TopBand: ground losses (long boring antenna theory discusion)

[Eric Gustafson]

Jim,

Sorry to have taken so long to get back to this.  I've been very
busy here at work and traveling much of the last month.

I don't have any hard measurement supported info for you on the
vertically polarized "magnetic field radiators" as differentiated
from "E-field radiators".  However, I have been doing some
modeling and a bit of (relatively informal) measuring and on air
testing to verify model results on minimizing the ground losses
associated with small antennas.  This includes resonant loops
which are less than 1/4 wavelength in circumference.

First, as radiators, the distinction between the types vanishes.
In the far field, they are both "both-field" radiators.  The
far-field radiation interaction with the ground surface is the
same for both kinds of antennas.  It is only in the near-field
where any distinction can be made.  So I don't really know what
to make of claims of "less low-angle loss" for one kind versus
the other over similarly poor uniform ground.  The losses which
are pattern or launch angle dependent are far-field effects which
are the same for both kinds of radiator.

However, over non-uniform ground, things can become a bit more
interesting.  By non-uniform I refer to conductivity improvement
which is local to the antenna but does not extend to infinity - a
local ground screen.  The reason that this is more interesting is
that differences in the types of antenna can be exploited to
inimize the area of local ground which must be improved to
preserve the effeciency of the system.

I will try not to bore everyone with a lot of detail.  So here is
the bottom line (which - trust me - is boring enough...) as I
interperet my findings so far.

1.  I am not convinced at this point that the good behaviour of
    properly constructed small loops is due to the fact that the
    near-field energy storage is primarily in the magnetic field.
    While it is true that this external field is mostly magnetic,
    there is also plenty of electric field involved in local
    energy storage.  It is just that the non-radiating E-field is
    mostly concentrated between the plates of the resonating
    capacitor where lossy material is not permitted access.

    However, the _size_ of the local external energy storage
    field for these antennas is quite small.  It is much smaller
    than predicted by the rule of thumb formula for predicting
    the size of the near-field zone shown in most antenna
    reference works.  For example, an 80 meter loop large enough
    to remain more than 80% effecient has a near-field region
    that extends only to a little more than 20 feet from the
    center of the antenna.  The small size of the near-field zone
    permits real effeciency gains to be realized with a
    relatively small area of improved ground conditions.  A 30
    foot radius ground screen is contemplatable for even the
    "average" ham.

2.  While shrinking the size of a linear vertical also reduces
    the size of its near-field zone, by the time the near-field
    zone is as small as a full efficiency loop's, the linear
    vertical is a very high Q structure with very low radiation
    resistance.  Extreme measures would be required to match it
    and to prevent radiator conductor loss resistance from
    swamping radiation resistance.  If radiation effeciency was
    restored to above 80%, the bandwidth would be extremely
    narrow (smaller than a SSB SIGNAL).

    However, it is worth considering _some_ reduction in size for
    a linear vertical in order to minimize the ground area that
    must be improved.  It is well known that smaller verticals
    require less ground screen area than full size versions to
    function to their capabilities.  I believe a reasonable
    compromise can be reached which preserves most of the system
    radiation effeciency while permitting the near-field losses
    to be controlled with a fairly limited ground screen.  This
    is probably at least part of the reason for the success of
    the battle creek special and similar compact verticals when
    operated over a fair number of relatively short radials.  The
    far-field pattern, however, will be whatever is provided by
    the ground conditions outside the limited ground screen
    radius.

    The squashed full wave loops (MU type) are really two short
    verticals fed in phase.  Its likely that a real improvement
    for these antennas could be had by placing a relatively small
    screen under each vertical element.

3.  Now, the _really_ good news.  The required ground screen
    radius to affect an improvement in the far-field pattern (and
    losses) is directly proportional to the height of the
    antenna's phase center above ground.  The lower the phase
    center, the less ground area must be improved.  The small
    loop's phase center is at the center of the loop.  For the
    previously mentioned 80 meter loop, that is only about 10
    feet up when the loop clears the ground by a foot.  A phase
    center this low requires only a 40 to 50 foot radus screen to
    significantly enhance the far-field pattern below 25 degrees
    (down to the 5 to 10 degree area) elevation.  When I get time
    to work on it, I will be building such a loop over a 50 foot
    radius screen.  I'll be on here crowing loudly if it works as
    good as it _should_.  The work I've done up to this point was
    with a one meter diameter loop on 10 and 30 meters.  This
    work so far indicates that the recent versions of modeling
    software can produce meaningful results for these small loops
    near earth.

73,  Eric  N7CL



>Date: Thu, 14 Aug 1997 09:22:26 -0600
>From: Jim Henderson <jhenders@tdrss.wsc.nasa.gov>
>
>Esteemed Reflectees:
>
>        Reducing losses in the ground system or counterpoise
under a vertical is one important consideration, and one most of
>us can address.  Reducing the power-robbing effects on low-angle
>radiation by rotten ground onditions a little farther away is
>another, and is something most of us cannot remedy as
>easily. Has anyone seen any hard numbers or other good data
regarding comparative ground losses between vertically polarized
>electric field radiators versus vertically polarized magnetic
>field radiators?
>
>        I have seen and heard much anecdotal evidence there is
>less low-angle loss from the magnetic radiator over poor ground
>than the lectric radiator, and my own experience tends to
>support this. But I have >never seen anything in Amateur circles
>resembling a real test. Work on the ag-U, DMS, and similar
>types, has been published in the ARRL Antenna compendiums and
>other sources, but does anyone know of any scientific studies,
>IEEE, or other work that would be applicable to Amateur designs?
>
>        Considering the current thread of verticals and their
>ground losses, I would think any practical antenna alternatives
>that might decrease the low-angle losses would be of special
>interest to Topband Reflectees just now, while there is still
>some prime antenna-building weather left.
>
>73 de Jim, KF7E
>


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