vhfuhf5.art

一个很小和天线计算程序

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 VHF 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 in 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 the 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 one, 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 
beam.  This side mounting configuration rotates with HF beam and
maintains a constant distance from the tower and HF beam.

       -------------------------------------------------------

           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