[TowerTalk] 2ele Yagi - driven element & director or reflector?

[L. B. Cebik]

On Fri, 20 Mar 1998, CP2235 wrote:
> Can anybody tell me why most commercial 2ele design are combinations of a
> driven element with a reflector whereas the ARRL Antenna Book states that the
> specifications on both F/B and gain are slightly better with a combination of
> driven element and director? Plus the advantage of a slightly shorter element
> (director instead of reflector)?
> Thanks, 
Con,

The reason for the prevalence of DE-Ref 2-element beams is operating
bandwidth.  At the design center frequency, it is possible to achieve
slightly higher gain and 17-20 dB F-B ratios from a DE-Dir 2-el Yagi with
a feedpoint impedance between 20 and 25 ohms (fit for a beta match or
similar), but only over a narrow operating bandwidth.  A spacing of about
0.07-0.09 wl will work with most common tubing sizes.

DE-Dir designs are well suited to the WARC band, which are narrow by
comparison to the harmonic bands.  For example, there is an open-sleeve
fed back-to-back pair for 12 and 17 using this design at my web site.
Also, if one has need for only a portion of a given harmonic band, say the
CW end, then this design is also a way of getting slightly more gain and
a full S-unit of F-B.  I designed a 10-meter version that was used in a
recent contest (but band conditions limited testing of the antenna in
intensive use).

Most commercial 2-element beams seek broader coverage on the harmonic
bands (80-10 meters).  A well designed full size 2-el DE-Ref Yagi can
cover the first MHz of 10 meters, and, of course, a similar frequency
ratio as the frequency is changed.  F-B will only be about 12 dB with a
spacing of 0.125 wl (feed Z about 30-35 ohms) and down to 10-11 dB as
spacing increases to about 0.16 wl (feed Z near 50 ohms). (By contrast,
getting 100 kHz at 17 meters with a DE-Dir design requires care.)

On the lower bands, where 2-element designs often use shortened elements,
the operating bandwidth decreases very rapidly.  Hence, the DE-Ref design
is about the only option.

Hope this is useful.

-73-

LB, W4RNL

L. B. Cebik, W4RNL         /\  /\     *   /  /    /    (Off)(423) 974-7215
1434 High Mesa Drive      /  \/  \/\     ----/\---     (Hm) (423) 938-6335
Knoxville, Tennessee     /\   \   \ \   /  / || /      (FAX)(423) 974-3509
37938-4443     USA      /  \   \   \ \       ||              cebik@utk.edu
         URL:  http://funnelweb.utcc.utk.edu/~cebik/radio.html




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