Title: VLF LF MF and HF ANTENNAS
1VLF LF MF and HF ANTENNAS
- CLASSIFICATION ACCORDING TO
- FREQUENCY BANDS
2- VLF/LF Antennas and Antenna Systems
3 VLF Band
- EM waves penetrate well into the sea water.
- (Communications with submerged submarines)
- Low atmospheric attenuation.
- Appropriate for long range communication.
4VLF Antennas
- Ground and Sky waves
- Frequeny range 3-30 KHz
- Antennas very large
- Power kW levels and even more
5Some 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.
6LF Antennas
- Ground and Sky waves
- Frequeny range 30-300 KHz
- Antennas large
- Power kW levels and even more
7Some Disadvantages
- High cost
- Large Dimensions
- Trouble with efficiency, power capacity, bandwidth
8- VLF and LF antennas are electrically small
antennas - problem high capacitive reactance and small
antenna radiation resistance - remedy top loading
9Top-loading
- Top-loading
- increases gain bandwidth (by decreasing
reactance) - In VLF large top-loading
- supported by towers
10A simple VLF/LF Transmitting Antenna
11VLF / LF Ground Systems
- Radial-wire
- radial wires buried in the ground
- Multiple-star
- small radial-wire systems forming a star
topology
12Basic Theory The Vertical Electric Monopole
Antenna
13Vertical Electric Monopole Antenna
- Assume uniform electric current I along a
vertical monopole of - effective height he
electric field
magnetic field
14Vertical Electric Monopole Antenna-Radiated
Power-
The vertical electric field in terms of radiated
power is
15Vertical Electric Monopole Antenna-Equivalent
Antenna Circuit-
16Vertical Electric Monopole Antenna(Radiation
Efficiency)
and
where
antenna total loss resistance
Effective power (power capacity of the
transmitter) x (antenna system efficiency)
17Vertical Electric Monopole Antenna-Antenna
Bandwidth-
- The 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
18Multiple Tuned VLF Antennas
- To have sufficiently large bandwidths
- Huge antenna systems must be built.
- or
- Several small multiple-tuned elements must be be
used.
19Multiple Tuned VLF Antennas
20Multiple 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.
21Triatic Type Antenna
22Cutler, Maine Antenna Installation
23Goliath Antenna
24Goliath Antenna
25References
(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 Pages1091 - 1096
vol.3 (3) Technology Conference,
1991.IMTC-91.Conference Record. ,8th IEEE , 14-16
May 1991 Pages330 - 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
26MF ANTENNAS ANTENNA SYSTEMS
27INTRODUCTION
- Usually Vertical radiators operating in the MF
band (300-3000 kHz). - The towers may be guyed or self-supporting.
28APPLICATION AREAS
- AM Broadcasting
- Maritime Radio
- Coast Guard Communication
- Direction Finding
29CHARACTERISTICS OF RADIATORS
- Maximum radiation in the horizontal plane
- Antennas taller than one-half wavelength have a
minor lobe
30Characteristics of the Radiators
- Requirement for metallic ground plane to minimize
losses - Vertical polarization is preferred due to
superior propagation characteristics
31Other features of the radiators
- Shunt fed radiators
- Top loaded radiators
- Sectionalized radiators
32Circuits for MF antenna systems
- Antenna tuning units for matching purposes
- Phase shifter networks for directional antenna
systems - Power dividing networks
33Ground 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.
34Ground Systems
- A typical ground system for a two-element
directional antenna
35HF Antennas Antenna Systems
36HF Antennas and Antenna Systems
- Frequency Range 3 to 30 MHz
- ( 10 to 100 meters in wavelength)
- For medium- and long- distance communications
and broadcoasting
37Characteristics 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.
38Types 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
39Non-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
40Long-wire Antenna
- A long terminated wire radiator
41Vee 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.
42Rhombic 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.
43Resonant HF Antennas
- Monopole Antenna
- Elevated-feed Monopole
- Double-cone Monopole
- Inverted-L and T Antenna
- Dipoles and Slot Antennas
- Loop Antennas
44Monopole Antennas
- Outside half-wave resonance, elevation pattern
breaks up into main - lobes as input impedance becomes very high.
Efficiency decreases
45Dipole Antennas
46Loop Antennas
- Usully used for reception and direction finding.
47The Log-Periodic Antenna
- Fed from the vertex.
- Signal travells along the structure until reaches
its resonant region. - The signal radiates from the resonant region
48Directional 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
49End-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
50Broadside Arrays
- Beam steering by phase variation is possible.
51Circular 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
52References
- 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.