Part 11
Welcome to Part 11 of our discussion.
We're going to begin taking a look at antennas. I don't think there's any subject in Ham radio that's more controversial. Nor is there any one so cloaked in mystery, misinformation, magic, miracles, and more. Here are some hard and fast facts about antennas. Always keep these in mind...
1. Antennas operate according to the Laws of Physics. Always. Without Exception.
2. A low SWR does not mean you have a good antenna. It means your antenna is well matched to its feedline.
3. A high SWR does not mean you have a bad antenna. It means there is a significant mismatch between your antenna and its feedline.
4. An antenna's performance (very loosely defined as "how well does the other guy hear me?") has nothing to do with SWR. Note that "an antenna's performance" is not the same as an antenna system's performance.
5. Antenna modeling software will correctly reflect an antenna's performance if (and only if) an accurate model is created.
6. Almost every antenna installation is different, and results in different antenna performance. The difference can be dramatic, or negligible.
7. If you work everyone you hear, it just means you don't hear enough.
8. There is no single "best" antenna. Every antenna is a compromise in one way or another.
9. Antennas are reciprocal -- that is, their radiation patterns are the same on transmit and receive at a given frequency.
10. Once Again -- Antennas operate according to the Laws of Physics. Always. Without Exception.
We'll start with the (not so) humble 1/2 wave dipole.
The half-wave horizontal dipole is not only a good performing antenna in and of itself. Along with its close cousin, the 1/4 wave vertical monopole, it also forms the basis for most other Ham Radio antennas in use today. If you've read anything at all about antennas, the following picture should be familiar -- a 1/2 wavelength of wire cut in the middle and fed with either coax cable or some type of twin-lead (ladder line, for example).
The basic half-wave dipole antenna
You may have also seen pictures similar to those below that depict radiation patterns. They show the directions where your signal will be stronger, and weaker. Here are antenna radiation patterns for three "different" dipoles:
Antenna #1 Antenna #2 Antenna #3
Maximum Gain = 2.1 dBi @ 90° Maximum Gain = 7.22 dBi @ 30° Maximum Gain = 5.72 dBi @ 65°
Along the top row are azimuth slices. Think of viewing the antenna from above or below, with the antenna wire running left and right. Looking at antenna #2, your signal will be somewhat stronger in front and in back of you; and somewhat weaker to your left and right.
Along the bottom row are elevation slices. Think of looking at the antenna from the side, end-on -- in other words, the wire is now running from your head, through your monitor and out the back of it. Note that the azimuth slices represent the antenna pattern at the elevation angle of maximum radiation.
Antenna #1 is a 1/2-wave dipole in free space. You've probably seen that azimuth pattern before in antenna books. If you install your antenna in free space, it will perform exactly like the picture. However, it's likely that the FAA (and NASA!) would take a dim view of such a project. Furthermore, the loss in 100 miles or so of feedline would be significant. And I doubt that Home Depot stocks 100 mile ladders.
Antenna #2 is a more down to earth <pun intended> antenna. It's the same 1/2-wave antenna, but now it's installed at an altitude of 1/2-wave over real ground. Looking at the plots, you can see that the strongest signal will be broadside to the wire, at an elevation of 30 degrees. Antenna #3 is again the same antenna, but this time at a height of only 1/4-wave. Your signal is now almost omnidirectional, with a maximum strength at 65 degrees elevation.
Let's take a closer look at those pictures, focusing on Antenna #2. The numbers along the vertical axis represent reduction in signal strength, in dB, from the maximum. On the azimuth pattern, you can see that the trace of the pattern crosses the -3 dB point at just about 45 degrees. Similarly, at 90 degrees, the signal drops off by 10 dB. Recall from our last installment -- it's right below this one, if you'd like to look it over -- that 3 dB is double or half, and 10 dB is times 10, or a tenth. In other words -- let's assume for a minute that you could easily rotate your dipole; and that another station is located exactly in the direction of maximum signal. Turn your antenna 45 degrees, and it will sound (to the other station) as if you had cut your power in half. Turn it 90 degrees, and it will sound like you had cut down to a tenth of your original power. So what does the receiving station see on his S-Meter? Well, the standard says that one point on an S-Meter is a 6 dB difference. Therefore, at 45 degrees, the other guy sees a drop of 1/2 of an S-Unit. At 90 degrees, it's a bit less than 2 S-Units. Unfortunately, not many radios adhere to the standard, but this should give you a general idea of how things work.
There's a lot more to tell about those simple dipoles, and we'll continue looking at them next time. Until then,
73 for now
John Bee, N1GNV
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