Shortwave communication operates within the frequency range of 3 to 30 megahertz and relies primarily on skywave propagation. This propagation method utilizes the ionosphere to reflect and scatter shortwave signals, enabling their transmission and reception. Shortwave communication is favored for its mobility, simplicity of equipment, low cost, ease of reestablishment, minimal terrain impact, and strong resilience against destruction. Consequently, it has played a significant role in international communication services, being widely applied across various sectors including government, diplomacy, meteorology, and commerce for transmitting voice, text, images, data, and other types of information. This article primarily focuses on the impact of shortwave antennas on shortwave communication.
Common types of shortwave antennas mainly include whip antenna, loop antenna, trifilar antenna.
To understand the transmission impact of antennas on communication, it is essential to first comprehend the transmission modes of shortwave, which primarily consist of skywave and groundwave propagation.
Skywave is the primary mode of communication for shortwave. After shortwave signals are transmitted, they are reflected back to the ground by the ionosphere, unaffected by obstacles.
Groundwave, on the other hand, propagates along the ground. Skywave propagation experiences minimal attenuation but is unstable, whereas groundwave propagation is stable but experiences rapid attenuation.
Inland areas with low ground conductivity strongly absorb radio waves. In northern China, groundwave propagation diminishes within about 20 kilometers due to this absorption. Skywave propagation, with its radiation pattern from the antenna, inevitably covers a greater distance upon reflection from the ionosphere back to the ground. For vehicle whip antennas and most other vehicle-mounted antennas, the distance from the transmission point to where the skywave returns to the ground is typically around 80 to 120 kilometers. Within this range (20 to 120 kilometers), neither groundwave nor skywave coverage is effective, resulting in a communication blind zone.
Loop Antenna
The loop antenna is designed to address communication blind zones in shortwave by utilizing a loop structure and tuning to generate high-angle radiation. The unique structure of this antenna creates a "fountain-like" upward radiation pattern concentrated between 90° and 40° elevation angles. Therefore, after reflecting back to the ground from the ionosphere, the coverage extends completely within a radius of 500 kilometers, effectively eliminating any blind zones.
Whip antenna, also known as a vertical antenna.
Mainly used for ground wave transmission, the antennas commonly referred to as "whip antennas" and "loop antennas."
Trifilar antenna
The trifilar antenna employs a combination of impedance transformation and loading techniques at the end of the radiating element, enabling an expanded frequency range for the antenna. Depending on the installation method, it can facilitate omni-directional communication without the need for tuning, ensuring high efficiency. It is primarily utilized for skywave transmission.
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