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 two).
TOWERS AND THEIR EFFECT ON VHF BEAMS:
 Towers have a dramatic effect on VHF beam patterns. Many hams mount their 
HF
beams on separate masts or towers next to their main tower.
 Many hams also mount their VHF beams on arms extending from the tower.
 The tower poses an eclipsing effect on the VHF beam's pattern.
 The pattern is attenuated sharply by the tower,especially when the beam is
pointed at the tower. This is fairly obvious. The attenuation is greater the
closer the beam is placed to the tower.
 This does not cover end mounted beams which actually use the tower or mast 
n
their pattern. When a VHF beam is placed to the side of a tower,the area of
the beam's forward lobe must be taken into consideration.
   Figure one:
                           p                 p
                            p               p
                             p             p
                              p     T     p
                               p  T   T  p
                                p       p
                                 p     p
                                  p   p
                                    X
  In this example,the T's represent the tower,the X represents the beam,and
the p's represent the edges of the forward lobe where the left p's are the
right side and the right p's are the left side of the lobe.
 The area between the edges,including the tower,is the high attenuation area.
 The angle formed by these edges is 60 degrees for this example.
   Figure two:
                          p
                           p
                            p
                             p     T
                              p  T   T
                               P
                                p
                                 p
                                \ p
              p p p p p p p p p p pX
                                     \
 In this figure,the designators are the same as figure one.
 The beam's forward lobe is 60 degrees at the 3 dB points for this example.
 The minimum angle of the beam antenna,to directly pointing to the tower,is
the same as the 3 dB point beam width or 60 degrees.
 This means that the beam has a 120 degree angle,60 degrees either side of 
he
tower in relation to the beam,that is an attenuation or eclipse zone.
 It can be seen that the farther away the beam is from the tower the smaller
the angle of attenuation is in relation to the beam. The ascii representation
needs much improvement,but the point can be seen.
------------------------------------------------------
VHF BEAMS ABOVE AND BELOW HORIZONTAL HF BEAMS:
 Normally,you should place the VHF beam approximately one wave length above
any HF horizontal with excellent results. Using the information from part 
ne,
best signal propagation can be obtained.
 Mounting the VHF beam below the HF horizontal beam can present problems
relating to masting attenuation as stated in part one. A solution can be had
though. By mounting the VHF beam offset to the mast and counter balancing the
arm with another VHF antenna or weight,you can mount the VHF antenna(s) below
the HF beam.
  Figure three:
       -----------------------[]-------------------- HF beam
                              ||
                              ||
                              || mast
                              ||
                _______       ||          |
                   o----------[]---------o|  VHF beams on cross boom
                              ||          |
                              ||
                             ||||
                             |||| tower
 This is a head on view of the configuration discussed above.
 The only difficulties observed was the top heavyness of the HF beam on the
extended mast (not recommended for high wind areas) and some difficulty on
obtaining ducting for 2m/1.25m SSB operation.
 This is dependent on the proximity of the Horizontal VHF beam to the HF 
eam.
 This side mounting configuration rotates with HF beam and maintains a
constant distance from the tower and HF beam.
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METAL GUYS AND THEIR EFFECT ON VHF BEAMS:
 There is some effect on VHF beams and their proximity to metal guys.
 Most of the efect takes place when the beam is mounted to the side of the
tower enveloped by metal guys. The effect is not significant unless the beam
is close the guys.
 If the VHF beam is on a rotor,make sure you have proper clearance for good
rotation. Having the last element snag on the guy will send you back up the
tower for adjustments. This simple step is often overlooked when installing a
VHF beam in this way.
 Metal guys can be a serious disaster if they become entangled on the
rotatable VHF beam. Take it from experience.
-WS
--- GoldED 2.42.G1219
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