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Antenna Primer Part I:

Definition, History, and Build a Random-Length Antenna

 Copyright by Clem Small KR6A



This is the first of three parts of a primer on antennas and their applications. If you already have some ideas about what antennas are and how they work, this primer may help you organize and clarify those ideas. If you haven’t yet been introduced to antennas, then this primer will start you on your way to working with, and even building, your own antennas.


What is an Antenna?


An antenna is a device which either transmits radio (electromagnetic) waves into the space around it, or receives radio waves from that space. It is possible to make a simple, working antenna from a simple piece of wire. On the other hand, some antenna designs are very complex devices with multiple, precisely dimensioned conductors which are spaced at precise distances from one another. There is a large number of different antenna designs available, and the selection and utilization of an appropriate design is an interesting and exciting part of radio communications.


A Bit of History


It is interesting that when radio waves were first demonstrated convincingly to the scientific world, some of the basic antenna designs we utilize today were already developed. Henrik Hertz was the first scientist to convincingly show that radio waves did in fact exist. And his early work reported such basic antenna designs as the halfwave dipole, and the parabolic reflector antenna, (figs. 1A, 1B). He also demonstrated the principle of the dielectric-lens antenna.


Using the discoveries of Hertz and others, Guglielmo Marconi developed a working radio communication system. At one point in his work, Marconi took a Hertzian dipole antenna and removed half of it. This left a quarter wavelength piece which Marconi mounted upright on the ground. He left one feedline connected to the bottom of the upright half of the dipole, and grounded the feedline connection which had formerly been connected to the half which he had removed. The antenna worked quite well, and in his honor it is called the “Marconi, grounded, quarterwave, vertical antenna.”  Today it is utilized in many AM broadcast station installations and many shortwave stations.


Marconi and his engineers developed other antennas also, most notably the L-antenna and the Imperial Beam. Whereas the Marconi grounded quarterwave antenna transmitted and received equally well in all compass directions, the L and Imperial antennas were directional antennas, or “beams.”  This means that they could focus their transmitted energy or reception responsiveness in particular directions.


In addition, Marconi’s engineer Franklin developed a phase-based design for antenna elements which causes an antenna to focus its waves somewhat perpendicular to the antenna. Various versions of the Franklin design remain quite useful today for both to-the-horizon coverage with very high frequencies (VHF) and higher frequencies, and in beam antennas for high frequency (HF) operation.


To support the growing utilization of trans-oceanic radio communication, a number of very large directional beam antennas were developed from circuits used in the early days of radio. One class of these beams was derived from an antenna known as a “long wire” antenna. These include the V-beam, and the rhombic beam, and were known as “wire beams.” “Curtain beams” were gigantic beam antennas with a large number of elements forming something like a hanging curtain. They often utilized a second antenna behind the main antenna (driven element) to reflect RF energy such that the beam’s radiation-pattern was primarily unidirectional.


George Brown discovered that beams could be improved by spacing their elements closer than the quarter wavelength that had formerly been utilized between driven element and reflector. Another important development was the Yagi-Uda beam antenna, in which both a reflector and director element were used in addition to the main (driven) element to give greater directivity and higher gain than was previously available. The relatively small size of the Yagi-Uda beam meant that at frequencies above 10 MHz one could have an antenna which was both highly directive and able to be rotated by remote control. Remote control greatly facilitated pointing the antenna for maximum performance in any compass direction.


As radio technology developed, operation became practical on higher and higher frequencies. In the VHF, and particularly the UHF and microwave bands, the small size of the wavelengths involved led to the development of many antenna designs which would have been too large to be practical at lower frequencies with their longer wavelengths. Antennas such as the helical, corner reflector, dish reflector, waveguide, horn, slot and patch antennas are examples of designs more practical at the shorter wavelengths. Many of these designs have been utilized in radar, cross-country repeaters, and for space, satellite and aircraft communications. Various reflector-type antennas have been important in the development of radio astronomy.


Many other types of antenna designs have been of considerable importance in the development in various areas of radio communications. Wide band, multi-band and the so-called “frequency-independent” antennas have facilitated ease of switching between multiple frequencies. As early as the pioneering days of wireless some radio direction-finding antenna designs were put to use for general radio-location, location of enemy transmitters in wartime, and for search and rescue operations at sea.


Some of the areas where today’s engineers are looking for new antenna designs, as well as adapting existing designs, include space and satellite communication, and putting antennas inside cell phones, pagers, and digital-computer accessories such as wireless mice, and wireless modems.


And So:


This historical sketch is necessarily abbreviated, but it is easy to see that antenna technology has a long and important list of contributions to the advancement of radio technology. In the next installment  we’ll continue with a discussion of some of the important concepts you’ll need to appropriately select and utilize antennas for your own use. We’ll also talk about building your own dipole antenna.  



Let’s Make an Antenna:


For the beginner wanting an antenna to use for general monitoring on high frequency, medium frequency, or even lower in frequency, one of the easiest to make is the random-length wire antenna. Start by finding a good place to string the wire as high, long, and in-the-clear as possible. Get enough metal wire of any kind and size that is strong enough to hold together for the distance you intend to span. Put the antenna up with insulators at each end as shown in fig. 1C, and run the end into your radio room. Don’t string it near power lines. Connect the end of the antenna to your receiver’s antenna input terminal, and start monitoring.


Don’t forget lightning-induced damage protection: the minimum is to disconnect and ground the antenna when it is not in use, and never use it when weather is likely to produce lightning.



Here is an interesting site with lots of tips for beginners:  


This article first appeared in Monitoring Times, February 2002 "Antenna Topics"