Something that has always confused me is how the same antenna can receive 2.4 and 5GHz signals. I thought an antenna was designed to pick up a specific range of frequencies, and that range was determined by the length of the antenna element, which is a multiple of the received signal's wavelength. If someone could help explain, or point me toward some reading material, that would be awesome.
The most critical part of antenna design is frequency selection, which you correctly say is related to its length - its resonant length.
It is possible to make antennas that have multiple resonances, and yet have one physical length. A simplified example would be an antenna that is resonant for some frequency in the 5 GHz range and somewhat less resonant for exactly 1/2 that frequency in the 2.4 GHz range. Of course in reality its much more involved than that.
In this case the antenna would probably be designed originally for the 5 GHz range, with modifications made to use the lower band too. In this way the 5GHz signals would get the better match and higher gain, which would be better for the higher frequency.
A Network Analyzer (electrical network analyzer) can be used to produce a plot which will show the Return Loss and VSWR at various frequencies for an antenna connected to it. The analyzer is first initialized by calibrating it with Open, Short, and Load standards for the frequency range selected and the impedance value of the antenna connection. In our case that's usually 50 ohms.
The graph produced by the analyzer will usually be an undulating line with highs, and hopefully sharp lows. The lows correspond to the more resonant frequencies, i.e. greater return losses, of the antenna. A return loss of -16 dB would be so-so. A return loss of -28 dB would be pretty darn good.
I am sorry I don't have any of these to post at the moment. I'll see if I have one to post at work.
If you search on a combination of Antenna, VSWR, Return Loss, and Network Analyzer, I'm sure you can find some very good examples of these plots.
I have an older Agilent Analyzer at work, but it only goes to 3 GHz. It will also display Smith Charts for the antenna at the push of a button.
If you ever get a chance to work with one, go for it.
See the attached file for an example of an antenna displaying multiple resonances. It is of a questionable (i.e. cheap) Ham Radio antenna, but the plot shows the resonant frequencies. The bigger the dip, the better the antenna is at that frequency.
The plot was made over a wider frequency range than a Ham would normally be concerned with. At least with the same antenna - 300kHz to 3 GHz.
OOPS. When I first attached the file, in the reply above, I didn't realize its name would be changed automatically.
If you want to view the plot, download the file, and change the file name to end with ".bmp" rather than "-bmp". Then you can use Paint, or other viewer, to open the file.
Thanks for this example Howard. I really appreciate it.
Here is an example of a base station tri-band antenna for ham radio. Notice the electrical wavelengths and how they correspond to frequency, and also their respective dB gain.
Frequency (MHz):52-54 1/2 wavelength, gain 2.15dBi
Frequency (MHz):144-148MHz 2-5/8 wavelength 6.2dBi (2x5/8 means two 5/8 wavelength stacked collinearly)
Frequency (MHz):440-450MHz 4-5/8 wavelength 8.4dBi (4x5/8).
Overall length 8.3 feet.