AN OIRT ANTENNA

A suitable 2-element Yagi with folded dipole can be made from commercial antenna parts. The dipole shown here was made
by grafting the 'folds' from a Band II antenna onto some new elements, and the reflector used 2 more elements supported by
a dipole insulator with a shorting wire across the terminals. A 4:1 balun was made using a ferrite bead

OIRT aerial

The driven element length is 2 1.00m
The reflector length is 2 1.075m
The spcaing is 0.54m

element dimensions

Polar responses were modeled using MMANA-GAL and as shown below the gain remains reasonably constant across the OIRT band
The F/B ratio is lowest at the bottom end. The impedance shows a rather poor match to 300 Ohms,
but the design cannot be improved much, for a 2 element antenna with wide bandwidth

polar response at 66MHz

polar response at 70MHz

polar response at 74MHz

The details below show the pre-formed radius end-section joined to the straight section, the construction of the ferrite-bead balun,
and the mounting of the balun and connector in the dipole connector compartment

construction details

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J.Hardstone June 2010