2. TURNSTILE ANTENNA
 A turnstile antenna is a set of two dipole
antennas aligned at right angles to each
other and fed 90 degrees out-of-phase.
 It has been developed primarily for
omnidirectional vhf communications.

3. TURNSTILE ANTENNA –
cont’d
 Antenna Gain - The gain of a turnstile
antenna is actually about 3 dB less than
that of a single dipole in its direction of
maximum radiation, because each of the
elements of the turnstile receives only
one-half the transmitter power.

4. TURNSTILE ANTENNA –
cont’d
 Turnstile antennas are often used for
FM broadcast reception, where they
give reasonable performance in all
directions without the need for a rotor.

5. RADIATION PATTERN
 When these two antennas are excited
with equal currents 90 degrees out of
phase, the typical figure-eight patterns
of the two antennas merge to produce
the nearly circular pattern.

6. RADIATION PATTERN

7. EXAMPLE OF TURNSTLE
ANTENNA

9. MARCONI ANTENNA
 Marconi Antenna is usually a quarter –
wave grounded antenna or any odd
multiple of quarter wavelength.
 It is used primarily with frequencies
below 2 MHz .
 Marconi type requires a conducting path
to ground.

10. GROUNDED MARCONI
ANTENNA

11. EFFECTS OF GROUND
REFLECTION
 The transmitter is connected between
the antenna and ground.
 When an antenna is installed within a
few wavelengths of the ground, the
earth acts as a reflector and has a
considerable influence on the
radiation pattern of the antenna.
 Image antenna – the reflection from
the ground looks like it is coming a
quarter wavelength section beneath
the ground.

12. EFFECTS OF GROUND
REFLECTION – cont’d
 Half-wave operation may be obtained.
All of the voltage, current and
impedance relationships characteristics
of a half-wave antenna will also exist.
 The effective current in Marconi antenna
is maximum at the base and minimum at
the top, while voltage is minimum at the
bottom and maximum at the top.

13. EFFECTS OF GROUND
REFLECTION – cont’d
 If the conductivity of the soil in which the
Marconi antenna is supported very low,
the reflected wave from the ground may
be attenuated.
 This problem is alleviated by installing a
counterpoise.

14. COUNTERPOISE
 It is a portion of an antenna system that
is composed of wires or other types of
conductor arranged in a circular pattern
at the base of the antenna at a certain
distance above ground.
 Counterpoise is a grounding grid
established where the earth grounding
cannot satisfy electrical requirement for
circuits completion. It designed to be
non-resonant at the operating
frequency.

15. COUNTERPOSE

17. RADIATION PATTERN

18. TOP LOADING
 It is used to increase the current at the
base of antenna, and also to make the
current distribution more uniform.

20. GROUND PLANE ANTENNA
 A ground-plane antenna is a variant of
the dipole antenna, designed for use
with an unbalanced feed line such as
coaxial cable.
 It resembles a coaxial antenna whose
lower section consists of straight
elements called radials instead of a
hollow conducting cylinder.

21. GROUND PLANE ANTENNA -
cont’d
 There are two or more radials, each
measuring ¼ wavelength.
 The main element can be any length,
but it must be adjusted to function at and
near a specific frequency. This
adjustment is done using a tuning coil.

22. GROUND PLANE ANTENNA –
cont’d

23.  Ground-plane antennas are favored at
frequencies above approximately 10
MHz where the dimensions are
manageable.
 This type of antenna is especially
popular among Citizens Band radio
operators for fixed-station use in the
class-D band at 27 MHz.