Average Q-factor

Average Q-factor and available bandwidth

Straight split dipoleOld gain optimized 2M7N50New 2M7N50LY low Q design
VSWR 140 -150MHzVSWR 140 -150MHzVSWR 140 -150MHz
Average Q-factor plot of a straight split dipole2M7N50 old style gain optimized Long YagiNew 2M7N50LY Low Q Long Yagi by G4CQM


VSWR bandwidth is often used as an indicator of antenna Q which in itself can seem somewhat abstract. However, when defined with a well known reference (example straight split dipole) offers a meaningful way to compare designs. Seen to the left above, the average Q-factor plot of a straight split dipole, boasting a very wide bandwidth. Comparison with old style 2M7N50 gain optimized yagi and new 2M7N50LY low Q design. Average Q-factor is calculated from R and X values against frequency.

When a driven element is set amongst the yagis parasitic elements mutual coupling will usually raise the average Q-factor and so narrow bandwidth. By how much is determined from the level of coupling between the elements. This complex dynamic dependent on the build tradeoffs when optimising.

Yagis with a low average Q-factor are less sensitive to construction tolerances and bad weather.


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