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[ rfc791.ORG : Ham Help : Antennas ]


Harry Nyquist, born in Sweden in 1899, figured out in 1927 that in order for a certain piece of audio to make it through an analog to digital conversion, its highest frequency must be at most half the sampling rate of the analog-to-digital converter. He figured this out in a time when the only sound recording technology was wax. He published this idea in Certain Topics in Telegraphy Transmission Theory in 1928. Nyquist is credited with 138 telecommunications-related patents, and was involved in the invention of telephotography, or the "fax machine".
Commercial antennas can cost hundreds of dollars. So why not save a few bucks and learn some stuff in the mean time by building your own?

Antennas are components that work on RF. The term RF is frequently used to refer to one of two things: Radio Frequency Alternating Current, which is current that occurs in a wire, and Radio Frequency Electromagnetic Radiation, which is radiation that occurs in free space. I'll refer to these two things as RF AC and RF Radiation when the need arises to differentiate.

RF AC is generated in a transmitter, sent to an antenna through a feedline and converted by the antenna into RF radiation. RF radiation propagates through free space and is converted back into RF AC by a receiving antenna, which sends it down a feedline to a receiver for demodulation.

RF radiation

RF radiation is composed of two inseperable fields at right angles to each other. These fields are the magnetic and electric fields. Each wave of the electric field produces a new wave of the magnetic field, and each wave of the magnetic field produces a new wave of the electric field. This amazing phenomenon is known as propagation, and only works if each field creates its counterpart before its counterpart has completely collapsed. This is why only Radio Frequency energy, or energy above about 100KHz, successfully propagates further than one wavelength out.

Our transmitters energize our antennas with electricity and create an electrical field that creates a magnetic field that creates an electrical field that creates a magnetic field, etc, until eventually the magnetic field created through the receiving antenna causes current to flow in it.

Radio Frequency AC

AC stands for Alternating Current. This means that electrons in AC flow one way, then back the other, unlike DC, which goes one way constantly.

Unlike DC, if you add 12 volts AC to 12 volts AC, you don't necessarily get 24 volts AC. The phase relationship of the two AC waveforms also plays a role in the sum. If and only if the AC waveforms are exactly in phase with each other, that is to say that their peaks and troughs line up exactly, then you will end up with 24 volts AC. If the AC waveforms are 180 degrees out of phase, that is to say the peaks of the first wave line up with the troughs of the second wave, then you wind up with 0 volts. Vector math can be used to figure out sum voltages of two waves in other phase relationships.

A force resisting AC is called reactance. Because reactance operates on sine waves, it is also measured as a vector quantity. There are two types of reactance, called inductance and capacitance. Understanding inductance and capacitance is easiest if you understand the basic components that produce inductance and capacitance, the inductor and capacitor.

An inductor is just a coil of wire. When you start pumping current through an inductor, it first opposes the flow as it builds up a magnetic field. When the magnetic field is built up, the current is no longer impeded and flows freely. If you shut off the current going to an inductor, it will essentially act as a voltage source and keep the current flowing until its magnetic field collapses completely.

A capacitor is two plates of conductor seperated by a dielectric (essentially an insulator). When you start pumping current into a capacitor, it builds up electrons on one plate. Electrons vacate the second plate, and head back to your power supply. This attracts more electrons to the first plate by electrostatic force through the dialectric. So DC current appears to flow through a capacitor until it becomes saturated on one side and empty on the other side (essentially). The current flow slows down until this point of saturation, and at this point stops, as there are no more electrons to flow in the plate connected to the positive side of the voltage source, and the side connected to the negative terminal is essentially full.

At DC, these devices make a difference in what happens when you turn the current on, or when you turn it off. This event doesn't happen much at DC, but when you're dealing with AC, this happens all the time. Think for a few moments about what would happen to a capacitor if you hooked it up to a battery one way around, then flipped it backward, and kept repeating these steps over and over. You'd eventually drain the battery by carrying all the electrons over in the capacitor just like a bucket. An inductor would be constantly building and collapsing its magnetic field.

What happens at AC with inductors is they cause the current to lag the voltage by 90 degrees, or one quarter of a cycle. Capacitors cause voltage to lead the current by 90 degrees.

In a DC circuit, Ohms law says V = IR, or voltage equals current multiplied by resistance. In AC circuits, ohms law says V = IZ, or voltage equals current multiplied by impedance. Impedance is the combination of all reactance (capacitance and inductance) and resistance in a circuit.

