Topband: Tower Shunt Feed

[K4SAV]

K3BZ wrote: "I want to put the feed caps in a plastic box at the 4' 
level so the shunt won't interfere with tilting the tower over.  The 
tower grounds and the radial system would connect at ground level to a 
flexible copper line running from ground level up to the feed box.  
Would that work OK at 160M?"


Placing the feedpoint 4 feet above ground presents some problems.  That 
4 feet between the feedpoint and the radials will still act as part of 
the vertical section of the antenna and it will have high currents.  
That also means there will be large RF currents on the coax shield 
unless a very good choke is used.  All other lines exiting the tower at 
this same point will also require chokes.  This problem is made much 
worse because the tower is very short and voltages and currents at the 
tower base will be very high.  Normally no chokes are required when the 
feedpoint is placed at the same level where the radials are attached and 
the shields of all lines exiting the tower are also connected at this 
point.  Another other option is to start the radials from 4 ft above 
ground and slope them to the ground, but that creates a mess around the 
tower.  Another option is to move the feedpoint to ground, route all 
cables and shunt wires to ground, and then remove all of this when 
laying the tower over.

If the SteppIR elements are isolated from the boom, I would recommend 
retracting them when feeding the tower.  I doubt there will be any 
problem with the stepper motors, but the control box is sensitive to 
small levels of RF and feeding the tower 4 feet above ground increases 
the chances of getting RF back into the control box.  You could 
disconnect the control box when feeding the tower.  Some people have 
been successful at feeding a tower with a SteppIR on top without any 
problems, others have not.  Power level and the exact implementation of 
all the wiring will probably make a difference.

If you have any inverted vees or other antennas hanging off the tower, 
those will also load the tower and may produce some unexpected results, 
depending on the chokes used on those other antennas.

Jerry, K4SAV
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