Topband: Monopole Radiation Patterns, takeoff angles etc

[Guy Olinger K2AV]

Hi, Rich.

I guess one could ask why bother with this mid-field analysis.  It IS
TECHNICALLY TRUE what you say, no argument, but of little use since you
don't get to keep it, UNLESS you can get it over salt water, or off a
mountain top.  The far field code in the various programs is adding up the
gains AND THE LOSSES and presenting you with the sum, rather than giving
you the loss in one presentation, the gain in another and making YOU add
them up in your head.  For myself I will stick with the convenience of
letting the program do the subtracting.  You are talking about salary
before taxes.  I can only spend take-home pay, and I can only make QSO's
with the take-home pattern.  I don't see anything wrong with using the
take-home takeoff angle as the item of conversation -- it's the one you get
to use.

73, Guy.

On Sun, May 6, 2012 at 8:44 AM, Richard Fry <rfry at adams.net> wrote:

> All vertical monopoles of 5/8-wavelength __and less__ radiate (launch)
> their
> maximum relative field (E/Emax) in the horizontal plane.  This is true no
> matter what the loss in the r-f ground connection they use.
>
> A lossy ground connection will reduce the gain of the antenna system, but
> it
> will not change the relative fields they radiate.  IOW, their pattern
> shapes
> remain the same regardless of the loss in the ground connection, be that to
> salt water, or dry sand.
>
> The link below leads to a plot of the radiation patterns and directivities
> of
> several monopoles.  These are the shapes of the radiation patterns leaving
> the monopole as they exist at the beginning of the far field of the
> radiator.
>
> These patterns were calculated for two ohms of loss in the r-f ground
> connection - which is about the loss that 120 x 1/4-wave buried radials
> provides even in poor soil.  If fewer/shorter radials are used, then loss
> increases and the directivities (gains) of these patterns would be
> reduced -- but the radiation pattern shapes would remain the same.
>
> Many amateur radio operators consider only the far-field pattern of a
> monopole antenna as shown by NEC and in textbooks, without realizing that
> this is not the shape of the radiation leaving the monopole.  It leads to
> the concept of a "takeoff angle" where radiation apparently was maximum
> from that monopole.
>
> However the elevation field radiated by a monopole always is maximum in the
> horizontal plane, and always is less than that at the elevation of an
> assumed takeoff angle.  A NEC analysis including the surface wave from the
> monopole will show this.
>
> Some of that low-angle radiation can reach the ionosphere and produce
> skywave service, even though according to a NEC far-field analysis, the
> fields are approaching zero at those low angles.
>
> This doesn't mean that radiation at and near the "takeoff angle" does not
> provide significant skywave service, but it does mean that significant
> skywave service can be generated by radiation at much lower angles than
> commonly believed.
>
> http://i62.photobucket.com/albums/h85/rfry-100/MWElPatComparison.jpg
>
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