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Log Polar Antenna

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Thejas Kesari

on 9 November 2013

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Transcript of Log Polar Antenna

Rumsey's Principle
The dimensions of antenna, when expressed in terms of wavelength, vary with frequency, antennas usually exhibit different radiation properties at different frequencies.
Variations of radiation characteristics with frequency limit the bandwidth of the antenna and thus the information carrying capacity of the communication link to which the antenna belongs.
The issue of frequency independent antennas, which ideally provide an infinite bandwidth, was first addressed by Rumsey, who explained the following basic requirements for these antennas


Log Periodic Antenna
Invented by Dwight E. Isbell, Raymond DuHamel of the University of Illinois at Urbana-Champaign (and variants by Paul Mayes).
Prelude
Self-complementary antenna is an arbitrarily shaped antenna which is constituted with a half of an infinitely extended planar-sheet conductor such that the shape of its complementary structure is exactly identical with that of the original structure with two terminals for the simplest case.
Log Periodic Antenna
Antennas & Propagation Seminar
November 9, 2013
Overview
Intro
Background Check
Frequency Independent Concept
Log Polar Dipole Antenna
Construction & Components
Working
Design & Parameters
Applications & Advantages
Limitations
Recent Developments
Exam Perspective

The self-complementary antenna has constant input impedance independent of the source frequency and the shape of the structure.
Mushiake Relationship
Z = Zo / 2
Angle Specified Antennas
Rumsey proposed that if the shape of a lossless antenna is such that it can be specified entirely by angles, its performance such as pattern and input impedance would remain unchanged with frequency. In other words, the dimensions of this class of antennas, when expressed in terms of wavelength, are the same at every frequency.

The implication is that electrical characteristics of the antenna do not change with frequency. This is a very simple and powerful idea for the design of broadband antennas, which are referred to as frequency independent antennas for the ideal case.
 


Self - complementary Configuration
In addition to the angle dependence, a second principle was used in the early development of frequency independent antennas.
This principle states that if an antenna has the same shape as its complement empty part, its impedance is constant at ALL FREQUENCIES.

Motivation
The advent in information technology and wireless communications had created a continued demand for data transmission at higher rates and over longer distances.

Transmission of data at higher rates requires wider bandwidths.

These requirements for the antenna necessitated rigorous research in the field. Rumsey’s work became a much needed shot in the arm.
Thus, we got the Log Periodic Antenna!
Now, what exactly is Log Periodic Antenna?
It is a self-complementary antenna that can operate on a wide frequency band and has the ability to provide directivity and gain. It has radiation and impedance characteristics that are repeated as a logarithmic function of excitation frequency.
Types:
Zig-zag log periodic array
Trapezoidal log periodic
Slot log periodic
V log periodic
Log Periodic Dipole Array
The most common is the log periodic dipole array basically consists of a number of dipole elements.
These diminish in size from the back towards the front.
In operation, as the frequency changes there is a smooth transition along the array of the elements that form the active region.
Working
There are three regions:
Active region
Reflective region
Directive region

The active region is the region of LPDA in which electromagnetic waves will resonate within the operating frequency band.
Here the length of the dipoles is lambda/2.
Thus at the wavelength lambda, radiation will occur from the middle portion where the dipole elements are lambda/2 long.

Reflective region is the part of the array which acts like reflector for the resonated elements.
Here the length of the dipoles is greater than lambda/2.
This region will introduce inductive reactance to the line, hence the current is small. Therefore, the radiation will be small.
When the wavelength is increased the radiation zone move towards the right

Directive region is the part of the array which acts as director for the resonated elements.
Here the length of the dipoles is less than lambda/2.
This region will introduce the large capacitive resistance to the line, hence the current in the region is small and radiation is also small.
When the wavelength is decreased the radiation zone move towards the left.
Parameters
Bandwidth - is the range of frequencies over which it is effective, usually centered around the resonant frequency.
Gain - is the comparison of transmitter and receiver EM waves. Gain of Log Periodic antenna is very high.
Polarization - A plane electromagnetic (EM) wave is characterized by travelling in a single direction. In this case, the electric field and the magnetic field are perpendicular to each other and to the direction the plane wave is propagating.
LP antenna has linear polarization.
Impedance - As the electric wave travels through thedifferent parts of the antenna system,it may encounter differences in impedance.

SWR - The ratio of maximum power to minimum power in a wave.

Radiation Pattern - shows us that how an antenna transmits and receives electromagnetic signal or energy in different directions.

Construction
Applications & Advantages
Ham Radio
Cellular and Wi-Fi Networks
Mechanically simple - no risk of charring electronics due to a lightning strike
Very wide band application
No need of stepper motors to change frequency
Not extremely expensive
Easy on the maintenance front
Limitations
Low gain
Only a few of the elements are active on a given frequency, the rest exist along for the ride
The dipole element for the lowest operation frequency in the HF range may become too long to be conveniently handled in the environment of application
Recent Developments
Antennas are constructed with insulated 14ga stranded electrical wire (500ft for $16, enough for two antennas), 4-inch pieces of 1/2-inch PVC electrical conduit for end insulators (10ft for less than a dollar), 450-Ohm ladder line (100ft for $15, enough for four antennas) and a 4:1 balun ($15-25). It is fair to say that a complete antenna costs less than $50.
Dipoles don't have to be straight and parallel. To allow inexpensive support, dipoles can angle down or sag with no practical radiation or feed problems. This installation uses two 10ft masts spaced 32ft either side of the main support mast and equipped to gather 10 dipole support lines. Get the longest highest dipole as close to 35ft as possible, spread the elements out as best you can (but in the same plane), add a 50-Ohm coaxial transmission line and have at it.

Key References
V. H. Rumsey, "Frequency Independent Antennas", IRE national convention record, Part I, pp. 114-118, 1957.
D. Isbell, "Log periodic dipole array", IRE Transactions on Antennas and Propagation, Volume 8, Issue 3.
Y. Mushiake, "Self-Complementary Antennas", Springer-Verlag, Berlin, 1996.
R. L. Carrel, "The Design of Log-Periodic Dipole Antennas", IRE Int. Conv. Rec., Part I, pp. 61-75, 1961.
R. S. Elliott, "A View of Frequency Independent Antennas", The Microwave Journal, Vol. 5, pp. 61-68, December 1962.
Jeong Il Kim, "Log Periodic Loop Antennas", Virginia Polytechnic Institute Theses Archives, 1999.
The standard parallel-wire line is shown in Fig A. There are a number of possible LPDA structures using booms. The booms serve both to support the elements and to create relatively low-impedance (under 200 ohm) phase-lines. Fig B shows the basics of a twin circular tubing boom with the elements cross-supported by insulated rods. Fig C shows the use of square tubing with the elements attached directly to each tube by through-bolts. Fig D illustrates the use of L-stock, which may be practical at VHF frequencies.

Wire Log-Periodic Dipole Arrays for 3.5 or 7 MHz: Parts and construction

Cut the wire elements to the proper sizes and mark them for identification beacause after the wires are cut and placed aside, it will be difficult to remember which is which unless they are marked. When you have finished fabricating the connectors and cutting all of the wires, the antenna can be assembled. The elements are made of 14 stranded copper wires. The two parallel feed lines are made of 12 solid copper-coated steel wire, such as Copperweld. Copperweld will not stretch when placed under tension. The front and rear connectors are cut from 1/2-inch thick Lexan sheeting, and the feed-line spacers from 1/4-inch Plexiglas sheeting.
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