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[TowerTalk] Takeoff Angles and Long Boom Yagi's

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Subject: [TowerTalk] Takeoff Angles and Long Boom Yagi's
Date: 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

At  heights of ONE  WAVELENGTH  and above (>70 ft. on 20M),
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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