about the bisector, one major lobe of each leg points along the bisector, and these lobes add in phase there (the antiphase feed and mirror geometry make them additive). Lobes in other directions cancel partly, so the radiation concentrates along the bisector. Gain is roughly double (3 dB more than) a single long wire.

Types and patterns

1. Resonant V (unterminated): ends open; standing waves; bidirectional pattern along the bisector (both forward and backward).
2. Non-resonant V (terminated): each end grounded through R ≈ Z₀/2; travelling wave; unidirectional pattern.

 Resonant V                 Terminated V
     _.-.                         _.-.
 <==(    )  <V>  (    )==>   <V>(    )====>
     '-'_            '-'          '-'
 equal front & back lobes    weak back lobe

Advantages

• Simple and cheap; only three supports needed.
• High directivity and gain (≈ 10–15 dBi for long legs).
• Wide bandwidth (especially terminated type).
• Easy matching to an open-wire line.

Disadvantages

• Large space needed; side lobes; fixed beam; power lost in terminations.

Applications

• HF (3–30 MHz) point-to-point sky-wave communication and reception, amateur and military radio links.