SUBJECT:  Vertical Beams on VHF and the effect of metal masts
          on propagation patterns, gain, and bandwidth (part one).


       I have 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 89, 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 metal guys encompassing the beams.

       In part one of this series, the discussion will cover the first 
three studies.

*** 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