Bob,
the base of the tower needs to be ungrounded. The tower can be fed at the
base via a parallel LC or it can be fed at the junction of the tower and the
decoupling wires or tubes. The disadvantage of the elevated feed is bringing
the coax across the insulated tower base. But you have that issue anyway
with the cables going to the Yagis on top of the tower.
VSWR bandwidth suffers with short decoupling wires. Two 25' wires can be
resonated with about 75 uH. The 3:1 VSWR bandwidth is 50 kHz. For wires
would be better of course.
For those with a poor ground system this feed/decoupling scheme offers an
advantage. The tower current distribution is similar to a halfwave vertical
with maximum ground current occuring far from the tower base. Therefore the
quality of the near-field ground is much less important than for a grounded
90 degree monopole.
Dave WX7G
On Sun, Sep 27, 2009 at 8:58 AM, DAVID CUTHBERT <telegrapher9@gmail.com>wrote:
> Bob,
>
> 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@yahoo.com> wrote:
>
>> Hi
>> 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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>
>
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