created: Aug. 2019
A full length l/4 monopole in the centre of the FM broadcast band (98 MHz) is ~0.8m long; i.e. too unwieldy for hand-held use. For portability, rubber duckies or normal-mode helixes can be used instead. Helixes allow the aerial to physically shortened by continuous loading of the radiating element.
The aerial is constructed by continuously wrapping an insulated length of copper wire on a polyurethane tube that is typically sold for pneumatic (fig. 1). Approximately, 72 turns of wire were evenly spaced-wound over a 285-mm long tube with an outside diameter of 8 mm. Prior to winding, the wire has a linear length of 1.7 m, or approximately l/4.
Fig. 1 construction detail
As the continuously wound helix has an inductive component in its impedance, an external shunt capacitor (56pF) is used to match it to 50 ohm (fig. 2).
Fig. 2 Equivalent circuit of aerial and external matching capacitor
To achieve portability, the trade-off is that the useable bandwidth is sacrificed. In our example, when the antenna is physically shortened to l/11, then bandwidth of VSWR >2 is less than 2% of the centre frequency. This represents a total bandwidth of 2 MHz.
Fig. 3 Impedance (top) and VSWR (bottom)
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