An Antenna System Primer

This is a work in progress.

Technical Terms

Many of the measurements discussed here will be expressed in terms of decibels, abbreviated dB. Decibels are used as a way to describe one quantity relative to another using logarithms, or powers of ten. (The Bel was a unit devised in the 1920s for making comparisons to the amount of loss in a mile of telephone cable. As it worked out, the numbers became cumbersome with Bels, so deci-Bels, or tenths of Bels, became the norm.)

Decibels are calculated using the formula dB = 10 x Log10( x / y ), where x and y are the quantities being compared.

In the sections below, you'll see dB with a letter afterward, such as dBm or dbi. These express decibels relative to some fixed standard. For example, say you decide that $1 will be the standard by which all other quantities of money will be measured. Let's call that a dBB, for decibels relative to a buck.

If someone else has $2, you plug those numbers into the formula, 10 x Log10(2/1), and get 3.010299957. You can say that person has 3 dB more money compared to a dollar, or 3 dBB. Here's a table with some interesting amounts:

Amount How Much?
$0.01 -20 dbB
$0.10 -10 dbB
$0.25 -6 dBB
$0.50 -3 dBB
$1.00 0 dBB
$2.00 +3 dBB
$4.00 +6 dBB
$8.00 +9 dBB
$10.00 +10 dBB
$20.00 +13 dBB
$50.00 +17 dBB
$100.00 +20 dBB
$1000.00 +30 dBB

Even if you don't understand the mathematical details, just remember that with every change of 3 dB, you get twice or half as much depending on whether it's 3 dB more or 3dB less.

Parts of the Antenna System

The Radio

Your radio will put out a specified amount of energy (power) at the antenna connector. Marketing usually measures this in Watts, but electrical engineers measure RF power in dBm, or dB relative to a milliwatt (0.001 Watt). You'll see why this is shortly. This table shows the power output of a number of typical UHF radios in Watts and dBm:

Radio Watts dBm
FRS Handheld 0.5 26
GMRS Handheld (Low) 1 30
GMRS Handheld (Med) 2 33
GMRS Handheld (High) 4 36
GMRS Mobile 25 44
GMRS Mobile 50 47

You'll notice that even as the Watts make large jumps, the dBm figures don't. That's the power of logarithms at work: they keep the numbers to a manageable size.

Feed Line

In mobile installations, the spot where the radio is installed is rarely the ideal place for an antenna, and the best place for an antenna is rarely a good place to mount the radio. The solution to that problem is a feed line, which is a length of cable that runs between the radio and the antenna. In most land-mobile installations, the cable is some variety of coaxial (coax for short) cable, which has a center conductor to carry the signal and a shield around the outside that serves as a ground and a way to prevent external noise from entering the center of the cable.

Wire of any kind has some amount of electrical friction that converts some of the energy passing through it into heat. This conversion is called loss, and how much there is depends on the construction of the cable and the frequency being passed through it. At DC (0 MHz), cable has almost no loss. As the frequency increases, so does loss. Manufacturers of coax cable (the good ones, anyway) provide a table that shows the loss in dB per hundred feet at a number of frequencies. For GMRS, we're interested in one frequency, 462 MHz.

As a general rule, cables that are physically larger have less loss at a given frequency than smaller ones. There's a variety of cable called LDF6-50 that has only a couple of dB loss per hundred feet at 462 MHz, which sounds great until you realize that it's over an inch in diameter, can't be bent in curves with a radius of any less than 15", weighs 0.63 pounds per foot, costs $11 per foot and requires connectors on the ends that run about $30 each. It's great cable, but not for land-mobile applications. The other end of the spectrum is RG-174, which is very thin and flexible and loses a whopping 20 dB in 100 feet.

