[TowerTalk] Takeoff Angles and Long Boom Yagi's
T A RUSSELL
n4kg@juno.com
Wed, 26 Nov 1997 14:30:35 -0600
TAKEOFF ANGLES and LONG BOOM Yagi's de N4KG
I stand CORRECTED. Previously, I stated:
"Takeoff angle is determined ONLY by antenna HEIGHT,
not type of antenna." Height is NOT the ONLY factor.
Jim Lawson, W2PV summed it up pretty well in Chapter 5
of his Yagi Antenna Design book when he wrote:
"Thus, we see that the main lobe of an antenna occurs at an angle
PRIMARILY determined by its height over ground, but secondarily
by the natural antenna directivity."
First, some housekeeping.
I apologize for posting a reply without first obtaining permission.
I thought the question was a good one and unfortunately, I missed
clearing the sender's identity from all places in the copy.
Second, I want to THANK all of you who gently pointed me to a
higher understanding of the complex interrelationships between
takeoff angle, antenna height, antenna gain (boom length), and
terrain. Thanks go to K3WW, W3AB, KC4ZXX, NT5C, K0HB, K0KR.
Of the above parameters, TERRAIN may have the greatest influence
on the final far field radiation pattern from any antenna. All of my
comments assumed a LEVEL foreground and reflecting plane.
Terrain analysis is a complex issue. N6BV and K6STI have some
interesting new software that examines these effects.
(Thanks to W3AB for the reminder!)
TAKEOFF ANGLE Clarificaton:
The GROUND REFLECTION Coefficients for a horizontally polarized
antenna are determined by the HEIGHT of the antenna above FLAT
ground. These are shown in the ARRL Antenna Handbooks.
(K3WW pointed out that in the strictest sense, this is true only
for isotropic sources. More importantly, he kept me digging
until I was more fully enlightened. Thank You Joe.)
The FAR FIELD radiation pattern of a horizontally polarized antenna
is the product of it's FREE SPACE pattern and the Ground Reflection
Coefficients.
At heights of ONE WAVELENGTH and above (>70 ft. on 20M),
the TAKEOFF ANGLE is INDEPENDENT of BOOMLENGTH.
A dipole, 3 element Yagi, and 6 element Yagi all show peak
radiation at approximately 14.5 degrees at one wavelength.
At heights BELOW one wavelength (WL), long boom Yagi's
force more radiation at low angles than do short boom Yagi's
and therefore their maximum radiation occurs at a lower angle
than for a shorter boom Yagi or dipole. This is achieved at
the price of reducing radiation at higher angles.
(Thanks to NT5C for transcribing the W2PV 3 and 6 element
Yagi data vs. height which reveals these interesting points.)
Placing a LONG BOOM Yagi at a low height to achieve
lower angle radiation seems a mis-application to my mind.
The FULL gain of a LONG BOOM Yagi is only realized at lower
angles which are optimized at heights GREATER than 1 WL.
Lower antennas are useful for providing coverage of the
higher angles that can be supported by the ionosphere
during daylight hours. Therefore, it is best to use shorter
boom Yagi's (3L, 2L, or even dipoles) at low heights (<1 WL)
for high angle coverage and Long Boom Yagi's at greater
heights to take advantage of their higher gain at low angles.
Contesters like to use stacked arrays with capability of
selecting Upper, Lower, and Both antennas for wave
angle selection.
de Tom N4KG
.............................
In W2PV's book, "Yagi Antenna Design", pages 5-8 through 5-11,
the following are shown for the main (lowest) lobe angles
for 3-el and 6-el Yagis:
Main (lowest) Lobe Angle (deg.)
Antenna Ht (WL) 3-el 6-el
WL 0.1 55 45 degrees
0.25 37 30
0.5 27 23
0.75 20 18
1.0 15 15
1.25 11 11
1.5 9 9
1.75 8 8
2.0 7 7
2.5 6 6
3.0 5 5
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