[Towertalk] 40m beams

[n4kg@juno.com]

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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