Let’s begin at the REAL basics. I realize that most people understand this already, or don’t really care, but I thought a few might have just memorized this stuff for their exam and really didn’t take any time to “roll it around in their heads”. It’s surprisingly simple common sense stuff! You electrical engineers out there feel free to “fast forward” through this!
What is Voltage? It is the “the electromotive force” due to different electromotive potentials on either side of a given circuit, measured in Volts, sometimes abbreviated “E” or “V”. You can think of it as “electric pressure”, not unlike the pressure at the end of your garden hose. Increase the “pressure” (voltage) and you increase the flow of the water through your hose (current). Current is just the flow of electric charge, measured in Amps, abbreviated as “I”. Resistance is the opposition of electric current through a conductor, measured in Ohms, abbreviated as “R”. You can think of it like the diameter of your garden hose – if you use a narrow garden hose you get less water out of it, regardless of the pressure in the pipes! How do they all relate to each other? Well, that would be Ohm’s Law, that you might have felt forced to remember for the exam, but it’s very simple and very useful:
Or if you enjoy silly pictures to get the point across:
Resistance is relatively easy, in that it is not affected if the current is not constant (i.e. “Direct Current” DC) but fluctuating. The most usual form of fluctuating electrical current is one that changes with a very consistent pattern – a “Sine Wave”!
As I mentioned, the property of resistance doesn’t care if your current is steady or always changing, it supplies the same resistance to current no matter what, but there are some electrical components that are not that “accepting”…
Capacitors, usually made up of a series of conducting plates store electric charge. An Inductor, usually made from a coil of wire can be thought of as storing a magnetic field. They both respond in the exact opposite to alternating current. If you place a capacitor in the path of alternating current you will find that the sine waves of the voltage and the currents no longer line up perfectly – current LEADS the voltage, in the case of a circuit with an inductor in it, it is the current that LAGS the voltage.
This opposition to flow is called “Reactance” and is also measured in Ohms -it comes in two polarities – Inductive Reactance (from an inductor) is considered positive, while Capacitive Reactance is considered negative. What’s all this talk then about Impedance, that any antenna topic seems to keep mentioning? Well, that’s just the way Reactances and Resistances combine together. Unfortunately Impedance isn’t calculated by simply adding everything together. You can sum the different Reactances and get a “Sum of Reactance” but in order to combine Reactance with Resistance you have to take a little memory trip back to Junior High School Geometry – remember The Pythagorean’ Theorem, that the hypotenuse of a right triangle was equal to the square root of the sum of the other two legs “squared”?
So, as an example, a circuit that has 3 ohm of resistance and 4 ohms of reactance has:
5 ohms of Impedance!
That wasn’t so hard, was it? And this is the kind of stuff on the Extra exam that seems to scare everyone!
Why did I spend all this time and blank space to hammer the idea of Impedance? Because it is the one singular, solitary thing that every Ham seems most interested in – give him a light bulb that has a nominal 50 ohm impedance and a Standing Wave Ratio (SWR) of 1:1 on their meters and they are happy as a lark. Unfortunately Impedance matching or a 1:1 SWR by another name is only PART of what makes a good antenna! Also, an antenna is a just a tuned circuit with a given resistance, reactance, and impedance which also determines its bandwidth (Q factor) and resonant frequency. All these factors determine the characteristics of every antenna that has ever been built.
There are a few more factors you should be aware of that determines whether an antenna design will suit your needs.
The gain can be very important – some people misinterpret this as something like an amplifier’s gain, but antennas are passive devices and their total output power is always less than the power you put into them – the loss is usually dissipated in heat. Antennas don’t work like a simple incandescent light bulb, shining its light all around (in “Physics talk” that’s called an Isotropic pattern of emission). Antennas are directional to one degree or another and “gain” refers to the directionality of its reception and transmission patterns (which should be the same – Engineer types call this the “Reciprocity Theorem”). The lower the transmitted signal flying off of its back, the more of the available power is radiating from the front, the higher the dB gain that the manufacturers can boast of! Directionality is usually a good thing because of the nature of radio wave propagation which favors more steerable directed emissions. Some antenna designs are optimized for “shooting” most of your RF straight up! Some people call them “Cloud Burners’ while others have been known to call them “Near Vertical Incidence Skywave” (NVIS) Antennas. They are great for short distance, emergency local communication, but not so useful if you are interested in distance (DX) communications. That is not to say it is impossible to DX with a NVIS Antenna – it just is VERY difficult to accomplish and a lot of luck is required. When most people talk about the angle that their antenna signal goes off to the horizon, they use the term “Elevation Angle” (or in Engineer-speak “Elevation Plane Pattern”) most antennas also have a specific direct of peak energy in the North, South, East or West directions depending on the orientation of your antenna (or in Engineer-speak “Azimuth Plane Pattern”). It would all make better sense if I could “cut and paste” a holographic 3D image right here! There is Antenna Efficiency to consider – how much of your power will go into heat due to a given antenna’s resistive loss, any insertion losses, ground coupling/absorption or transmission line losses.