TopBand: Top loaded vertical or Tee for 160

[George Guerin]

     To K9UWA, ON4UN and Other Topbanders:
     
     In the past there have been discussions on the effectiveness of top 
     loading a vertical element with horizontal wires.  
     
     One school of thought says the effect of one or two horizontal wires 
     is the same, so that an inverted L or a Tee antenna has the same 
     loading wire length from attachment at the top of vertical element to 
     end of top loading wire(s), and to shorten the top loading, three or 
     more wires are needed at the top of the vertical element.
     
     Another school of thought says using two wires provides more top 
     loading, so each side should be about 60 - 70 % as long as a single 
     inverted L horizontal top wire, and adding more top loading wires 
     will simulate a solid disc, which may make the length of each top 
     loading wire about 50% of a single wire length from an inverted L.
     
     Over this past weekend W8UVZ, W0CD and I did some work on a top 
     loaded wire vertical element as part of a multi-element array for 
     160 meters.  We thought we should share the dimensional information 
     for future reference.
     
     The Tee element is part of a cantenary wire from the top of a 126.5 
     foot tall tower to a ground anchor 247.5 feet away.  The feed point 
     for the wire is 67.5 feet from the tower.  The tower is insulated 
     at the base, and is guyed with Philystran (r) to avoid extra metal 
     in the near field.
     
     We started with a 78 foot vertical wire connected to the center of  
     a 96 foot long Tee top loading wire.  Both are made of 3/16 
     galvanized EHS guy wire.  The approximate resonant frequency was 
     1465 Khz, with a feed point impedance of about 26 ohms.  
     
     After twice trimming the top loading wire of this Tee element we 
     ended up with the same vertical wire, 78 feet high, and the Tee top 
     wire 67 feet long.  Since it is part of a cable from the top of a 
     tower to a ground anchor, it is tilted about 33 degrees from 
     horizontal.  
     
     The ground system is 120 radials as much as 210 feet long on the 
     outside perimeter of the array under construction.  The interior 
     radials all tie to buss wires and radials from future element feed 
     points.
     
     The resonant frequency is 1811 khz with Z ant = 30.8 + 0j ohms.
     
     Note: the Tee top length is about two feet longer than a computer 
     model that seems to agree with the second school mentioned above. 
     
     Incidentally, the tower fed against ground comes in at 1811 or 1812 
     Khz, with base insulators one foot above ground, and 125.5 feet actual 
     height of Rohn 25G from feed point to top of the plate on the top 
     section.  
     
     Hope this is of interest.
     
     George, K8GG for W8UVZ and W0CD.
     
     PS, my home modem took a hit, so I am sending this from the office.
     


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