Travelling wave antenna and V antenna

Travelling wave antenna

A travelling wave (non-resonant) antenna is one in which the current travels along the conductor in one direction only, because the far end is terminated in a matched load (equal to the characteristic impedance). There is no reflected wave, so no standing wave; the current amplitude is nearly uniform and the phase changes progressively along the wire. Such antennas are unidirectional, wideband and have nearly constant resistive input impedance. Examples: terminated long wire (Beverage), terminated V, rhombic.

 feed            travelling wave  →            R = Z₀
 ~ o==========================================/\/\/--⏚
   max radiation tilted forward, towards the load end

V-antenna: construction

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

• Two long horizontal wires forming a "V", fed at the apex in antiphase by a balanced open-wire line.
• Apex angle chosen as 2θₘ, where θₘ is the main-lobe angle of a single long wire: cosθₘ ≈ 1 − 0.371λ/l (e.g. l = 4λ → θₘ ≈ 25°, apex ≈ 50°).
• Far ends open (resonant) or terminated by resistors to ground (non-resonant).

Working principle

Each leg radiates a cone of radiation at angle θₘ to its wire. When the apex angle equals 2θₘ, one major lobe of each leg lies along the bisector of the V, and because the legs are fed in antiphase and are mirror images, these lobes add in phase there. Lobes in other directions are partly cancelled. Ground reflection sets the elevation angle of the beam.

Characteristics

• Directive, with gain about twice (3 dB more than) a single long wire of the same leg length; ≈ 10–15 dBi for long legs.
• Horizontal polarization, HF band (3–30 MHz).
• Input impedance high (≈ 500–800 Ω); wide bandwidth, especially when terminated.
• Requires large space but only three supports.

Types of V-antenna

 Resonant:    <=== V ===>      Terminated:   V ====>

Applications: HF point-to-point communication and reception.