> But what about F/B or F/S ratio? Aren't they of any importance?
Of course they are. If you spend a little time working with the
modeling programs (or reading the ARRL Antenna Book), you will find
peak F/R is relatively narrow on most yagi designs and most fixed
tuned antennas fail to achieve real pattern performance over even
half of a given band. Often the F/R of an antenna set up for CW
will fall to less than 10 dB on phone. For example, a three element,
16 foot boom 20 meter yagi might show F/R > 20 dB below 14.100 but
at 14.200 it's down to 11 dB and only 7 dB at 14.300. Similarly,
SWR is greater than 2:1 shortly above 14.100.
> Personally, I can't take too seriously the manufacturer claim
> that "a long boom is not needed to achieve near optimum gain
> and front to back ratios on 20 - 10 meters since the SteppIR
> can control the element lengths" !!!!
The adjustable element lengths allow the SteppIR antennas to achieve
a very nice balance between the gain available for a given boom
length, F/R and SWR. Quite simply, modeling will show that, within
limits Gain and F/R are nearly constant flat as one changes the element
spacing (e.g., the position of the driven element in a three element
yagi with a fixed boomlength) as long as the element tuning is
adjusted to compensate.
With the SteppIR antennas, the boom length has been compromised
slightly in favor of higher gain on the low end of the range to
the detriment of best possible F/R on 10 meters. That is a trade
that many are willing to make.
... Joe, K4IK
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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