In Practice: Care and feeding of VHF/UHF long Yagis

摘要

We have reached the stage where all competently designed and computer-optimised long Yagis for VHF/UHFare pretty darn good. Raw forward gain isn't really a deciding factor any more, because all good designs deliver the amount of gain that we should expect from their overall boom length, within a few tenths of a dB. The differences are now in more subtle areas such as minor lobe suppression and the ways that the forward gain, pattern and VSWR vary with frequency. These are all tradeoffs, depending on the combination of properties desired.rnTurning to the specific point about feed point impedance, if you follow up the references [1] you will see that it isn't a major factor in the overall performance of these Yagis. Although many designs come out in the 20-30Ω region, Yagis with equally good overall performance can be designed for direct 50Ω feed and also forfeedpoint impedances as low as 12.5Ω.