Antenna Basics (Pt. 2)

Antennas Basics for Survival – What is an Antenna? — Message Two

Basic basics: The most basic antenna I can think of is the quarter-wave, vertically polarized whip antenna. You might see these on vehicles for radio communications. You’ve seen CB antennas on cars, and these are pretty typical of such antennas. The technical name for these antennas is “isotropic radiator”. An isotropic antenna is a non-existent, theoretical (perhaps even MYTHICAL!) antenna that radiates omnidirectionally (as, say a light bulb, or the sun might). All antennas are “tested” against this format basically and it isn’t really necessary you know this, but it might be helpful later if you’re building something to understand how to read charts, graphs or other information related to something you’re building or using.

There are many types of antennas, all have cool sounding names, and many behave differently than other antennas can’t do. You will need to understand different antenna configurations for doing things like aiming in a certain direction, getting your signal out in all directions or just getting an antenna set up properly to best receive a particular signal. I’ll try to cover some of these aspects over these next few posts, but right now, the title says it all. What is an antenna?

More Advanced Basics: Essentially, an antenna is very simply an electrical element in a radio circuit that can pick up the desired radio frequency signal and deliver it to the radio circuitry. That’s the simple explanation.

The difficult explanation is that it is a wire that is “excited” by the passing of an electromagnetic waveform, which in turn induces current into the wire. That current is then fed to a circuit to be used in some way, usually by tuning the desired signal, detecting the intelligence in the radio signal, amplifying the detected audio and then feeding it to a transducer (that’s another name for a speaker or headphone).

Let’s see here. Electromagnetic signals consist of basically, an electrostatic field flowing away from a ‘radiator’ (another name for an antenna!) combined with a magnetic field that is at “right angles” to the electrostatic field. The two components of the EM field are required for the signal to become a “radio wave”, or dissipated Radio Frequency (RF) energy. We will not worry about things like how the signal is generated, but will concentrate on the simple reception of signals, and assume we’re not going to build a whole transmitter at this time.

In physics, if you have a piece of wire that is just a large loop, and the ends of the wire are attached to a volt meter (a device to read voltages, obviously) and you take a magnet and point one pole of the magnet at the wire and hold it there, nothing will happen. However, if you move that magnet back and forth across the wire, you will cause the magnetic lines of force from the magnet to cut across the wire. In so doing, you will force what we call the “induction of current” into the wire, which can then be measured by the meter.

This is the very basic premise for how radio waves are collected by an antenna. The RF energy is traveling around, passes over a wire (the antenna), induces a current into the wire. Of course, in physics you have to have a “complete circuit” as well for current to flow, and this means you need a “load” (simply a resistance into which the current will be dissipated as heat energy usually – but we’re going to make it do something else), you need a path and you need to obviously have an energy source.

For a radio receiver you have an antenna, you have a complete path, which will take the induced current to “ground” and you have a load, which in the case of a receiver will be a “tuned circuit” (a coil and capacitor) which will act as the load. (Technically, this is an INDUCTIVE load, because there is a coil.)

You need two more things to make a radio receiver besides the antenna and a tuned circuit. You need a DETECTOR (which in our simple radio will be a diode, a device which passes current in ONLY ONE DIRECTION) and you need a transducer, which as I explained elsewhere is a simple high impedance earphone or headset. If you can locate an old, old telephone, circa 1940-1970ish with a regular hand set (not the newer, fancier stuff) you can usually unscrew the ear phone portion and take out the little earphone piece. It is a “high impedance earphone” and a single one is all you need.

I’ll teach you a bit about building a simple radio receiver later, so collect some parts if you can when I mention to do these things.

Now, let’s talk about Medium Wave (or Medium Frequency) and the AM band for awhile, since everyone is familiar with it, and the simplest radio receiver you can build and make work will be for that particular band. The AM band as we call it falls around 535-1600 kilohertz (KHz) and is probably one of the oldest “bands” of frequencies that was used in history. Marconi’s famous “first transmission” across the Atlantic was at 500 Khz, and was using a device called a spark-gap transmitter. It’s output power was in the 35 kilowatt range, but probably had peak power outputs in the tens of MEGAWATTS range. The intelligence consisted of three “dits” (the dots in morse code). The signal was picked up by a simple receiving device in what is now Canada at a distance of roughly 1550 miles, and the signal had to have bounced off the ionosphere twice.

Amazingly, one doesn’t need megawatts to broadcast across the ocean. I’ve had CW QSOs (Morse code conversations) with people in Japan, France and several other countries, as well as within the United States using a battery powered, QRP (low power, under 5 watts) radio. The secret was my antenna. Which is what this is all about.

The AM band has a wavelength running from about 560 meters down to 187.5 meters. What I just told you was “math” (Ack, I know, math, why does this have to be so difficult, you might ask). Actually, this is pretty simple. Basically, the formula was given in the previous message. l = C/f where C is the speed of light, and f is the frequency in Hertz. The answer (Lambda, a greek letter for the confused) is the WAVELENGTH in meters. C= 300,000,000, or 300 million meters per second, that’s the speed of light. 535 kilohertz is 530,000 hertz. Simply divide the F into the C and your answer is in meters.

Now, 560 meters is actually, well, pretty long. It is roughly 1690 feet or so. Most people do not have back yards that big, especially if you live in a city. This is where some thinking comes in. In antennas, there are divisions of the wavelength that will work well with antennas. In other words, you could divide this number several times by multiples of two, and get a working long-wire antenna, or better a dipole antenna. I’m going to steal someone else’s diagrams for a dipole (and provide the link to the site) because I don’t really have time to draw right now.

Here is a working model (ok, it is picture of something that would work):

(Found here: http://www.electronics-tutorials.com…nna-basics.htm and he has a good article on antennas)

What you’re looking at is ANY antenna for ANY radio on ANY frequency, as long as it is cut to the right length (or approximate length). The truth is, if there are signals out there, ANY antenna will work, some will just work better than others.

You could construct such an antennas out of thin or thick wire, and you can use about anything that insulates as insulators. I’ve used old pill bottles, and nice, glazed ceramic insulators made for just such a thing. The thing called a “balun” is simply a simple transformer. Let’s just ignore that device for now, and pretend it isn’t there, since we don’t really NEED it for most receiving antennas. It’s a device to match the impedance (the internal resistance, both inductive, capacitive and resistive – known together as reactance – which using some formulas give you something called impedance – never mind, it’s math again). Simply, we can ignore the balan for most purposes right now.

Ok, that’s about it for right now. The last thing I want to do in this message is post a link to an article I wrote many years ago. It’s even got pictures (WHOO HOO!) that you can see and from it, I hope, you might be able to build a simple radio with a couple of junk parts or something you can pick up from Radio Shack. (By the way, I don’t necessarily ENDORSE Radio Shack, and I’m sure that the site in general doesn’t either, but hell, it’s a quick place to get parts… so I use them.)

 

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