antenna-theory.com

[xempt]

I would greatly appreciate any help with the following questions:

1. Given the famous expression for the time-averaged Poynting vector of a finite-length electric dipole antenna (where r is the unit vector in spherical coordinates):

<S >=(No [Io]^2)/(8π^2 rd^2 ) [(cos⁡(kL/2 cos(Th))-cos⁡(kL/2))/sin(Th)]^2r

find the ratio L/lambda and the angle theta_s of the first side lobes when their intensity just exceeds that of the broadside lobe at theta = pi/2.

Given this same expression, find the radiation resistance of a full-wave dipole, L = lambda

2. Stripline Dipole. At microwave frequencies and above it becomes advantageous to integrate resonant printed-circuit antennas monolithically on the top surface of the same substrate that support the semiconductor devices. The challenge is coupling the antennas to free space since the radiation tends to go into the lower-intrinsic-impedance, substrate side. A useful starting point for dipolar strip antennas).

a. Find the intrinsic impedance N [in ohms] in a substrate of semi-insulating (SI) GaAs. Find the average dielectric constant at the air interface as an arithmetic weighting of the two dielectric media [clue: the dielectric constant of SI GaAs at RF frequencies is practically the same as the dc value].
b. Calculate the interface velocity as the speed of light reduced by the average refractive index. Use this to design the physical length of two printed dipole, one that will approximate a half-wave circuit resonance at 60 GHz, and the second one a full-wave resonance at 60 GHz
c. Representing the antenna as an ac current generator at the resonant frequency, located at the interface, and in shunt with the radiation resistance RA, estimate what fraction of the available power radiates into the air (clue: consider the current-divider method in circuit theory, but be careful how you construct the load resistors !}.

[bigSteve]

What class is this for?

The first couple are just plugging in numbers.

For the last one, part (c), tell your teacher that the intrinsic impedance of the material surrounding the antenna does not produce mismatch loss. His method of calculating/estimating the antenna efficiency is nonsense. A 50 Ohm antenna doesn't radiate worse or better than a 377 Ohm antenna in free space (intrinisic impedance 377 Ohms). Reference antenna-theory.com if he has a problem with that

[xempt]

This is an antenna theory class with mostly grad students. I am an undergrad having some trouble with understanding. You say that the first problem is just plugging in numbers but I am not certain I even understand the question being asked.

If you don't mind going through with me to explain more of these concepts or even recommend me something to read where this would make a little more sense I would appreciate it.

Thank you very much for you reply.