To put it as succinctly as possible:
A wire is an antenna because of the electromagnetic laws of physics.
If attached to a device that can use what the wire picks up then the signal moves from the wire to the receiver, radio, TV, or other receiving device, to select the correct frequency and decode the information.
We can see why almost any wire may work as an antenna. (This was tested in my last blog “Any Antenna May Do.”) In fact, almost anything metallic you can attach to a radio receiver will act as an antenna. It may not be a good antenna for your purposes, but it will work to some extent.
Story 1: A ham radio operator, Jeff Horlick, KZ2F, once took two cars and attached them as a dipole antenna to his ham radio. He called his article, ““The Dodge Dipole – Not just another ordinary $22,000 antenna” see photos below. He was able to contact people using this two-car-dipole antenna. He published his article in the April 1995 issue of QST magazine pages 53 and 54 – see http://www.arrl.org/ for more about ham radio. NOTE: This was an April issue article and I find it a little funny because it shows how even something so ridiculous can still work!
Used with permission, April 1995 QST; copyright ARRL.
Story 2: While a teenager I experimented with almost anything metallic I could attach to my CB radio to see how well it would work. Some of the stranger things I tried were a fold-able music stand and the screen in my bedroom window. Back then, in the 1960s, window screens were metallic – today’s plastic screening material would not work. Interesting note: I also discovered that this screen window would also spark whenever there was a close lighting strike. The lightning would flash, the spark would jump, and then the thunder sound would reach me a few seconds later. The closer the strike, the louder the spark.
Now that we know that any wire can be used as an antenna, why do we use all this expensive wire/tubing in the air as an antenna?
The answer is again based on the electromagnetic laws of physics. These waves come in all sizes, which also means all frequencies. The basic relationship of electromagnetic wave to frequency is f=c/λ or frequency equals the speed of light divided by the wavelength. Footnote: Light is also an electromagnetic wave.
In the most basic terms then, an antenna is any wire attached to the input of a radio frequency-receiving device. Things which use radio frequencies are cell phones, pagers, TVs and radios, just to mention a few. All of those things mentioned, as well as all other devices that use electromagnetic radiation below the light spectrum, are “radios” by definition. If it receives radio waves, it is a radio. How we decode the imparted information on the radio wave is either data, picture, sound, or any combination of those three things we can read, see, or hear.
Back to our last question, stated a little differently: why are there so many different antennas if any piece of wire will do? Answer: wavelength or (put another way) frequency.
Those of you who caught that little equation I gave may have noticed two things:
All electromagnetic radiation, all frequencies all wavelengths, travel at the same speed. Yes, but ONLY in a vacuum. This is the case in air but that’s mainly because the distances involved for receiving radio waves is so short that we don’t have to worry about minor spreading.
If f=c/λ, does this mean any wave follows the same equation? Yes, it does. If we replace c with v for the velocity/speed of the wave of interest, then we have the equation for sound.
The second question above then leads us to another interesting observation: the lower sound frequencies travel slower than the higher frequencies do.
Stuart Sweet has written a few articles to help explain antenna parts and to show/explain some different antennas. You can read about this in the following three articles:
- What is a “driven element?”
- “What is a Yagi Antenna?”
- “What’s the difference between a log periodic antenna and a Yagi antenna?”
The gist of those articles is that, since TV stations travel on different sets of frequencies, the antenna for each station is a bit longer or shorter than the last station. (Thus the “funny” look of a log periodic antenna.) All log periodic TV antennas MUST receive all stations as equally as possible because the manufacturer does not want to design a new antenna for every different location that receives a different set of TV stations.
Also, the FCC in the past has not allowed any location to use two consecutive TV station frequency sets. This was probably because it’s best not to have two close cities using the same TV station frequencies, and it also might have been that two strong stations with frequencies close together from the same location might interfere with one another.
Last, there is something called polarization. This works for light as well as for TV frequencies – think of polarized sun glasses. So, just as we can cut out unwanted light with polarized lenses, we can also use polarized antennas to our advantage in TV transmission and reception.
Originally, I believe, all TV stations used horizontally-polarized antennas. As more and more people began using TV receivers in cars and other things that move, the horizontal antenna was found to be a problem. For cars we need vertically-polarized radio waves. Now almost all TV stations are using elliptically-polarized antennas so either a horizontally or vertically polarized antenna will work. The big problem here is that we can’t really travel with a multi-element log-periodic vertically-oriented antenna any better than a horizontally- oriented antenna, for two reasons:
They are too big, have too much wind resistance and they will eventually fall apart.
Log-periodic antennas have a narrow viewing angle so if the car, boat, turns for example, the antenna must also turn to continue to face the TV transmitter antenna.
This means we have to compromise when using an antenna that moves – in planes, cars, ships, etc. We have to use simple vertical antennas with lower gain but with a broader view for receiving stations from any direction. This also means that we have to be closer to the station to receive it well enough to use it.
