Long Wire antennas Vee antenna and Rhombic antenna Book sources: K. D. Prasad, J. D. Krauss

Long wire antennas - Theory Consider a thin wire of length L is horizontally placed at a height from ground plane. At one end – a RF source is connected and other end is terminated by the characteristics impedance of the wire Z0. When the wire is excited with a sinusoidal signal, the wave should travel along one direction and will not create any standing wave – so called “Travelling waves”. We are gonna derive the expression for electric field at a point P - located at a distance of R from the wire and angle of θ with respect to the length of wire. Zo Wave direction

Travelling wave structure – E field The retarded current in the wire shall be described as Where: v=p.c or p=v/c “p” is the ratio of the wave velocity at the wire to the free space, called “Relative phase velocity”. Used to vary with the attenuation constant. Zo Wave direction P θ R

Radiation pattern We can find the radiation pattern as similar to the dipole case, will result: Here, η is wave impedance.

For 3λ and 5λ

Bidirectional long wire antenna The maxima direction oriented with the direction of wave in the wire, so – by introducing wave on both the direction – we can introduce “Bi- directional radiation”. This can be possible by “Open circuited long wire” or “Un-terminated”, so called “Stand wave antenna”.

Vee antennas γ=2β β By terminating with Z 0, we can get Uni-directional radiation

Rhombic antenna Based on the principle of “Traveling wave” and “Vee antenna”, Rhombic antenna is a very high directive antenna – has Diamond or Rhombus shape β

Working of Rhombic antenna By properly selecting the tilt angle, the rhombic antenna will give additive effect of radiation pattern of each long wire antenna The radiation mechanism is basically depends on two factors: – Tilt angle (φ) – Height above the ground (h) These are design parameters of antenna Due to ground effect, the maximum radiation is elevated about an angel (β) Z0Z0 β

Design equations of Rhombic antenna BBL field equation: (Bruce, Beck and Lowry – 1935) From this equation, we can deduce the condition to get the maximum power direction with respect to height h and length of line L

Maxima with height

Maxima with Length L

Design formula Finally, the design formulae are:

Example-1 If we assume the average beamwidth of rhombic antenna as 10 o, then design the rhombic atenna such that it radiate maximum power over the ranges from 10 o to 40 o for the operating frequency of 100MHz