EZNEC antenna design tips

A few quick start tips for using EZNEC antenna design software for yagi antennas. This is assuming a yagi antenna for 440 MHz band.

  • Ground: start with “free space”. If the antenna model is too close to the ground, the simulation can be unrealistic. Once satisfied in free space, consider real ground (non-free space)
  • Number of segments: suggest 5 per wire, with free EZNEC license
  • Number of yagi elements: 3..5 elements are possible with free EZNEC at 440 MHz.
  • Source: 1 source, middle (50%) of “driven” wire

Example antennas

Typical yagi performance

How good is good enough?

  • Gain: > 6 dBi
  • Backlobe (front/back ratio) (opposite direction of desired mainlobe): < -15 dB or so
  • Sidelobes (off side of desired direction): < -15 - -20 dB or so
  • SWR (439-441 MHz): Ideally less than 2.0. 3.0 is perhaps tolerable.

SWR is a measure of how efficient an antenna is at coupling energy to/from the air to a radio device.

Γ = (SWR - 1) / (SWR + 1)

  • Mismatch loss [dB] (energy loss due to SWR): -10 * (1 - Γ)^2

Example

Suppose: SWR = 3.0

  • Mismatch loss: -10 (1 - ((3-1) / (3+1))^2) = 1.25 dB

Power loss (gain) factors:

  • 3dB ~ factor of 2
  • 10dB ~ factor of 10
  • 20dB ~ factor of 100