If you hook an inductor up to a capacitor in a circle, you make a tank circuit. Electrons on the more negatively charged capacitor plate will try to push through the inductor, building up its magnetic field. The inductor will keep pushing to move the electrons as the electron flow stops as one plate begins running out of electrons. Eventually the magnetic field will collapse and the electrons in the now fully charged plate will start looking to go the other way. If it were not for resistive losses in the circuit (the electrical force being converted to heat in the wires), this could go on forever with no voltage source. Hook it up to an amplifier (which requires a voltage source, this is not a perpetual motion machine!) and you have an oscillator.

Enough electronic theory, we now understand that capacitors and inductors are special components that operate at AC. This is important because antennas are essentially a combination of a capacitor and an inductor. They are also a member of a class of devices called transducers, devices which convert one type of energy into another. A speaker/microphone is a transducer. It converts electrical energy into kinetic energy in the air, and vice-versa. And it generally works both ways. An antenna is a transducer. It converts electrical energy into electromagnetic waves in space, and vice versa. And it generally works both ways.

Antenna Types

Your basic antenna is a dipole. You take two long pieces of wire and stretch them out in opposite directions. The center, where the two pieces of wire come together, is called the feedpoint. One of the pieces of wire is connected to one of your feedline conductors, the other to the other.

That's the basics of it. From someone who's used to thinking in terms of DC, it may seem weird that two pieces of wire not connected to each other going in opposite directions can support an electric current, as they don't look at all like a circuit. But remember we're dealing with AC, the magic kind of electricity that can pass through capacitors with ease.
Another important thing to note is the wavelength of RF. 14MHz radio waves are 67 feet in length. 60Hz AC, which is the frequency found in the AC in your wall sockets, has a wavelength of about 3,105 miles. RF can actually be fit in a reasonable length wire. That means you can have a wire 33 feet long with 14MHz RF in it and actually have the voltage be exactly the opposite on one side from what it is on the other! Whoa!

And it just so happens that a half-wavelength wire dipole works really well. If you take 468 and divide it by your operating frequency in MHz, you get about what a half-wavelength of that frequency RF would be, and can cut your dipole about that length.

The device we use to test if an antenna is resonant usually is some sort of SWR meter. An SWR meter tells you the Standing Wave Ratio of the antenna at the frequency you pump into it. The SWR meter essentially tells you if the antenna is too inductive or too capacative. A long dipole is more inductive and a short dipole is more capacitive. A resonant antenna is essentially as inductive as it is capacitive, and in this case, the inductance and capacitance cancel out, and the antenna just appears as a resistance.

Devices called antenna tuners have tunable capacitors and inductors in them, and can be used to compensate for too much inductance or capacitance in the antenna by cancelling it out with the opposite type of reactance. Many hams make antenna not resonant for any particular ham band frequency, it just happens to be as big as they can get it, hook it up to a tuner and get several frequency bands with that antenna.

So that's the amazing dipole. Next on the menu is the loop. A loop is essentially a wire in a big circle. At one point in the circle, the feed line comes in. To the uninitiated, this looks like a short circuit. To DC, it would be a short circuit. But to AC, it's essentially an inductor. Loops work better in situations where the antenna cannot be raised very high off the ground. I have one at roof level on my first story house, and I've talked to spain on it. The formula many hams have found for an optimal loop circumference in feet is 1005 divided by the operating frequency in megahertz.

Other types of antennas are basically variations on these themes. Most of them are variations on the dipole. If you take your horizontal dipole and rotate it such that it's perpendicular to the ground, you have a vertical dipole. If you feed this with coaxial cable, you want the shield to be connected to the leg that points toward the ground.

If you take your vertical dipole, throw away the bottom element, and replace it with a ground plane of some sort, you have a vertical monopole. These are mostly referred to as just "verticals." A ground plane is just some kind of conductor to connect to your coax shield, to make a completed circuit. It can be a circle of wire mesh, a group of "radials", or wires that radiate out from base of the vertical, it can be a car body (that's how car radio antennas work), it can be a cookie sheet sitting on your roof.

Another variation on both the dipole and the loop is the beam. You have one dipole or loop hooked up to the feedline which we call the driven element, then you have multiple other dipoles or loops which are hooked into the transducer's circuit through induction and modify the radiation pattern causing the radiation of the system to go in one direction. We call those other elements "parasitic" elements.

One example of this is the yagi. To figure out exact numbers for calculations on how to build a working yagi, you'll have to consult other references on the web, but basically the way it works is you have your driven element with a larger parasitic element behind it we call the reflector, and zero or more smaller parasitic elements in front we call directors. The radiation going from the antenna travels in the direction the directors are pointing. Radiation received by the antenna favors this direction also.

