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This is my first attempt, mistakes can happen. This is a work in progress.

VHF crossed yagi

To get an orthogonal polarisation (near circular), two identical yagi share the same boom with a 90° polarisation angle (H/V), each of the two yagis are physical 90° apart on the bom (this will enable the use of a symmetrical phasing harness). Each of the yagi must have a 50 ohm feed point.

Then a power divider is needed to connect both antennas to the transmitter.

Design of a yagi with a 50ohm feedpoint

To my knowledge there is no easy formula to get a yagi with a direct 50ohm feed point. The only way is simulation.

A parametric model in CST will be very convenient :

The goals for this design :

  • gain of the antenna should be at least 12dB
  • front to back ratio should be at least 10dB
  • RL at feed point should be under -15dB

Parameters to adjust to get this result :

  • reflector length (max : 120mm, min : 70mm)
  • reflector spacing (max : 34mm, min : 15mm)

Obviously to lower the reflector spacing is the key to success. Unfortunately to get a decent front to back radio we need to adjust the reflector length.

Here is a result that match the criterias :

First design

  • reflector spacing : 20mm
  • reflector length : 99.98mm

This is not perfect. The side lobes are still strongs, but to get something better we need to have a very different feed, so for now it will be enought.

wiki/projects/antennas/vhf_crossed_yagi.txt · Last modified: 2020/05/14 14:48 by f4hvx

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