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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