[TowerTalk] re; QUESTION/2 ele 40M

[n4kg@juno.com]

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
> 
> 
> 
> 
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