All:
 Original article from: Wayne Sarosi 
 Reposted by Rob Dennis,Moderator HAM_TECH,for use of the FIDO HAM_TECH echo.
  THIS POST/SERIES NOT TO BE USED IN ANY CDROM COMPILATIONS FOR COMMERCIAL
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SUBJECT:  Vertical Beams on VHF and the effect of metal masts on propagation
         patterns,gain,and bandwidth (part one).
 I've complied some information on vertical beams used in VHF communications.
 Yagis were used for this test. This information was taken from studies done
at the Kennedy Space Center from 1982 thru 1988 and from 1990 thru 1991.
 Also,the studies include information gathered here,at my QTH,field day 
9,90,
& 91,and other amateur stations from 1987 thru 1991.
 These studies include the effects of metal support masts to the boom of the
beam,past the boom of the beam,and short of the boom by a wave length with a
non-conductive support,towers near the antenna,VHF beams above horizontal
antennas,below horizontal antennas,side mounted beams,and the effects of 
etal
guys encompassing the beams.
 In part one of this series,the discussion will cover the first three 
udies.
*** VHF beam antennas and metal support masts ***
 Metal masting past the beam: (metal mast at earth ground)
  Figure one->                  || <--  Metal Mast
                                ||
                                ||
                                ||
               |    |    |    | || |   |   |   |   |
               |    |    |    | || |   |   |   |   |
               |----|----|----|-[]-|---|---|---|---| <-- VHF Yagi
               |    |    |    | || |   |   |   |   |
               |    |    |    | || |   |   |   |   |
               R    DE   D1   D2|| D3  D4  D5  D6  D7
                                ||
                                ||
 Testing: Yagi antenna pattern and gain over a vertical dipole at
          the same frequency.
  Figure two->
               R                                   D7
               |    |    |    |    |   |   |   |   |
               |    |    |    |    |   |   |   |   |
               |----|----|----|-[]-|---|---|---|---| <-- VHF Yagi
               |    |    |    | || |   |   |   |   |
               |    |    |    | || |   |   |   |   |
                                ||
                                ||
                                || <-- Metal Mast at earth ground
  Testing: Same as in Figure one.
  Figure three->
               R                                   D7
               |    |    |    |    |   |   |   |   |
               |    |    |    |    |   |   |   |   |
               |----|----|----|-[]-|---|---|---|---| <-- VHF Yagi
               |    |    |    | !! |   |   |   |   |
               |    |    |    | !! |   |   |   |   |
                                !!
                                !!
                                !!
                                !! <-- Non-conductive mast (PVC)
                                !!
                                !!
                                !!
                                !!
                                ||
                                || <-- Metal Mast at earth ground
 Testing: Same as in Figure one.
 Results of testing from test set-ups in figures one,two,and three:
  1) Figure one results:
  o Pattern is mis-shaped,forward lobe is divided and severely distorted.
    Side lobes are enhanced,back lobe is enhanced,but contains three
  distinctive lobes.
  o Gain over a vertical dipole is 2dBd at the lowest central point of the
   forward lobes,gain is 3.7dBd at the highest points of the forward lobes
   which are at 59 and 124 degrees respectfully from a 90 degree center lobe
   location. Best gain is to the back center lob at 177 degrees,5.3 dBd.
  2) Figure two results:
  o Pattern appears near normal with some distortion to the forward lobe.
  o Gain over a vertical dipole is 10.2dBd at the peak of the central forward
   lobe which is located at 93 degrees.
  3) Figure three results:
  o Pattern shows near typical to standard patterns with better shaping
   than both previous tests.
  o Gain over a vertical dipole is 10.9dBd at the peak of the central
   forward lobe which is located at 89.5 degrees.
 In each case,the same dipoles,coax,signal,and yagi were used for testing
purposes. The distance between the dipole and yagi was 100-ft.
 Height off the ground was 20-ft to the antenna feed.
 Signal was 10mW at 146 MHz CW.
 Clearly,this test shows that mounting your yagi mid-mast is not an effective
way to achieve the desired results of the yagi. Mounting the beam on top of
the mast achieves near commercial results while the insulated mast achieves
the best results for performance from the antenna.
 The discussion will continue,in part two,to elaborate on the
effects of figure two vs figure three with a grounding wire.
 This test was conducted at the Kennedy Space Center,Spring of 91 using the
KB4YLY gamma modification to the yagi and dipoles.
 All test equipment used was by Hewlett-Packard.
 These tests were incorporated into antennas and antenna systems at the
Kennedy Space Center and are presented here for public information.
-WS,KB4YLY/AFA2GN,Sr. EMC/RF Engineer,MDSSC,EMLab,KSC,FL
--- GoldED 2.42.G1219
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