[Lowfer] flat top

[John Davis]

>Those of you operating flat tops:  how are you figuring the flat top into
>the part 15 regulations?  The ones I've seen would seem to exceed the
>limits.  I've seen some discussion on verticals and loops, but what
>about flat top verticals?


Same principle as any top loading, really.  If length is taken, in the
simplest definition, as the measure from one end of a thing (the attachment
to the transmitter terminal, for instance) to its farthest point (the far
end of the tee section), then you have the same situation for a flat top/tee
as you do for a top hat or inverted-L or any other top-loaded vertical.

The idea is to maximize capacitance from the top of the antenna to the
ground system, with the objective being to make the current in all parts of
the vertical section as large and as uniform as possible, thus increasing
the effective height of the antenna from no more than half of the actual
height (with no loading) to nearly its full physical height (with maximum
top loading).

With a typical LowFER top hat, we try to maximize capacitance with an
adequate number of radials of sufficient length all around, perhaps wired
together spiderweb style.  The fact that we can physically arrange the
tophat 360 degrees around the top of the vertical section maximizes its
capacitance to ground, given the comparatively small overall dimensions we
are allowed.

Most NDBs, on the other hand, have no such length limitation.  Some smaller
ones do use tophats.  But as the size of the antenna system increases, it's
no longer physically practical to do radials in a disk-like arrangement at
the top.  Instead, it's easier to stretch out the top loading into a flat
top, forming a tee.  This can be scaled up to a substantial size, using
nothing more exotic for support than a couple of power poles.

The dimensions of a typical tee at an NDB, therefore, far exceed Part 15
limits, but are entirely within reason for ham operation in the pending
136kHz band.

73,
John

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