Topband: electrically shortening a tall grounded tower
w2xj at nyc.rr.com
Sun Sep 27 08:05:51 PDT 2009
1/4 wave stubs do work. They are frequently used in broadcast work. The
problem is with the modeling. You really need a professional (expesnive)
program preferrably employing NEC 4. This is one of the things the the
ham friendly programs do not do well (or at all) in many cases.
DAVID CUTHBERT wrote:
>rather than using a series hi-Z (trap) to decouple the upper portion of the
>tower a low-Z shunt element can be used.
>At the 130' point a loaded 'radial' sysatem of sorts is attached. An example
>is two 25' aluminum tubes extending horizontally and 180 degrees apart. A
>loading coil is used to resonate them. The tower is now fed at the junction
>between the loaded 'radials' and the tower. The current distribution on
>the tower portion from GND to the 130' point is reversed; that is, the
>current is maximum at the elevated feedpoint and minimum at GND.
> Dave WX7G
>On Fri, Sep 25, 2009 at 9:50 PM, Bob Kupps <n6bk at yahoo.com> wrote:
>>Is there any effective way to electrically shorten a tall grounded tower to
>>1/4 wave? I tried modeling a 1/4 wave cage surrounding the tower but it
>>still shows a lot of current in the protruding section with resulting high
>>angle radiation. Then I tried modeling a 1/4 wave stub above the cage
>>grounded at the top to the central tower which appeared to turn it into a
>>dipole and still significant current above the stub. I was also thinking of
>>using an external parallel resonant trap/choke but would this actually work
>>when situated at a zero current point at the far end of a 1/4 wave monopole?
>>Thanks and 73 Bob N6BK/HS0ZIA
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