Frequency Independent Antennas ECE 5318/6352 Antenna Engineering Dr. Stuart Long Chapter 11 Frequency Independent Antennas

Important characteristic THE BEGINING Before 1950’s no antennas with bandwidth > 2 : 1 V.H. Rumsey – University of Illinois 1955-1958 Important characteristic Antenna performance is typically a function of “ length / wavelength “

SCALING PRINCIPLE Scale size by ratio of frequency and get similar behavior e.g. decrease antenna dimensions by factor of 2 and increase frequency by factor of 2 same performance (both are the same size in wavelengths) depends on some “characteristic length”

SCALING PRINCIPLE (CONT) Basic structural feature for frequency independent operation is the absence of any “characteristic length”  self -scaling

ANGLE PRINCIPLE need a structure completely described by angles

TRUNCATION PRINCIPLE but practical structures need to be finite in size somehow currents need to fall off rapidly enough so that terminating the structure at some finite length does not alter its characteristics “TRUNCATION PRINCIPLE”

TRUNCATION PRINCIPLE (CONT) Example infinite biconical infinite bi-fin (bow tie) Why need such broad band? e.g. TV. channels Jammers

EQUIANGULAR SPIRAL (log spiral) Boundaries given by Except for a rotation in space it looks the same for all frequencies

EQUIANGULAR SPIRAL (CONT) Earlier work by H.G. Booker on impedance relationship between a slot and its complementary dipole

EQUIANGULAR SPIRAL (CONT) if could somehow make an antenna and its complement the same, then the impedance would have to be constant, independent of frequency called “ SELF-COMPLEMENTARY ”

EQUIANGULAR SPIRAL (CONT) Dyson tried to build a self-complementary spiral; found  a constant and radiation pattern over several octaves; also had circular polarization  current “ I ” fell off faster than over a frequency independent band before truncation

EQUIANGULAR SPIRAL (CONT) found  lost frequency independent behavior; frequency was lowered to point where diameter (upper limit on frequency depends only on the accuracy of the feed region) But needed linear polarization (LP) as well as circular polarization (CP)

EQUIANGULAR SPIRAL (CONT) Fig. 11-2 Spiral plate and slot antennas

EQUIANGULAR SPIRAL (CONT) Fig. 11.4 Conical spiral metal strip antenna