VLF LF MF and HF
ANTENNAS
CLASSIFICATION ACCORDING TO
FREQUENCY BANDS
VLF/LF Antennas and
Antenna Systems
VLF Band
EM waves penetrate well into the sea water.
(Communications with submerged
submarines)
Low atmospheric attenuation.
Appropriate for long range communication.
VLF Antennas
•
Ground and Sky waves
•
Frequeny range: 3-30 KHz
•
Antennas : very large
•
Power: kW levels and even more
Some Problems Associated with VLF
Antenna Systems
•
Small Bandwidth (usually less than 200 Hz)
•
Small radiation resistance.
•
High cost.
•
Antenna system covers a large area.
•
Need for very high power levels for transmission.
LF Antennas

Ground and Sky waves

Frequeny range: 30-300 KHz

Antennas: large

Power: kW levels and even more
Some Disadvantages
 High
cost
 Large
Dimensions
 Trouble
with efficiency, power capacity,
bandwidth
VLF and LF antennas are
“electrically small” antennas :

problem: high capacitive reactance and
small antenna radiation resistance

remedy: top loading
Top-loading
 Top-loading
increases gain bandwidth (by decreasing reactance)
 In
VLF large top-loading
supported by towers
A simple VLF/LF Transmitting
Antenna
VLF / LF Ground Systems

Radial-wire:
radial wires buried in the ground

Multiple-star:
small radial-wire systems forming a star topology
Basic Theory: The Vertical Electric
Monopole Antenna
Vertical Electric Monopole Antenna
Assume uniform electric current I along a vertical monopole of
effective height he :
electric field
magnetic field
Vertical Electric Monopole Antenna
-Radiated Power-
The vertical electric field in terms of radiated power is:
Vertical Electric Monopole Antenna
-Equivalent Antenna Circuit-
Vertical Electric Monopole Antenna
(Radiation Efficiency)
where
and
antenna total loss
resistance
Effective power = (power capacity of the transmitter) x (antenna
system efficiency)
Vertical Electric Monopole Antenna
-Antenna BandwidthThe 3 dB bandwidth b in (c/s) for a single resonant circuit is:
f : resonant frequency
Q: the circuit reactance resistance ratio X/R0
R0: Total series resistance
Multiple Tuned VLF Antennas
To have sufficiently large bandwidths:


Huge antenna systems must be built.
or
Several small multiple-tuned elements must
be be used.
Multiple Tuned VLF Antennas
Multiple Tuned VLF Antennas
 Ground losses are reduced.
 Radiation resistance and efficiency are increased.
 Instead of one and vulnerable antenna, several and smaller
elements can achieve the same bandwidth-efficiency product.
 If one element is out of service, the others can still
operate.
 The effective ground loss with multiple-tuning will be less
than for a single element.
Tuning and retuning the system is difficult.
each antenna has to be matched to the transmitter.
Triatic Type Antenna
Cutler, Maine Antenna Installation
Goliath Antenna
Goliath Antenna
References
(1) “VLF Radio Engineering”, A. D. Watt, Perg. Press, 1967
(2) “High Power Very Low Frequency/Low Frequency Transmitting
Antennas”, P Hansen, Military Communications Conf., 1990. MILCOM
'90, Conference Record, 'A New Era'. 1990 IEEE, 30Sept.-3Oct.1990
Pages:1091 - 1096 vol.3
(3) Technology Conference, 1991.IMTC-91.Conference Record. ,8th IEEE
, 14-16 May 1991 Pages:330 - 334
(4) “Multiple Tuned VLF Antennas”, Manfred Schopp, IEEE Transactions
on Broadcasting, Vol. 39, No.4, Dec. 1993.
References for the photos & figures:
[1] http://hawkins.pair.com/nss.shtml
[2] http://www.tpub.com/neets/book17/77.htm
MF ANTENNAS &
ANTENNA SYSTEMS
3/15/2016
26
INTRODUCTION
•
Usually: Vertical radiators operating in the
MF band (300-3000 kHz).
•
The towers may be guyed or selfsupporting.
APPLICATION AREAS
•
AM Broadcasting
•
Maritime Radio
•
Coast Guard Communication
•
Direction Finding
CHARACTERISTICS OF
RADIATORS
•
•
Maximum radiation in the horizontal plane
Antennas taller than one-half wavelength have a
minor lobe
Characteristics of the
Radiators

