In the world of amateur radio, the search for the perfect balance between size and performance often leads to innovative designs. One such standout is the VHF Slingshot Antenna. Characterized by its “U” shaped geometry, this antenna is a variant of the Moxon or modified dipole, engineered specifically for the VHF spectrum. Its name comes from the distinct upright arms that resemble a slingshot, but its purpose is far more technical: providing a directional, low-noise signal for the 144-148 MHz range.
VHF Slingshot Antenna – Understanding the Dimensions
The efficacy of the Slingshot antenna lies in its strict adherence to geometric proportions. According to the technical schematic, the antenna features an overall width of 1118mm. This total span is comprised of two horizontal segments, each measuring 541mm, separated by a crucial 14mm air gap at the feed point. The vertical elements—the “arms” of the slingshot— stand at 1016mm. These dimensions are not arbitrary; they are calculated to align with the wavelengths of the 2-meter band, ensuring that the antenna resonates efficiently without excessive SWR (Standing Wave Ratio) issues.
Assembly and Feed Point Mechanics
Constructing this antenna requires attention to detail at the central mounting plate. The two main elements are secured to a non-conductive or insulated base plate, which is then mounted to a vertical mast. The feed point is the heart of the system. A 50 Ohm coaxial cable is used to connect the radio to the antenna. Specifically, the center conductor of the coax attaches to one horizontal element, while the outer shield (braid) attaches to the other. This
creates a balanced-to-unbalanced transition that allows the radio to “see” a stable load.
Performance Expectations
Why choose the Slingshot over a standard vertical whip or a simple dipole? The Slingshot design offers a more compressed radiation pattern. By folding the ends of a traditional dipole into these 1016mm vertical sections, the antenna achieves a form of “top loading” or end-loading. This results in a slightly broader bandwidth and a more rugged physical profile that can withstand wind better than thin wire alternatives. It is an ideal choice for stationary home stations or portable “Summits on the Air” (SOTA) operations where gain and portabilityare both valued.
Final Tuning and Installation
Once assembled, it is vital to test the antenna using an SWR meter or antenna analyzer. Small adjustments to the 14mm gap or the height of the vertical arms can shift the resonant frequency. Ideally, the antenna should be mounted as high as possible on a sturdy mast, away from metal obstructions. Because of its polarized nature, ensuring the arms are perfectly vertical (or horizontal, depending on your target signal) will significantly impact your ability to hit distant repeaters or engage in simplex contacts across the VHF band.
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