Topband: 160 Meter Antenna Help

[k3ky@erols.com]

These lowband transmitting antenna discussions sometimes
give me a headache. Is it possible that two different (but related)
subjects are being intermixed here? One seems to be antenna
efficiency in relation to nearby ground _losses_ (especially for
verticals 'ground driven' with current max at ground level). The
other seems to be the _fresnel (minimum) angle_ for vertically
polarized radiation, as determined by ground quality out well away
from the antenna. Over this, we have no control other than the
choice of  where we live.

I believe matters get complicated because many (or most) low
band transmitting antennas radiate a combination of vertical and
horizontally polarized energy- I am talking about _practical_
antennas here. With the exception of classical verticals, including
loaded types, most antennas should have both polarity components
to some degree. This should even be true of a classical wire
dipole, due to sagging of the wire from its own weight and the
weight of the feedline (to at least some small degree).
 
There is enough anecdotal evidence to strongly support the notion
that putting a large, local ground screen under any practical low
band transmitting antenna usually helps efficiency and thus
field strength at a distance- whether or not that antenna happens
to be a 'pure' vertical fed against ground. Think for a moment about
one major undercurrent in this thread, however- the idea of the "T"
antenna. Here we are seeking to balance the current in any
horizontally oriented conductor so that  _in the far field_  (at great
distance), those horizontally polarized components cancel. This
is the same general idea as used in the bobtail family of IGP
(inverted ground plane) antennas. Whether it is a 1, 2 or 3 element
bobtail (1/2 wave T, half square, or bobtail) we are also doing
the same thing- deploying lengths of horizontal wire along with the
vertical element(s) such that there is cancellation of any horizontal
component. I think the important thing to realize is that the
current max need not necessarily be in the middle of the vertical
element- it can also be at the top or the bottom. We can allow
plenty of current in a horizontal conductor, just so long as it
cancels in the far field. Then we have the maximum possible vertical
component, which is what we are after. Beyond that, we run into
the fresnel limit on our minimum vertical angle of radiation, and we
have no control there (except to move to a better location) because
the fresnel zone is out well away from the antenna- tens or
hundreds of feet, or even miles.

I think it all might depend on whether or not our "flattop" is really
flat and/or symmetrical. In the case of a T flat top, a current
max right at the vertical/horizontal point of connection could be
a _good_ thing, I believe- due to cancellation plus the benefits of
moving the current max up away from lossy earth. Look into 1/4
wave IGPs to better understand this benefit (but how many hams
are able to put up a quarter wave vertical for 160m?)
http://www.angelfire.com/md/k3ky/page49.html

With an inverted L, we have an entirely different situation.
In that case, it makes sense to keep the current max down
in the vertical section. As always, it makes sense for low
band DXing to strive to maximize the vertical component as our
primary aim. If we can move the current max up away from
ground as a secondary goal, so much the better, as this should
reduce ground losses under the antenna. There are a lot of 
inverted L antennas on top band that work very well. I suspect
the best performing ones do indeed have the current max down
in the vertical section, plus a very good radial field or large
area ground screen.
 
It can help to draw out a diagram of a proposed antenna with
arrows showing current direction in each half wave section. For
classical IGP antennas it is very easy to visualize cancellation of
the horizontal component and any reinforcement of the vertical
component from multilpe vertical conductors. This would not be so
easy with many odd fractional wavelength wires, sloping wires,
and the like, however. I don't know whether antenna modeling 
programs provide this simple information or not. I believe they do
not. Plots of azimuth and elevation patterns are quite informative
but do not, I believe, directly give us this critical detail. You can
only infer it from the angles of the vertical lobe plots.

BTW In case anybody wants to argue that maximizing vertical
polarization is not important, Tom has pointed out that his vertical
beats his very high horizontal dipole on top band most of the
time. I believe his dipole is up 300 feet (?) 

I understand that inverted vees generally work DX better than
dipoles on the lowest two bands, 80 and 160m, for any practical
antenna height. It is the vertical component that does the work,
while the horizontal component is wasted on high angle 'cloud
warming'. The same should hold true for a V-style "T" atop a
vertical or verticals with V- or T-shaped capacitance hats, etc.,
so long as there is reasonable balance and symmetry and we
achieve maximum possible cancellation of any horizontal 
component.

My own experience on 80 meters: an inverted vee at 70 feet
worked well, and when I raised it to 110 feet, it worked even
better. When I built a 1/4 wave wire vertical with 55 random
length ground radials, I found that it was usually slightly better
than the 110ft vee in many direct comparisons. The greater the
distance, the bigger the difference.  73, David K3KY



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