Topband: Tophat? Does it have to be at the top?

[Ford Peterson]

My 42' tower and SteppIR 3 element beam at 45' are shunt fed at the thrust bearing.  A single 120' horizontal loading wire is also attached to the thrust bearing plate.  

Very clearly, tuning the elements of the SteppIR effectively changes the feedpoint impedance of the tower on 160M.  Even though the SteppIR elements are electrically 'floating' above the boom, changing their length does change the feedpoint.  The 20M position on the SteppIR places the 'sweet spot' at about 1825.  The 15M position places the sweet spot at about 1860.  Retracting the elements completely is not useful as the 'effect' of the elements is nil once retracted to the 10M position or so.

While the system is far from ideal for a number of reasons, it does work.  And the configuration illustrates that the 'loading' can take place in spots other than the top of the vertical, which in my case is an inverted L arrangement.  It is apparent to me that I do not fully understand the workings of the system--discovered using serendipitous methods.  A similar stunt is performed on 80M using a separate shunt (tap at 33' instead of 42') and a horizontal hat wire of only 20', but the position of the SteppIR elements has no effect on feedpoint impedance on 80M.  Go figure...

Your mileage WILL vary...

Ford-N0FP
ford at cmgate.com