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What's an antenna?

posted Jun 9, 2012, 11:15 AM by Charles Boling
Q: What is an antenna?
A: Any chunk of metal or other electrical conductor

If it took something special to qualify as an antenna, there sure would be a lot fewer RFI (radio-frequency interference) problems in the world!

Pretty much any conductor carrying an alternating current will radiate some of its energy in the form of electromagnetic waves. 

In most electronic circuits, the length of the conductor is minuscule compared to the  wavelength of the current.  (For example, a typical audio-frequency tone of 1 kHz would have an electromagnetic wavelength of about 300 km.)  As the relative length of the conductor becomes larger, more of the energy is radiated (and, as a receiver, more energy is absorbed and turned into electric currents along the line).  As you're probably aware, a wire that is 1/4 wavelength or longer can make a fine antenna indeed!  At 3 GHz -- certainly not an unheard-of frequency today, particularly within the CPU of a modern computer -- a 1/4-wave radiator would only be 1 inch long!  This is why anti-EMI (Electromagnetic interference) shielding is important in modern equipment, and why radios and computers can make poor shack neighbors if care isn't taken!

EMI issues aside, radio amateurs usually want to know what makes a good antenna!  We've already established that -- at least within certain bounds -- bigger is better, and this is particularly true of receiving antennas.  (You could think of an antenna as a big net, catching the radio waves flying by.  The bigger the antenna, the more you can catch at one time.) Anything electrically smaller than 1/4-wave makes a poor radiator, and doesn't do well as a receiver, either.

Another aspect of antennas is impedance.  A good topic for a future discussion, for now we'll just say that impedance is the AC equivalent of resistance.  Every AC circuit has a characteristic impedance, and one stage of a circuit (e.g. radio) can transfer energy most efficiently to another part (e.g. antenna) if the two are designed to have the same impendence.

Antenna systems aren't the only transducers (or other devices) connected to electronic circuits where impedence is a concern.  You have doubtlessly heard impedence expressed in the audio-frequency world, relating to speakers and microphones.  If an audio amplifier is designed to drive an 8-ohm load and you attach a 16-ohm speaker to it, it will likely only put out about half its rated power, and the frequency response may change, perhaps making the sound more "tinny", for example.  Load it down with a 4-ohm speaker and turn up the volume, and you will overload the amplifier, causing distortion and possibly damaging it by causing it to dissipate more heat than it is designed to.

Getting back to radios, nearly all amateur radios are designed with a 50-ohm impedance at the antenna socket.  Feed line (e.g. coax) has its own characteristic impedance, too.  If your 50-ohm radio is treated to a 50-ohm antenna fed with 50-ohm coax, it will be happy.  Energy will transfer efficiently from the radio into the ether, and received signals will do the same in the reverse direction.  Any place there is an impedance mismatch in the system (say you decided to use 72-ohm TV coax between a 50-ohm radio & antenna -- you'd have two impedance changes), some of the signal will be reflected back and ultimately probably wind up warming your wires instead of doing good.  The greater the impedance mismatch, the more energy is lost.

A real-world half-wave dipole (or 1/4-wave ground-plane) antenna typically has an impedance of roughly 20-100 ohms, so life is good when it's operated near its resonant (we'll define that word more exactly at a later date) frequency, i.e. the frequency where it does indeed measure half (or 1/4) a wavelength.  If you were to try that same antenna on a frequency that's twice as high, i.e. use it as a full-wavelength dipole, the impedance is extremely high -- theoretically, it approaches infinity -- the mismatch is so horrible that you might as well just leave your coax laying on the ground not attached to anything!  Between those two extremes are antennas that have impedance values from, say, 5 ohms to 10,000 ohms.  These can be impedance-matched to the radio and/or feed line using a circuit containing inductors/capacitors to "translate", or change, the impedance that each device sees.

The antenna itself can also be modified to do this; for example, it's common to "load" antennas with coils to allow them to be physically shorter than 1/4 wave, while electrically they look longer than they are.  Nearby conductors (either part of the antenna or the environment) can add capacitance and thus alter its impedence.

A curious and useful fact is that antennas that are 3/2-wavelengths long have the same impedance as a 1/2-wave.  Same for 5/2, 7/2, 9/2, and so on.  (Hey, the 70cm/440MHz band is roughly 3x the frequency of the 2m/144MHz band!  e.g. 147x3=441.  Hmmm...) The larger antennas end up having more complex radiation patterns, with more energy going in some directions, and some less.  A 5/8-wave ground-plane antenna fed with a matching system will likewise have a different radiation pattern than a 1/4-wave.  Whether that's good or bad depends on where you want your signal to go!  Designing antenna systems that radiate (and receive) more power in wanted directions while rejecting it in others is where the science really gets challenging and rewarding, and makes a difference between a good antenna and a great one for a particular application.

Bottom line: a good antenna is one that makes your radio happy, and directs radiation where you want it (e.g. "out there", not into the ground or merely heating itself up), and helps you get the signals you want, while rejecting those you don't.  There are lots of variations of these considerations, and real-world limits of size and money always play a major role in antenna selection.