This article documents a design prototype for a two element Yagi for Fox Hunting / portable direction finding of Radiosondes on 401MHz.
Design criteria:
- compact;
- good symmetry;
- good front/back ratio;
- deep nulls at the side;
- easy DIY construction; and
- low cost.
The design is for a two element Yagi using a Half Folded Half Wave Dipole driven element and reflector. The driven element was covered by US Patent 6307524 which may have expired.
A model was constructed in NEC-4.2 and adjusted to meet the design criteria.
Above is a diagram of the Yagi elements. The driven element feed arrangement has an integral balun and 1:4 impedance step up ratio allowing a close spaced configuration matched to 50Ω coaxial feed line.
Overall, it is about 400mm wide and about 100mm front to back, quite compact.
Above are the VSWR and Reflection plots from the model.
Above are the impedance plots from the model.
Above is a sweep of Gain and F/B.
Above is the gain pattern, front/back is 11dB. There are deep nulls at exactly 90° to boresight. Front/back is used to determine the approximate bearing of the target, the nulls are used for a more precise bearing.
WIRE NO. OF FIRST LAST TAG NO. X1 Y1 Z1 X2 Y2 Z2 RADIUS SEG. SEG. SEG. NO. 1 -0.16983 0.00000 0.00000 0.00000 0.00000 0.00000 0.00150 30 1 30 1 2 0.00000 0.00000 0.00000 0.16983 0.00000 0.00000 0.00150 30 31 60 2 3 0.00000 0.00000 0.01000 0.16983 0.00000 0.01000 0.00150 30 61 90 3 4 0.16983 0.00000 0.00000 0.16983 0.00000 0.01000 0.00150 3 91 93 4 5 0.00000 0.00000 0.00000 0.00000 0.00000 0.01000 0.00150 3 94 96 5 6 -0.18341 -0.06114 0.00000 0.18341 -0.06114 0.00000 0.00150 60 97 156 10 THE STRUCTURE HAS BEEN MOVED, GM COMMAND DATA IS - 0 0 0.00000 90.00000 0.00000 0.00000 0.00000 0.00000 1 1 0 156
Above is an extract from the NEC report that shows the dimensions of the GW elements used to construct the model. They can be used as a guide to find the dimensions of the Yagi elements and spacing.
Note that the element ends should be treated to reduce the risk of injury to persons.
Next: construction, measurement and evaluation.