Topband: Top Loading of Shunt-fed Tower

[Ford Peterson]

Jerry wrote:

> I'm considering shunt feeding of a 45 foot freestanding tower with a 3L SteppIR on top for 160 and 
> 80. The SteppIR is NOT electrically connected; the tower IS grounded,,, each leg has a 32 feet of #4 
> solid copper with ground rods at 16 and 32 feet.  The possibility of using the SteppIR as a tunable 
> cap is interesting... has anyone tried this?  If so, have you run full power thru it?  Did you use a 
> relay to connect the SteppIR?
> 73,  Jerry K3BZ 

Jerry,

I use a 3 el SteppIR at 45' on a Rohn 25G tower.  The tower is also loaded for 160m using a top hat wire of about 125'.  A second shunt, loads the system on 80m using a 25' top hat wire.  The two hat wires are switched with a vacuum relay at the thrust bearing of the tower.

I tuned the 160m with the elements set at 20m on the SteppIR.  This resonates the system at 50 ohms at about 1825 or so.  By moving the steppIR to 17m, the sweet spot moves up to about 1840 or so.  Moving the SteppIR to 15m pushes the sweet spot to about 1860.  No further tuning is noticed by retracting the elements.  No tuning is noticed on 80m whatsoever--I have no clue as to why.  Without tuning the SteppIR, the system Q is pretty high.  Tuning the hat allows me to run without the auto tuner over about 1815 to 1870 or so.

I can only run about 800w or so with my one holer (SB1000) but have not noticed any sort of weird behavior.  The SteppIR is unmodified, so the elements float electrically over the boom.  I suspect the only loading is happening with the driven element, due to coupling of the feedline running up the inside of the tower.  All tower feedlines run through a well-grounded bulkhead mounted at the base of the tower.  I should check to see which elements are actually acting as the hat by retracting the reflector and director.  I'm only speculating at this point.

Many have posted grave warnings about doing what I do, indicating that degradation of the housings 'could' result.  I run QRO so seldom, that I cannot imagine how the 2" separation of the elements and the boom can degrade the air and fiberglass, which is what is used to insulate the elements from the boom.  I figure that if it takes a dump on me, I'll just replace the defective pieces some day.  As Joe noted, the elements can be electrically connected to the boom without degrading performance (in theory).  I have attempted to model this antenna, and noted that when other elements are present in the system, and interaction is present, the floating elements seem to be more forgiving of the situation than if they are electrically connected to the boom.

Good luck.

Ford-N0FP
ford at cmgate.com