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

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

3. 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”

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

5. ANGLE PRINCIPLE need a structure completely described by angles

6. 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”

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

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

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

10. 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 ”

11. 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

12. 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)

13. EQUIANGULAR SPIRAL
(CONT)
Fig Spiral plate
and slot antennas

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