Usable (10dB) F/B bandwidth is only about 30 kHz on the short Yagi. Also the short Yagi reverses direction quickly below the design frequency.
The need for band switching is obvious !!
PERFORMANCE COMPARISON TO A FULL SIZE MODEL
The plots below show how the shortened Yagi would compare to one using full size 80m elements.
The Full Size model uses the same design as the shorten Yagi discussed on this webpage, except the 80m elements are lengthened to full size and the inductors are removed. The boom length is the same and the 40m elements do not change. The full size model was optimized for max. F/B at close to the same frequency as the shortened model. It may be possible to design a 2 el Yagi with better numbers by making the boom longer, but this comparison is meant to show how these inductor loaded elements compare to one using full size elements using this same short boom design.
The short Yagi sacrifices 1 dB in Gain and has actually better F/B, but over a **VERY** narrow bandwidth. As long as some form of band switching is implemented, the Yagi performance can come close to a full size design. The feed point impedance of the short Yagi is around 20 ohms compared to 31 ohms for the full size design, so great care must be taken at the feed point and element connections to minimize losses and maximize efficiency.
All model results shown are run over real, average ground at a height of 30m.
The shortened model uses 1 ohm series resistance at each loading inductor to represent coil loss.
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This plot shows that the F/B on the short Yagi is
actually better than the full size design, but this is only true for a
**VERY** narrow bandwidth !!!
Usable (10dB) F/B bandwidth is only about 30 kHz on the short Yagi. Also the short Yagi reverses direction quickly below the design frequency. The need for band switching is obvious !! |
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The Full size design has almost 4 times greater bandwidth
than the short Yagi. The need for switching is very obvious.
As discussed in the tuning section, the bandwidth can be increased by lengthening the reflector a little to move the max F/B frequency down below the design resonance frequency. This will sacrifice some gain and F/B however. |
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The short Yagi at its peak gain point is only 1 dB down from the full size model, but it's usable bandwidth is significantly less than the full size model. |