Bottom Line: 40 ft is pretty low for a 40M beam.
It will perform MUCH better between 60 and 100 ft.
At 50 ft, you will realize some benefit but not all.
The far field radiation pattern is the product
of the free space pattern of the antenna and
the ground reflection coefficients.
The free space pattern of a beam is centered at the horizon.
A 2L Yagi has a pretty broad pattern in the vertical plane,
up to plus/minus 60 degrees each side of the horizon.
The ground reflection coefficient pattern at 0.25 WL high
(35 ft on 40M) is centered straight UP. See the *old*
ARRL Antenna Books for plots of ground reflection coefficients.
There is little overlap between the free space antenna pattern
and the ground reflection pattern at 0.25 WL which means
that there is little benefit from the directivity of the antenna.
To realize the benefit of the antenna directivity, the free space
pattern and the ground reflection pattern need to have large
overlapping coverage. For the higher HF bands, this typically
requires a *minimum* height of 0.5 WL (35 ft on 20M).
On the lower bands, useful gain can be realized down
to 0.375 WL (50 ft on 40M).
Tom N4KG
On Tue, 18 Dec 2001 "William F. Beyer Jr." <n2wb@arrl.net> writes:
>
>
> OK to put a 2 element @ 40 Feet?
> Will have C.C. 3 ele WARC beam abt 8 feet up from 40m yagi.
> Thanks....
> --
> *See you in the pileups!
>
> William/N2WB
>
>
>
>
> AN Wireless Self Supporting Towers are now available! Winloading
> tables,
> foundation diagrams and charts, along with full details are now at
> the
> AN Wireless Web site: http://www.ANWireless.com
>
> -----
> FAQ on WWW: http://www.contesting.com/FAQ/towertalk
> Submissions: towertalk@contesting.com
> Administrative requests: towertalk-REQUEST@contesting.com
> Problems: owner-towertalk@contesting.com
>
________________________________________________________________
GET INTERNET ACCESS FROM JUNO!
Juno offers FREE or PREMIUM Internet access for less!
Join Juno today! For your FREE software, visit:
http://dl.www.juno.com/get/web/.
AN Wireless Self Supporting Towers are now available! Windloading tables,
foundation diagrams and charts, along with full details are now at the
AN Wireless Web site: http://www.ANWireless.com
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|