[Towertalk] 40m beams

[Pete Smith]

At 01:52 PM 4/7/02 -0600, n4kg@juno.com wrote:
>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.


But the Force 12 does not have a direct feed -- there is a 3-turn hairpin 
matching coil across the feedpoint.