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## [TowerTalk] N4KG: Re: Take off angle

 To: [TowerTalk] N4KG: Re: Take off angle n4kg@juno.com (T A RUSSELL) Tue, 25 Nov 1997 15:58:50 -0600
 ```--------- 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 -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com Search: http://www.contesting.com/km9p/search ```
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