The T Antenna
The T antenna is a top loaded vertical. It has an omni-directional pattern that stays pretty much the same from 530 kHz to 1700 kHz. The pattern looks like a donut. This particular antenna has a 100’ top section, center fed with a single wire, 50’ long, hanging vertically. The crystal set is connected to the bottom of the vertical leg. At 530 kHz, the maximum gain is about 2.8 dbi at an angle of 20 degrees elevation angle anywhere along the edge of the donut, i.e., it is omni-directional. At 1700 kHz, the maximum gain ranges from about 1.6 to 1.8 dbi at 25 degrees elevation angle, and is again, omni-directional.
Efficiency comments: This antenna is actually short for the broadcast band, as is the inverted L shown in the preceding page. Antennas that are electrically short will have a radiation resistance less than a full size vertical (about 36 ohms resistive for the ¼ wave vertical). Radiation resistance can range from a fraction of an ohm at the low end of the band to a couple of ohms at the top end for these antennas. Since these antennas are worked against a ground system, the losses in that ground system are in series with the antenna, creating loss and inefficiency. A good ground system should be implemented as part of the overall antenna system.
Figure 1a, T antenna at 530 kHz
Figure 1b, T antenna at 530 kHz, elevation plot
Figure 2a, T antenna at 1700 kHz
Figure 2b, T antenna at 1700 kHz, elevation plot
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