Antenna Basics

Part 2 – In The Beginning…

 

How many different types of antennas are there?  The cowards on Wikipedia quote the number as “a plethora of kinds”.  I have heard a very experienced antenna designer as saying that there are over 5,000 DIFFERENT antenna designs presently known!   Where to begin, where to begin…

 

Let’s begin with what was probably the first transmitting antenna ever used, the center fed dipole.  Heinrich Hertz (the guy that the Hertz, ”cycles per second”, is named after) developed a wireless communication system by forcing a spark to occur in the gap of a dipole antenna back in 1886 (his receiving antenna was a loop antenna).

 

So, what is a half wave center fed dipole?  It’s two, quarter wavelength’s lengths of wires, mirroring each other, usually positioned horizontally with a feed line attached at their center “ends”

 

When you cut the wire lengths for the longest length you can mount, it is usually called a non-resonant Doublet or simply a Doublet. When it is measured and cut to be at a specific electrical resonance length for a favorite frequency, it is usually called a Dipole.  How do you find “the right length”?  Well, there’s a story about that…

 

Half wave dipoles are called half wave because they should be cut to be one half the wavelength (λ/2) of the radio wave that you are most interested in transmitting on.  If you wanted to calculate half the wavelength for a given frequency in free space, the vacuum of outer space, it would be:

λ/2 Length (feet) = 495/frequency (MHz)

The problem is that your antenna is not going to be launched into outer space.  Chances are that it will be close (but not too close) to your property’s ground.  A lot of things affect the electrical wavelength in a wire relatively close to the Earth ground.  To begin with radio waves don’t travel as quickly in metal wires as they do in a vacuum (that’s called its velocity factor).  Capacitive coupling at the ends of the wires has an effect and the distance of the wire from Earth ground has an effect.  As a rule of thumb you should try to keep your dipole, no closer than a 1/2 wavelength’s distance from the ground,  Any closer and it will couple with the ground, producing a great deal of energy loss and the diminished radiated signal will be going in the wrong direction for your purposes.  Most people want an elevation angle that points towards the horizon and gives radio waves a chance to be reflected and refracted by the ionosphere for long distance communications.  By placing a dipole too close to the ground the elevation angle becomes almost 90 degrees—straight up!  Instead of a great DX (distance) antenna you have a Cloud Burner, or a Near Vertical Incidence Skywave (NVIS) antenna that’s great for local communication but much more difficult to use for DX communications.

Back in 1929 the ARRL published the following equation in that year’s Handbook for the very first time.  It was based on practical experiments by Hams over the previous few years:

λ/2 Length (feet) = 468/frequency (MHz)

 

Many people have read about the magic number, 468, but few know where it came from—now you are one of the Hams that know that trivia!  It isn’t carved in stone—most books suggest that you use that equation to give you a rough approximation and probably add three or four more feet at either side just to be safe (you can always tie wire back on itself and effectively make it act as though it were shorter—it’s much harder to make an antenna that’s too short function like a longer antenna)!  When a dipole is at resonance its impedance is purely resistive—its capacitive reactance and inductive reactive is balanced out and is zero!

 

So… measure everything out right, cut it to the correct length, mount it at least 1/2 wavelength above ground and you have an antenna with a perfect 50 ohm nominal impedance (surely you remember when we wrote all about impedance in “Part One”?)…

 

Sorry, the theoretical nominal impedance of a half wave dipole at resonance in free space is 73 ohms, not 50 ohms!  That’s 73 ohms of pure Resistance and 0 ohms of Reactance. What the ******?!  What about 50 ohms?  How did we come up with 50 ohms impedance for all our transmitter final stages if a resonant dipole is supposed to be 73 ohms????  Time for another short diversion…

 

I guess most Hams think there is something magical about 50 ohms of impedance.  It’s the impedance of everyone’s final stage transmitting amplifier, and all the coax that we buy, and it’s a 50 ohm impedance that you dream for, so that your Standing Wave Ratio is 1:1.  Where did it come from?

