On Sun, 07 Apr 2002 14:35:16 -0400 Pete Smith <n4zr@contesting.com>
writes:
> At 06:11 PM 4/7/02 +0000, Barry Kirkwood wrote:
> >Sometimes wonder if the 40m reflector is worth the trouble and
> would be interested to hear from anyone who has moved from
> single element to two element combination ( or vice versa)
> (ZL1DD)
A 40M rotary dipole is a very effective antenna, especially
when placed above 70 ft high. There is a lot of benefit to
be derived from having horizontal elements (as opposed to
inverted vee elements) placed high and in the clear.
I worked my first 300 countries on 40M using various
rotary dipoles at 80 ft. My CC 2L40 shows a perceptible
gain over my other dipoles.
> I made this transition, and I think Barry has a valid point.
> Certainly, the jump to a rotary dipole made a big difference
> in my ability to put a signal where I wanted, and the
> front-to-back ratio of my EF-240S (essentially the same
> as the 40 meter section of the C-4SXL) is not impressive.
> (N4ZR)
Here are the ELNEC results for a free space 2L Yagi design
using a reflector spaced 0.15 WL behind the DE for max gain,
max F/B, and minimum SWR.
Max Gain 6.95 dBi
F/B 6 dB
Feed Z 27.4 Ohms
Gain 6.12 dBi
Max F/B 13 dB
Feed Z 44 Ohms
Gain 5.58 dBi
F/B 10 dB
Min SWR 51 Ohms
Note the low F/B ratios for 2L reflector Yagis
and the loss of gain as the feed impedance is
raised to match 50 ohm coax. Maximum gain
designs almost always result in a low radiation
resistance at the DE which requires some sort
of matching network. Most split dipole fed Yagi's
have less than optimum gain which is the price
you pay for a direct 50 ohm feed.
Tom N4KG
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