Let's start a hypothetical motorcycle installation, which includes a four-Watt GMRS radio and six feet of feed line. At 462 MHz, these are the losses in six feet of a few popular cable types:

Cable Loss (dBm) 4W Becomes... Power Loss Comments
RG-8 0.174 3.843 4% Cable is too fat for motorcycle use
RG-8X 0.506 3.560 11% Smaller (~1/4"), more manageable version of RG-8
RG-58 0.602 3.482 13% Slightly smaller than RG-8X
RG-174 1.247 3.002 25% Very skinny cable

You'll notice that the RG-174 burns up a quarter of what's put into it, which makes it a very poor choice for UHF. RG-8X or RG-58 are both good compromises: reasonable loss, and small enough to be usable on a bike. Here's a useful calculator for determining the amount of loss in the most popular varieties of coaxial cable: http://www.ocarc.ca/coax.htm

Using decibels to express power and loss becomes very useful here, because the loss figure works no matter how much power you're dealing with. It's easy to take the power output of the radio, subtract the cable loss and get a figure representing how much power is coming out of the far end of the feed line:

Component dBm Unit
Output at end of feed line 35.398 dBm
Handheld GMRS Radio Output 36.000 dBm
Loss in 6' of RG-58 coax -0.602 dB

Converting back to Watts, we can see that our 4-watt radio has been reduced to 3.482 Watts by adding a feed line. That's not a bad thing, it's just a necessary side effect of putting cable into the system. Less cable means less loss, so keeping the feed line as short as possible is a good idea.

Antenna

The final part of the system is (drumroll, please) the antenna. There are many different types of antennas, some of which will be discussed later.

Like everything else in the electrical world, antennas have a standard by which everything else is measured. That standard is called an isotropic radiator, which is a point in space that radiates energy equally in all directions with 100% efficiency. If you're wondering if such an antenna actually exists, it doesn't, but its characteristics make it an excellent benchmark for comparing real antennas. If you were to draw a three-dimensional picture of how far out the signal gets from one of these antennas, you'd get a sphere. This shape is called the radiation pattern.

All other antennas have radiation patterns that aren't spheres, and because of that, each has some direction where it radiates the most energy. The term for how much more energy is radiated is directivity, and that figure is expressed in dBi, or decibels relative to an isotropic radiator. So, for example, an antenna that does twice as well as an isotropic radiator in its "best" direction can be said to have 3 dB of directivity or gain of 3 dBi. Antennas that do just as well as isotropics have 0 dBi of directivity or are said to be at unity. Those that can't do as well are said to have loss (e.g., one that does half as well has -3 dB of directivity or can be said to have loss of 3 dB). The gain and loss figures for an antenna are used to calculate effective isotropic radiated power, or EIRP, which is an indicator of how well an antenna does in its favored direction compared to an isotropic radiator. For example, if you put 36 dBm (4W) into a 3 dBi antenna, that system radiates the equivalent of putting 39 dBm (8W) into an isotropic radiator.

Another aspect of antennas is resonance. (To be continued...)

Selecting an antenna

You might think that isotropic radiators would be great antennas to have, because no matter how you orient one, it will radiate in all directions. In the real world, that can pose a bit of a problem. If you're trying to communicate with someone a few miles up the road, your isotropic radiator is also throwing energy straight up into the air and straight down into the ground. In both of those directions, that energy is wasted. Neither the ants or the moon men are listening.

Dipoles

One of the simplest antennas is called a dipole, which gets its name from its two elements. Unlike an isotropic radiator, its radiation pattern is shaped like a donut:
dipole-pattern.png
Notice that a dipole doesn't radiate very much energy straight up or down. That pattern is very useful in a mobile setting because more energy is radiated in directions where there are more likely to be receivers. Straight down and straight up aren't places where more signal would be useful. The gain in some antennas is sometimes expressed in decibels relative to a dipole, or dBd. Dipoles are 2.15 dBi (that's gain relative to an isotropic radiator), so if someone gives you a gain figure in dBd, just add 2.15 and you have dBi. Adding the gain of a dipole to our hypothetical system, we get the following:

Component dBm Unit
Effective Isotropic Radiated Power (EIRP) 37.548 dBm
Handheld GMRS Radio Output 36.000 dBm
Loss in 6' of RG-58 coax -0.602 dB
Antenna Gain +2.150 dB

In this configuration, we do about 1.5 dB better than we would with an antenna that gets unity gain, even with the 0.6 dB of loss in the cable.

Whips

Most of the antennas you see attached to cars and motorcycles are whip antennas, which simply means they're some sort of vertical

Helicals

Antenna Mounts

Reference Material

-- MarkFeit - 26 May 2007
Topic revision: r8 - 21 Sep 2007, MarkFeit
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