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 your antenna model is too close to the ground, the simulation can be unrealistic. Once satisfied with your design in free space, you can 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
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
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