To develop a better feel for Yagi performance vs.
boom length, number of elements, and bandwidth,
I highly recommend the YAGI DESIGN BOOK by
W2PV published by the ARRL.
In chapter 2, there are hundreds of curves of Yagi
gain vs. frequency offset from design center with
parasitic element tuning of +/- 2,3,4,5,6,7% for
3,4, and 6 element Yagi's.
Studying these curves, it becomes apparent that
gain is NOT constant with frequency and that
for most cases, gain and F/B do NOT peak at
the same frequency. Typically, most commercial
antennas sacrifice about 1 dB of gain to obtain
better F/B ratio and bandwidth. Another observation
I have made is that most commercial antennas have
one more element than the minimum necessary to
achieve a particular gain. Again, this is to provide
improved bandwidth and pattern.
An interesting side note is that very high gain (9.5dBi)
can be achieved using only 3 elements but over only
a VERY NARROW bandwidth. Such antennas might
be useful in the WARC bands and possibly 15M which
has just over a 2% bandwidth vs. operating frequency.
Such designs are NOT forgiving of tolerance errors!
The following table lists MAXIMUM gain vs. boom length
for a given number of elements. Most designs will
exhibit less gain in favor of F/B and bandwidth. Remember,
these are for FULL SIZE elements on MONOBANDERS.
TRAPPED TRIBANDERS will exhibit less bandwidth
and reduced gain due to trap effects and losses.
ELE MAX BOOM LENGTH SIMILAR
GAIN WL FT (20M) SIZED
2 6.5 - 7.0 .15WL 10' on 20M TH2,
3 6.7 - 7.5 .20WL 14' on 20M TH3, TA33, A3
3 7.0 - 7.7 .25WL 18' on 20M CL33, A4
3 7.5 - 8.5 .33WL 24' on 20M TH6, TH7,
Note that only when you go to a WIDE SPACED 3 element Yagi,
does the gain increase significantly over a 2 element Yagi. Accounting
for trap losses and reduced size elements, the smaller 3 element
Yagi's are hard pressed to deliver any more signal than a full size
2 element Yagi (such as the Force 12 C3). Many manufacturers
report that trap losses are small at the design center frequency,
but even a small loss adds up when you consider that there are
12 active traps in the small tribanders when used on 20M.
Disclaimer: I have no interest in any antenna manufacturer or
de Tom N4KG
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