Elliptically-polarized waves also explain why both a horizontal 40-80 meter antenna and a vertical VHF-UHF ham antenna both work for receiving some TV stations, though not always well for the same stations, again depending on the length, location, and height of the antennas used.
In a log-periodic antenna, each three-element Yagi antenna in that antenna is designed to receive one TV station. All together they comprise the log-periodic antenna. So while any length of wire will work as an antenna, any length of wire may not be the best length for the frequencies, or TV stations, you are interested in. If you are very close to all your TV transmitting station locations, almost any kind of antenna or length of wire might do just fine. Even the old rabbit ear antenna allowed changing direction, angle, and length of the two antenna elements.
In general, smaller antennas won’t work as well as larger, log periodic/Yagi-type antennas when the stations are far away. Stuart Sweet wrote an article about it. You can find it at:
Let’s consider an antenna I think is really interesting, the bow tie antenna. This antenna works really well for UHF frequencies. It can be small and it only needs to be rotated to pick up stations from different directions. This antenna was found to receive all TV UHF stations. And, it even works well enough, for some of us, on the remaining VHF stations. I’ve written several of my bow tie antenna experiments for this blog. See:
- Bow Tie Antenna Angle Tests
- Bow Tie Element Length Evaluation
- The Standard Bow Tie Antenna
- Simple Dipole and Bow Tie Antennas
You will notice that Solid Signal sells both the log-periodic and the bow tie antennas in a number of different configurations. To get gain with a bow tie antenna, Most designs use two or more vertically-stacked antennas:
properly phased to make the signal strength increase. Other designs have used the Yagi-Uda/log-periodic arrangement for reflectors and directors in the horizontal arrangement.
Why don’t we use just any old piece of wire, at any length, for our antennas?
There is something called wavelength that tells us that we should use a length of wire that matches the wavelength of the radio/TV wave/frequency we want to receive. This is actually the first place we make our receiver better at receiving what we want to see/hear than just any length of wire can do for us.
It turns out that we need to use the correct electrical length of wire to best receive the frequencies of interest. What is “electrical length”? It is the length of wire that will best receive the length of RF wave we want to receive. Even a single strand of wire has some inductance and capacitance and this changes the actual length of wire needed to receive a given wavelength of RF.
Another way of looking at it is that the wavelength of the radio wave is different when it resonates on a wire than when it is traveling in a vacuum or in the air. This different “best” length for this frequency/wavelength is called the electrical wavelength length for that wire.
Here’s another good article by Stuart Sweet about why we use the antennas we use for various services such as TV or Cellular: Can you use the same antenna for cellular and TV?
At times, we can have too much of a good thing. For instance, if you only want some frequencies, some TV stations, and not others, like cellular, then we do not want an antenna that is able to pick up both sets of frequencies well. I wrote an article that was published in Popular Communication in September of 2003 called – “Sometimes the Best Antenna is NOT the Best Antenna” that tried to explain this. I’ve put this article up on my website, with permission. It’s from the September 2003 issue of Popular Communications. – NOTE: Once you click on this link you need to then find the wanted article in the list, see the title above in bold.
A TV case in point would be if you lived less than five miles from your TV transmitting tower. In this case, just about any length of wire would do well enough to receive this signal. In fact, it might actually be dangerous to your TV set to use something like a log-periodic antenna because the signal coming in might be too strong for the TV and it could burn out the receiving amplifier.
A few things you can do to mitigate this situation is
Point the antenna away from the strong signal direction.
Use an attenuator (a simple resistor will do) to cut down that signal.
Use a random length of wire as an antenna.
Wire Length: The length of the wire used for your antenna and the particular arrangement will help you pick up the station or stations of interest. Conversely, it will be able to hurt the reception of other stations or frequencies. The rabbit ears antenna would be an excellent choice for anyone close to the TV station’s broadcast towers. Another really good antenna would be a single element bow tie antenna for UHF frequencies. You can see my bow tie antenna tests for the best length to use right here at: Bow Tie Element Length Evaluation.
For a simple piece of wire antenna, the best arrangement is the dipole antenna. A dipole of one-half the wavelength of the center frequency of the station wanted will pick up that station best. It will also pick up the stations better whose frequencies are closer to the dipole-designed frequency rather than those stations whose frequencies are further from the dipole design frequency.
The more complex antenna based on the simple dipole but for all stations, is the log-periodic antenna made for people living much further from the TV broadcasting towers. A very quick explanation of the log-periodic antenna: This is made up of multiple Yagi antennas. This is done by giving each element three jobs. Each element will be either a director, an active element, or a reflector. The most common type is called a log-periodic dipole array LPDA antenna. For more information you can read the article on how to build such an antenna: Build an LPDA Antenna
Stuart Sweet wrote an article about the difference between a Yagi and a log-periodic antenna. It is located at: What’s the difference between a log periodic antenna and a yagi antenna?
Until next time, I hope you have good OTA reception,
Phil Karras, KE3FL
All my blog articles are listed at: Karras’ Corner
Karras’ Corner Article Links on my Website
on my KE3FL web site.