How to choose the antenna system: T-antenna or umbrella antenna?
The medium frequency (MF) range and the upper part of the low frequency (LF) range are used for radio navigation. Differential global positioning system (DGPS) signals are broadcasted in the range of 283 to 325 kHz (1060-923 m). To broadcast the aircraft homing signals the frequency range of 190 to1750 kHz (1579-171 m) is used.
Antenna operation quality considerably depends on the antenna's structure and dimensions. Maximum efficiency and stability of the antenna may be achieved when using the antennas as high as possible. As for the dimensions, an important role is given to both the antenna's physical height that determines the effective height, and the dimensions of the horizontal part that ensures more uniform current distribution in the vertical part and consequently increases the effective height and reduces the reactance.
The most popular transmitting MF/LF antennas are the T-antennas and antennas with umbrella top structure (umbrella antennas). Technomarine manufactures T-antennas and umbrella antennas of various dimensions.
T-antennas essentially are the wire antennas with the flat top between the two masts, and with the downlead connected to the center of the top. Flat top may be made as single wire or two wire top, and the length may be up to 55 m. Depending on the masts used, the antenna may be mounted at height of 6 to 30 m.
Ground plane for the T-antennas is the set of 24 copper wires (counterpoise radials) located radially in relation to the antenna downlead within the ellipse whose axes are 60 and 40 m long or 120 and 60 m long.
The advantages of the T-shaped structure of these antennas are: fairly good performance, stability of parameters, and wide selection of the antenna system sizes. As for the disadvantages of these antennas, wide area is required for installation.
The umbrella antenna is a base-insulated vertical mast with capacity top radials (umbrella) on top of the mast. The umbrella includes six radials; the ends of these radials are linked by the framing wire. The height of the antenna is 8 m (AZM-8 antenna) or 20 m (AZM-20 antenna). The radials of the ground plane are within the 12 m radius for AZM-8 antenna, and 20 m for AZM-20 antenna.
The main advantage of the umbrella antennas in comparison to the T-antennas is the space saving. But the small size of the umbrella antennas is the reason why these antennas have all the disadvantages related to poor electrical performance.
As the main purpose of the antenna is broadcasting the signal so that remote users are able to receive it, first of all be guided by the antenna efficiency when choosing the type of the antenna. Maximum antenna efficiency and stability of operation may be achieved when using the high antennas. Among the previously considered antennas, the highest one is the 30 m T-antenna with 55 m two long wire top.
However, during design and engineering of transmitter site you may be limited by lack of space allocated for the antenna systems installation. In this case, a rather attractive option is to use the umbrella antenna. But when choosing this antenna, be ready for reduced coverage distance and reduced the depth of amplitude modulation in comparison to the T-antenna.
An important parameter for selection of the antenna type is the operational frequency. If considering the antenna electric properties only (without consideration of the signal propagation vs. frequency) for better performance and for suitable radiofrequency voltage on the antenna elements, higher operational frequency should be chosen. Using the umbrella antennas, especially AZM-8 antenna, for low frequency range (up to 300 kHz) is technically possible, but exceptionally undesirable, since in this case all disadvantages inherent to the short antennas will resulted. The most dangerous effect is the high radiofrequency voltage (exceeding 15 kV), which may destroy the antenna tuning unit or decoupling unit. High operational frequency (exceeding 1200 kHz) applied to T-antenna, in spite of the good expected efficiency, may lead to the difficulties when tuning the antenna circuit to resonance, since the operational frequency is approaching to the self-resonance frequency of the antenna, and the nature of the reactive term of the antenna impedance changes from capacitive to inductive. In the upper part of the frequency range, the wavelength of the operational wave becomes comparable with the physical height of the T-antenna, and as a matter of fact this antenna are not short antennas any more.
To compare the T-antennas and umbrella antennas, tables 1, 2, and 3 illustrate their electric properties estimated for values 300 kHz, 700 kHz, and 1200 kHz, correspondingly. These tables provide the efficiency, modulation depth attenuation at 1200 Hz modulation frequency (Bm), and maximum permissible antenna current at 15 kV (Ia).
Table 1 — Electric properties of the T-antennas and umbrella antennas manufactured by Technomarine for 300 kHz
Table 2 — Electric properties of the T-antennas and umbrella antennas manufactured byTechnomarine for 700 kHz
Table 3 — Electric properties of the T-antennas and umbrella antennas manufactured by Technomarine for 1200 kHz
Note 1 — Antenna type: - AZM — umbrella antenna, height 8 or 20 m. - Тx-y-z — T-antenna, where x — number of radials, y — top length (35 m, 45 m, or 55 m), z — height (8 m, 22 m, or 30 m). Note 2 — Some results are missing because the operation frequency is approaching to the self-resonance frequency and the short antenna calculation methodology is not applicable.
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