Question

The electric field E and the magnetic field H of a short dipole antenna satisfy the condition

• A. the $\theta$ component of E dominates the r component in the far -field region
• B. the $\theta$ and $\varphi$ components of H are of the same order of magnitude in the near field region
• C. the r component of E is equal to zero
• D. both r and $\theta$ components of H are equal to zero

Answer 1

Answer: (A), (D)

both r and $\theta$ components of H are equal to zero

Ans : (b & c)

Every dipole antenna has fields in directions of r, $\theta$, $\varphi$ with $E_r,E_{\theta },H_{\varphi }$.

These fields depend on 1/r, $1/r^2$ and $1/r^3$ with distance.

1/r terms are called as radiation fields and exist even at far away distance.

$E_{\theta }andH\varphi$ have 1 terms each depending on 1/r. $1/r^{2}and1/r^3$ terms are called as induction fields and exist at closer distances with zero values far away.

$E_r$ has only $1/r^{2}and1/r^3$ terms.

Option (a)-wrong -E exists

Option (b)-right-$E_{\varphi }$ only exists.

Option (c)-right- $E_{\theta }$ has 1/r terms which $E_r$ does not have.

Option (d)-wrong- $H_{\theta }$ does not exist.