 

It came from a compromise, like our founding fathers choosing to erect a new city, Washington, D.C., as our capitol, smack in the middle of the original 13 states.  You see, there ARE two important considerations in choosing a standard impedance—power loss and power handling ability.  For a dielectric filled coaxial cable the lowest power loss occurs when the cable’s impedance is somewhere in the 75-77 ohm range—this is why the cable TV industry that is only concerned with power loss runs nothing but 75 ohm coax cable!  The best power handling capability, for all you guys with your super-duper linear amplifiers, is from a coax cable with an impedance of 30 ohms!  The best compromise between power handling capacity and lower power loss is… 50 ohms!  (Well, that’s what everyone settled on as the explanation and they are sticking to that story!)  I could show you the derivation of how these two optimum impedances were calculated, but someone’s nose would start to bleed when they saw the differential equations involved.

 

I’m sure most of you have seen the graphic representation of the radiating pattern of a dipole:

 

Most people call it doughnut shaped, I prefer bagel shaped.  The maximum current is near the center feed and that’s where most of the signal is radiating from (that’s why an inverted “V” has a better elevation than a simple “V” shaped dipole.  The outer edges have the maximum voltage, so stay away from them when your transmitter is on.  RF burns can be painful!

 

The bottom line is that dipoles have a bit more gain at right angles to their axis and a little bit of loss in the directions that the dipoles are pointing at.  As an example a dipole oriented north to south would be a bit easier to work east or westward stations.

 

The gain of a dipole is usually given as 2.15 dBi.  What’s a dBi?  It’s a comparison to a radiation pattern similar to an incandescent light bulb, where the Radio Waves radiate equally in ALL DIRECTIONS.  The details of the ubiquitous decibels will be covered in the next part of this continuing series.  Anyone want to guess what a dipole’s gain is in dBd’s?

 

So you just plug it in and talk away?  Well, not quite yet.  Dipoles are a balanced antenna and coax is an unbalanced transmission line.  When you connect the two of these together very often you create a common mode current on the outer surface of your coax’s shielding—in effect your transmission line is no longer a relatively lossless RF shielded means of getting your transmitter’s RF to your antenna.  Your coax becomes an antenna and your shack becomes full of RF that usually causes all kinds of problems.  That’s where a balun comes in (BALanced to UNbalanced transformer).  The theory behind these very useful gizmos is presently beyond the scope of this article, but we may circle back to baluns in later parts.  The simplest type of balun, a voltage balun or so-called common mode chokes, can just be six to eight loops of coax bound together or a series of ferrite chokes arranged near the coax feed line connector.  Common mode current is “choked” and most of the RF is radiated far away from your shack, eventually to your dream DX station on the other side of the world!

 

Are there any variations on the basic dipole?  Sure!  You don’t have to orient it horizontally; you can orient it vertically, or at any angle.  Many Hams with humongous towers will attach a dipole to the top of their tower at one end and “slope” them to end near the ground some distance away from the tower’s base.  That’s why they call them Slopers!  There’s no rule that both legs of the dipole have to be strictly parallel, you can make them “V” shaped or inverted “V” shaped, but the angle shouldn’t be smaller than 90 degrees or each leg will start to couple with the other and losses and distorted radiation patterns will occur.  You can bunch dipoles for different bands together with a common feed point—those are called fan dipoles!  Trap dipoles are another design to get multiple bands using the same antenna.  You can fold your dipole, or use wire mesh instead of simple wire and build a Cage Dipole.  As you make your antenna electrically appear to have a larger diameter wire, as occurs with cage dipoles, the available bandwidth of the antenna widens, giving you more frequency options for good transmission on your Amateur bands.  Some different types of antennas us a dipole design as a part of their own design—as the design’s driven element.  These more complex designs will be discussed in a later part of this series.

 

If you want to try to “have your cake and eat it too” you can use a random but long non-resonant dipole with a very low loss balanced, open wire transmission line plugged into a very good antenna tuner and let your antenna tuner deal with the impedance mismatch.  Surprisingly, there is an even split among the tech wizards regarding antenna tuners.  Some think they are the greatest thing since sliced bread and should be used as often as needed.  Others think that they are toys, a magic trick to help fool yourself with a fake, better SWR and to fool your transmitter’s final power amplifier stage, that you really have a resonant antenna connected to it, when you really don’t!  In the good olde days most tube driven transmitters had a final power output tuning stage that was just another name for an antenna tuner, so if it was good enough for those old timers it should be good enough for us!  Some people swear by their non-resonant Doublets, some swear at them…