Rhombic antenna
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A rhombic antenna is a broadband directional antenna, mostly commonly used in HF (high frequency, also called shortwave) ranges.
It is named after its "rhombic" diamond shape, with each side typically at least onewavelength or longer in length. Each vertex is supported by a pole, typically at least one wavelength tall. It is fed at one of the sharp angles through a balun transformer, and is terminated at the opposite sharp angle with a non-inductive resistor. It is directional towards the resistor end, so it points towards the region of the world it is designed to serve.
The rhombic antenna can radiate at elevation angles close to the horizon or at higher angles depending on its height above ground relative to the operating frequency. Likewise, its beam can be narrow or broad, depending primarily on its length. A proper combination of size, height, and operating frequency make it fit for medium or long range communication.
Due to its considerable size, it is not very practical as the sole antenna of a radio station if geographic coverage requirements are expected to change rapidly. Moreover, it plainly requires a lot of land—especially if several must be combined to serve a variety of geographic regions at different distances and on widely different frequencies.
On the other hand, it is one of the best options for sustained long distance communications over point-to-point circuits. Its size gives it substantial gain, and allows it to capture energy from a wide area, thus making it a little less susceptible to sharply localized fading than smaller antennas.
[edit] Advantages of Rhombic Antennas
- Its input impedence & radiation pattern are relatively constant over a 2:1 range of frequencies.
- Multiple rhombic antennas can be connected in an end-to-end fashion to form MUSA (Multiple Unit Steerable Antenna). MUSA arrays can receive long distance, short wave, horizontally polarized down coming waves.