Polarization

Frequently, you may hear talk of the polarization of an antenna. This refers to the orientation of the electrical field the antenna produces most or is capable of receiving best, relative to the ground. If the electrical field produced is perpendicular to the ground, and the electrical field of the radiation best received by the antenna is perpendicular to the ground, the antenna is said to be vertically polarized. If the electrical field produced is parallel to the ground, and the electrical field of the radiation best received by the antenna is parallel to the ground, the antenna is said to be horizontally polarized. Many hams like horizontally polarized antennas, because most natural RF noise (like that produced by lightning) is vertically polarized.

Gain

The gain of an antenna is a comparison of the amount of radiation of a given antenna in a desired direction as compared to a fixed standard.

The amount of radiation in a desired direction can be modified by building the antenna such that it radiates less in one direction and more in another. Of course, the antenna cannot radiate more energy than that which is fed into it.

The desired direction can be a simple azimuthal direction, like North or East, or it can be a general direction like "up" or "toward the horizon in all directions (and not up at all)". Verticals are good at radiating toward the horizon and not up so much.

There are two fixed standards in common use. One is the isotropic radiator, a theoretical antenna that radiates equally well in all directions. Another is the dipole, which puts out more than twice as much radiation in some directions than the isotropic radiator.

Gain is measured in dB, or Decibels, which is a logarithmic scale. Logarithmic scales allow easier math. Instead of multiplying large numbers to figure out how a given antenna or amplifier affects a signal, the Decibel scale allows for addition of small numbers. If our reference is an isotrope, then we call this unit dBi, if our reference is a dipole, we call this unit dBd.

To figure out how much power comes out of an antenna in a given direction, you can take the gain of the antenna in dBi and multiply the input power of the antenna by ten to the power of the gain in dBi divided by ten. This number can be approximated by two to the power of the gain in dBi divided by three.

For example, imagine we have a dish antenna that has 24 dBi gain and that's hooked to a feedline that has 3dB loss and that's connected to a transmitter with 200mW output. We take the 24 dBi gain of the antenna and subtract the 3 dB loss of the feedline to arrive at 21 dB of gain. Since 21 is divisible by 3 and not 10, we'll divide 21 by 3 (7) and take two to that power (128) to arrive at out gain factor. Multiplying 128 by .200 W gives 25.6 W.

Frequently RF engineers will refer to power in dBm, or decibels over one milliWatt. 200 milliwatts is 23 dBm. 25 Watts is 44 dBm. 23-3+24 = 44. No multiplication required. Unfortunately some people find it more difficult to think logarithmically.

By the way, the easy way to convert from dB is this: Take the last digit of the decibel rating and convert that from dB to a factor. Then multiply by ten to the power of the remaining digits.

The easy way to convert to dB is to take the number of digits in your gain factor and multiply by ten, then add that number to log base 10 of your gain factor converted to db.

Feedline

Antennas can be fed several different ways, but there are two basic types, balanced and unbalanced.

Balanced line used to be the standard type of feedline for amateur radio stations. In balanced feeder, two conductors run in parallel from the radio to the antenna. Each of these conductors contains the RF AC, but the two conductors are 180 degrees out of phase. Balanced line is usually used to feed antennas in which this arrangement is optimal. These types of antennas, which include dipoles and loops, are also called balanced.

The common type of feedline used nowadays, primarily for its convenience, is unbalanced feedline, or coaxial line. Coax has one conductor surrounded by a dielectric, surrounded by another conductor, surrounded by a non-conductive jacket. The outer conductor is called the shield, and the other conductor is quite simply called the center conductor. Coax is unbalanced because the center conductor contains the RF AC, whereas the shield is connected to ground and maintains a constant zero voltage. Coax is usually used to connect to antennas for which this arrangement is optimal. These types of antennas, which include vertical monopoles, are also called unbalanced.

Many hams will feed a balanced antenna with unbalanced feeder because of its convenience and availability, or because their radio has an unbalanced connector, and connect the feeder to the antenna through a device called a balun. A balun is a device that transforms the 50 ohms impedance of the coax to the 400 ohms impedance of the balanced line.

The simplest balun is a piece of coax one half wavelength long, with the shield at one end connected to the shield at the other end. The shield of the incoming coax is connected to the shield of the coax balun. The center conductor of the incoming coax is connected to the center conductor of one side of the coax balun. Then the center conductor from each side of the coax balun is connected to each side of the balanced feeder, or each side of the balanced antenna. The result is that the RF appears at one conductor of the balanced feeder half a period behind when it appears at the other conductor, thus the two conductors end up being energized 180 degrees out of phase, exactly as they should be.

Usually, though, hams prefer broadband baluns, because coax baluns for HF would end up being inordinately large. Broadband baluns come in nice small cylindrical containers, usually about 6 inches in length and two inches in diameter, and work on all frequencies. Broadband baluns work very much like transformers.