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Bandwidth

Now how can you use SWR to tell how broad banded your antenna is? You can plot your SWR reading similar to the one in figure 3. Generally for base antenna use, a SWR over 2:1 indicates that the antenna is no longer performing efficiently - so the amount of band you can cover while keeping a SWR under 2:1 is considered your antennas bandwidth. You should center your lowest SWR reading on the channel you use most, or around channel 20 if you use them all. A few factors determine how broad banded an antenna is. One of the main factors that I always keep in mind is conductor size of the antenna. For instance, if you made an antenna from wire, it would be less broad banded than if you made it from larger tubing. The outside diameter is the primary concern, RF energy (on 11 Meters) travels on the "skin" or outside of the wire only. Another advantage to using larger conductor size on an antenna is it increase the amount of power it can handle safely (how many watts of transmit power it can handle).

Larger sized elements (larger tubing or wire) does not provide more gain. This is antenna manufacture propaganda and is totally false...thicker elements only serve to widen antenna bandwidth and up the power handling capability of an antenna system.

swrcurve.jpeg

Figure 3 - This is a way to chart your antenna's SWR. Make a graph similar to this. Then make a reading on channel 1, and record it. Do it for each of the the channels. This is a good method to look at your antennas bandwidth. The graph shows to sample readings that you could expect from a 4 element beam, and a 5/8 Wave antenna.

 

The 1/2 Wavelength Dipole Antenna

When the first radio experimenters were first trying out their systems, they would just hook their transmitters up to long, high pieces of wire. It was later discovered that relating antenna length to frequency which one was transmitting and receiving on resulted in an antenna that worked many many times better than random pieces of metal. One of the first resonant antennas discovered was the 1/2 Wave dipole. The basic 1/2 Wave dipole is pictured in figure 4. If you wanted to make a dipole antenna to use on the 11 Meter band (CB), you would just make each leg a 1/4 Wavelength, right? Almost! Notice above how, I always spoke of terms of the Wavelength if it was traveling through the air. In other mediums (wires, earth (CB waves do not penetrate the ground far), coax) the wave travels slower than if it was traveling through the air. However the wave still completes its cycle in the same amount of time, but it didn't travel as far to do so. In other words the wavelength of a signal is shorter when traveling through wire, coax and your antenna. This is not a major concern, just be aware that is why the following formula is readjusted.

1/2 Wavelength, in feet = 468 / Frequency, in MHz.

For Channel 40 (27.405MHz) 17.077 = 468 / 27.405

Each Leg of the dipole would be a 1/4 wave or 17.077 / 2 = 8.386

Notice, above in the air a 1/2 wavelength would be 492 (1/2 of 984), but since the signal is traveling through wire instead of through the air, we must adjust our formula for calculating 1 wavelength (of a wave traveling through #12 Copper wire, not the plain old air). We are going to use this antenna to demonstrate a few other properties antennas have. First, lets examine the impedance of this antenna. If you place the connections in the middle you actually end up with a feed point impedance around 70 ohms at its resonance frequency. So as you can see, using this antenna, you are already looking at a SWR no lower than 1:3:1 (because of the mismatch of mating 50 Ohm Coax to a 75 Ohm antenna). Bandwidth of this antenna is largely dependent on how large the wire or tubing's outside diameter is. Lets introduce a new important antenna property, its radiation pattern (also known as polar plot or near field pattern)

dipole.jpeg

Figure 4 - The 1/2 Wavelength dipole antenna.

Home | Radio Waves | How Antennas Work | SWR | Bandwidth | Dipole | Radiation Pattern | Polarization | Theory Index