The document discusses two types of broadband antennas: spiral antennas and log-periodic antennas. Spiral antennas can be conical or planar and are circularly polarized. Their bandwidth depends on the ratio between the base and apex diameters for conical spirals, and between the overall diameter and input terminal spacing for planar spirals. Log-periodic antennas use a logarithmic spiral shape to achieve frequency independence. They are constructed by rotating logarithmic spirals by 90 degrees to form a self-complementary pattern that provides broadband, bidirectional radiation.

1. Broad Bandwidth Antennas
Spiral and the Log-Periodic

2. Spiral Antenna
• Circularly polarized
• Flat spiral has been wrapped around a
dielectric cone.
• The conical spiral is fed by a coaxial cable
bonded to one conducting strip, with its inner
conductor joined to the other strip at the
apex.

3. Conical Spiral Antenna

4. Conical Spiral Antenna
• The lower-frequency limit
– when the base diameter is λ/2.
• The high-frequency limit
– when the apex diameter is λ/4.
• The bandwidth is in the ratio ½ base diameter
to apex diameter is about 7 to 1

5. Log-Spiral Antenna
bidirectional planar spiral

6. • The equation for a logarithmic (or log) spiral is
given by
r = aθ (or) (1)
ln r = θ ln a (2)
• Where,
r = radial distance to point P on spiral
θ = angle with respect to x axis
a = a constant
• The rate of change of radius with angle is
• dr/ dθ = aθ ln a = r ln a (3)

7. • The constant a in (3) is related to the angle β between
the spiral and a radial line from the origin as given by
(4)
From (4) and (2), (5)
• The log spiral in the above Fig. is constructed so as to
make r = 1 at θ = 0 and r = 2 at θ = π.
• These conditions determine the value of the constants
a and β.
• Thus, from (4) and (5), β = 77.6◦ and a = 1.247.
• Thus, the shape of the spiral is determined by the angle
β which is the same for all points on the spiral.

8. • Let a second log spiral, be generated by an angular
rotation δ so that (1) becomes
and a third and fourth spiral given by
and
• Then, for a rotation δ = π/2 we have 4 spirals at 90◦
angles.
• Metalizing the areas between spirals 1 and 4 and 2 and
3, with the other areas open, self-complementary and
congruence conditions are satisfied.
• Connecting a generator or receiver across the inner
terminals, we obtain frequency-independent planar
spiral antenna

9. Planar spiral antenna

10. Planar spiral antenna
• The arrows indicate the direction of the outgoing waves
traveling along the conductors resulting in Right Circularly
Polarized (RCP) radiation outward from the page.
• Left-circularly polarized radiation into the page.
• The high-frequency limit of operation is determined by
the spacing d of the input terminal
• The low-frequency limit by the overall diameter D
• The ratio D/d for the antenna of above Fig. is about 25 to
1
• If we take d = λ/10 at the high-frequency limit and D = λ/2
at the low-frequency limit, the antenna bandwidth is 5 to
1.