Rubber ducky for FM broadcast band

created: Aug. 2019

Introduction

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.

Material & method

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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