Basic Antenna Parameters
How would some of those antennas perform in your back yard or other environment? It would be really helpful to know that before you start running cables and shooting lines. This lesson will help you analyze key antenna performance parameters.
Antenna bandwidth is the frequency range of a given antenna. In the last lesson we mentioned “SWR bandwidth.” That’s the frequency range an antenna covers below a particular SWR range. An HT whip antenna has good bandwidth on 2 meters, but would be awful on 80 meters.
You may remember the “isotropic” antenna from your General course studies. You won’t build one in your backyard, it’s a theoretical antenna. An isotropic radiator is a hypothetical, lossless antenna having equal radiation intensity in all directions used as a reference for antenna gain.
Starting with this theoretical, isotropic antenna version, you can model real antennas. Use that for comparison of radiation patterns. You’ll also get to see what gain you can create versus the basic isotropic antennas.
One of the most common ham radio antennas is the half-wavelength dipole. Remember that it has a figure-8 pattern, so it has gain in some directions. We can compare that to the isotropic antenna.
Time to do some calculating for an exam question. How much gain does an antenna have compared to a half-wavelength dipole if it has 6 dB gain over an isotropic radiator? If we reword this question, it’s easier to solve. Think of it as “how much gain does a half-wavelength dipole with 6 dB of gain have, compared to an isotropic half-wavelength dipole?”
Since we know that a dipole has gain in some directions, we just need to know how much an isotropic dipole has. That theoretical dipole has 2.15dB of gain. Subtract that from the 6 dB given to get the answer of 3.85 dB gain.
Here’s another formula you need to know, but no math involved. To calculate antenna efficiency the formula is radiation resistance divided by total resistance. Radiation resistance is electrical resistance at the antenna’s feed point. It’s caused by the emission of a signal from the antenna.
Higher antenna efficiency is good. Ideally an antenna has a higher radiation resistance than loss resistance. That means more energy goes into the actual radio waves than is lost in the antenna.
Antenna Modeling
As you are thinking about what antenna might fit in your backyard, you may sketch it out on a piece of paper. At its most basic, that is antenna modeling.
Computer modeling can help you understand the real world performance of an antenna. As we said, it’s very good to know before you climb towers or shoot lines. The software displays expected reactions of your antenna in the real world environment.
What type of analysis is commonly used for modeling antennas? A principle called Method of Moments. Its principle is that a wire is modeled as a series of segments, each having a uniform value of current. Uniform is the key word here for the exam.
One popular modeling program for hams is called EZNEC. When creating an antenna model in software you should use at least 10 wire segments to represent each half-wavelength element. Using fewer than 10 puts you at a disadvantage. This is because the computed feed point impedance may be incorrect.
Antenna Radiation Patterns
Your antenna model will produce an expected radiation pattern in two planes. It will show the electric field and the magnetic field. They will be at right angles to each other. There are two types of radiation patterns to review. One is a polar pattern, the other an elevation pattern.
Let’s look at a polar radiation pattern. Think of it like you are looking down from above at your signal spreading out from your antenna.
This next figure could be on your exam. Figure E9-1 is a polar radiation pattern that represents the performance of a horizontal Yagi antenna. The rings are signal strength in dB and the radials are direction. While it’s not labeled, the outermost ring is 0 dB. That’s the point where the antenna reaches its strongest point of radiation.
Let’s read three figures for the exam.
The test asks, “What is the 3 dB beamwidth of the antenna radiation pattern shown in Figure E9-1?”
At the -3 dB ring, there are two points where the radiation pattern crosses the circle. They are roughly positive 25 degrees and negative 25 degrees. You can add those together to get the total beamwidth. 25 plus 25 = 50 degrees beamwidth of this radiation pattern. This means the antenna will have the response plus or minus 25 degrees from the direction it is pointed. After that the radiated power has dropped by at least half.
Another term to know for a Yagi antenna is front to back ratio. Front to back ratio is a good measure of how well the antenna focuses radio waves in one direction. To answer the next question, we’ll again read it from points on the chart.
What is the front-to-back ratio of the antenna radiation pattern shown in Figure E9-1?
We can see on the diagram that the front arrives all the way to the 0 dB line. The back of the antenna is halfway between the -12 dB mark and the -24 dB mark. Call it, -18 dB. Therefore the difference between the front and back is 18 dB.
Let’s try reading the front to side ratio now. The front is at 0 dB. The vertical side lobes are just inside the -12 dB line. It’s right at -14 dB. So the difference from the front to the side is in Figure E9-1 is 14 dB.
Switching to looking at an elevation antenna pattern now. We’re shifting our view by 90 degrees. Now it’s like looking at the antenna radiation from the side. It helps us understand the radiation from the top of the antenna.
Figure E9-2 on the exam is an elevation antenna pattern.
Reading the elevation charts is very similar to reading the polar charts. In this case the point at around 7.5 degrees is the front of the pattern and that is on the 0 dB line. The point nearest to 180 degrees is the back of the pattern. That reads -28 dB. Take 28 minus zero to get the front-to-back ratio of the radiation pattern shown. It’s 28 dB.
Let’s find the angle of peak response, which is the angle where the antenna has the strongest radiation. On the chart we want the point where the radiation arrives out to the outermost circle. Measuring the angle here, we get our answer of 7.5 degrees.
Gain as a Function of Pattern
Now that you have seen the patterns of a Yagi antenna, consider this. Max has two lossless antennas. One is an isotropic radiator and the other is a Yagi. If he feeds each with 100 watts from his radio, how much does the total power output change between the two?
The answer to this is zero. The total amount of radiation emitted does not change, it is just shifted in direction. When asked, “What is the difference in radiated power between a lossless antenna with gain and an isotropic radiator driven by the same power?” Your answer should be, they are the same.
You can also think about the radiation from energy in terms of near-field and far field. Near-field is close to the antenna, and, yes far-field, is further away. There’s a point as you get further away from the antenna. It’s where the shape of the radiation no longer changes with distance. The far field of an antenna is the region where the shape of the radiation pattern no longer varies with distance. The strength of the radiated field will still vary with distance. But, the shape becomes consistent in the far-field. The key word here is shape.
Another shape to look at is the Fresnel zone. It’s a term for the elliptical region between a transmit and receive antenna where loss can result. Small waves give you a smaller Fresnel Zone. Of some common ham radio options the 5 CM band, 5.6 GHz has the smallest Fresnel zone.
Near vs. Far field diagram
