--------- Begin forwarded message ----------
On Tue, 25 Nov 1997 09:18:42 -0600 k0hb@juno.com (Hans Brakob) writes:
>You said:
>
>> Takeoff angle is determined ONLY by antenna HEIGHT,
>> not type of antenna.
>
>Assuming the original poster from France meant angle of radiation,
>not many people will agree with you.
>
>A long boom yagi will have a significantly lower angle of radiation
>than (for example) a dipole at any equal height, high or low.
>
Au contrair!
The GROUND REFLECTION COEFFICIENTS are determined
by HEIGHT above ground for horizontally polarized antennas. They
correspond exactly to the coefficients for a dipole. Long Yagi's DO
cut off the higher angles, but do NOT change the ground reflection
coefficient. For very high antennas, the range of angles covered by
the first lobe(s) of a dipole and a long boom Yagi will be IDENTICAL,
the Yagi will just have higher gain.
Remember, the final pattern is the vector product of the FREE SPACE
ANTENNA Pattern and the GROUND REFLECTION Coefficient. The
-3 dB points of a 3 element Yagi in Free Space are around +/- 50 degrees.
For a 5 element Yagi, they are around +/- 35 degrees. Below about 10
degrees or so, there is very little cutoff from the Yagi pattern,
therefore
for antennas above 1.5 wavelengths high, the radiation ANGLES of a
dipole and a Yagi are nearly identical in the first lobe. If you have an
antenna modeling program, check it out. Look at the peak and -3 dB
points in the VERTICAL pattern of both a Yagi and dipole pattern
when both antennas are over 1 wavelength high.
Yes, the gain of a LONG boom Yagi will peak at a lower angle than
a shorter boom Yagi, but this is due to pattern cutting of the higher
angles in the vertical plane rather than actually forcing more radiation
into a lower angle. At higher angles (>20 degrees) long boom antennas
will produce no more signal than a 3 element Yagi.
Look at the same LONG boom antenna at greater heights and you
will see that the antenna gain appears to go UP as the angle goes
down. This is due to the closer alignment of the ground reflection
peak and the free space pattern peak of the Yagi due to less vertical
pattern loss. It is a suble but important difference to understand.
These remarks assume HORIZONTAL polarization and FLAT ground.
73, Tom N4KG
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