Antennas and Microwaves

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Reference Dipoles

The dipoles described here are simple half wave designs using a Pawsey stub balun. The design is most applicable in the frequency range 800-3000 Mhz. The air dielectric of the balun results in a very efficient antenna. The only losses are the Copper losses in the radiating arms and the insertion loss of the semi-rigid cable between the connector and feed point (typically <0.1dB)

Test Antennas

The dipoles are constructed from RG402/U semi-rigid microwave cable. The cable has silver clad steel centre conductor with PTFE dielectric within a seamless copper tube. The photograph to the right shows typical construction.

The starting point for the design is to aim for a tip-to-tip dipole length that is 95% of the free space half-wavelength (lo/2).The starting length of the Pawsey stub is 68% of lo/2. Dimensions for some popular mobile communication bands are tabulated in the drawing to the right.

For each of the designs tabulated above the return loss has been measured. The results are presented in the graph to the right.

Patterns for these antennas are obviously dipole like, in that they have the classic doughnut shape. However as with many practical dipole implementations the peak gain can be upto 1dB or so greater than the theoretical 2.15dB for a thin dipole.

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