Requirement for metallic ground plane to
minimize losses

Vertical polarization is preferred due to
superior propagation characteristics
•
•
•
Other features of the
radiators
Shunt fed radiators
Top loaded radiators
Sectionalized radiators
Circuits for MF antenna systems
•
•
•
Antenna tuning units for matching purposes
Phase shifter networks for directional antenna systems
Power dividing networks
FROM
TRASMITTER
PHASE CONTROL
NETWORK
T-LINE
ANTENNA
TUNING
UNIT
T-LINE
ANTENNA
TUNING
UNIT
POWER
DIVIDER
NETWORK
PHASE CONTROL
NETWORK
Ground Systems
•
•
•
•
•
120 buried ( /4 length) copper wires
Extending radially outward
120-180 cm depth is sufficient
Individual ground systems are required
for each tower of the array.
Copper-mesh ground system may also
be used.
Ground Systems
A typical ground system for a two-element directional
antenna
HF Antennas &
Antenna Systems
HF Antennas and Antenna
Systems

Frequency Range: 3 to 30 MHz
( 10 to 100 meters; in wavelength)

For medium- and long- distance
communications and broadcoasting
Characteristics of HF Antennas:

Signals are distorted as the ionosphere is neither
regular nor smooth.

High powers and high antenna gains may be
needed for communication.
Types of HF Antennas:
Non-Resonant HF Antennas
Long-wire Antenna
Vee Antenna
Rhombic Antenna
Resonant HF Antennas
Monopole Antenna
Dipoles and Slot
Antennas
Loop Antennas
Log Periodic HF Antennas
Early Log-Periodic Antenna
Logarithmic Dipole Antenna
Directional HF Antennas
End-fire Arrays
Broadside Arrays
Circular Arrays
Non-Resonant HF Antennas:
•
wave propagates along the radiator in one
direction only
• remaining power is absorbed in a matched load
TYPES



Long-wire Antenna
Vee Antenna
Rhombic Antenna
Long-wire Antenna
A long terminated wire radiator
3/15/2016
40
Vee Antenna



Single mast (one wire radiator terminated in a resistive load
at the far end).
Radiation pattern exhibits large side lobes near the main beam.
The efficiency is low (almost half of the total input power may be
exhausted in the matched load.
3/15/2016
41
Rhombic Antenna
•
•
•
•
4 radiating wires of equal length mounted on four masts
one of the wires are load-matched.
high directivity
the large rhombics are used for long-range communications.
3/15/2016
42
Resonant HF Antennas:

Monopole Antenna
• Elevated-feed Monopole
• Double-cone Monopole
• Inverted-L and –T Antenna

Dipoles and Slot Antennas

Loop Antennas
3/15/2016
43
Monopole Antennas
Outside half-wave resonance, elevation pattern breaks up into main
lobes as input impedance becomes very high. Efficiency decreases
3/15/2016
44
Dipole Antennas
Loop Antennas
Usully used for reception and direction finding.
The Log-Periodic Antenna



Fed from the vertex.
Signal travells along the structure until reaches its resonant region.
The signal radiates from the resonant region
Directional HF Antennas:

End-fire Arrays
• Horizontal Array of Dipoles
• RCA Fishborne Antenna
• Series Phase Array

Broadside Arrays
• Broadside Dipole Array
• Wide-Band Curtain Array

Circular Arrays
End-fire Arrays





Higher directivity.
Provide increased directivity in
elevation and azimuth planes.
Generally used for reception.
Impedance match difficulty in
high power transmissions.
Variants are:

Horizontal Array of Dipoles

RCA Fishborne Antenna

Series Phase Array
Broadside Arrays
Beam steering by phase variation is possible.
Circular Arrays



Used for direction finding.
Consists of 30 – 100 elements, with equi-spaced and fed
from a central source – goniometer.
Band-width seperation is possible:
References

JASIK, H.: Antenna Engineering Handbook; Mc Graw Hill,
1961

Y.T., LEE S.W.: Antenna Handbook; Van Nostrand Reinhold,
1988.

RUDGE, A.W., MILNE K., OLVER A.D., KNIGHT P.: Handbook
of Antenna Design (Volume 2); Peter Peregrinus, 1983.