Topband: More on 160 dipoles

[John B. Mitchell]

Bill, et al,

Interesting thread.  My experience still tells me that fairly high dipoles
or inverted vees are within a few db of single-element verticals over nearly
all openings, all conditions.  I've never, repeat never, seen a situation
that would remotely support a 20+db difference due to antenna polarization!
This includes both signals I've sent and those received, in comparison with
others in nearby areas, etc. etc.  Big differences CAN exist in path
conditions, for example, after trying to work ZS6 a few years ago, and
hearing them fairly weakly on the E. bev, calling unproductively, etc., a
couple of nights later they were suddenly an honest 589.  So was I to them!
I think everyone has experienced a 20 db difference in the path Top Band
provides two stations separated only by a couple of hundred miles.  But,
averaged over a season or two of very diverse conditions, one gets a very
good feel for the performance of one's antennas vs. other top band stations.
Contests, as you know, also wring out things pretty well.
I think there are several things working as far as understanding 160 meter
antenna effectiveness.  The first, of course, is efficiency.  With verticals
this means length of radiator plus percentage of near field coverage (radial
field) plus fresnel zone effects.  With horizontals this mainly means height
above ground (and surrounding terrain also becomes much more of a factor
since the reflective wave more often adds to the total signal); the dipole
design is self-contained, of course, and doesn't rely on a potentially lossy
radial field.  So, it is easier to build an efficient radiator in the
horizontal dimension, it would seem.  Verticals take more effort, but can be
made very efficient, as well.  Near field and fresnel zone losses are more
serious for verticals, at least when the horizontal is high enough to escape
the worst ground losses.
The question then becomes how high should a dipole/vee be on 160 to work
effectively?  I'm constantly amazed at the signals I get from Eu stations
running dipoles at 75-100 feet.  I can say with some certainty, that on
balance, they are always within an S unit or two of verticals and even
vertical arrays, of the biggest stations on that continent.  Rarely do I see
more than 2 S units of difference between stations from Europe when
conditions are peaking for their region.  That is, when the band is open to
Belgium (a relatively distinct and compact geographical region) the ON's are
all within an S unit or two of each other.  Doesn't matter whether it's a
big vertical or an inverted vee at 90 feet.  This has been borne out again
and again.  I conclude from all my observations that a dipole/vee at 100
feet gives up little to a vertical.  Put the dipole at about 3/8 to 1/2 WL
above ground (180-250 feet) and it should be even better. (I don't have
enough tower to experiment.)
I also conclude that, for practically all paths, takeoff angles below 20-25
degrees on Top Band do not help much.  I'm sure there are rare occasions
when stations near sea water gain DX advantages by packing more energy into
very low angles, but am equally convinced that most of the time the useful
angle stays between 25 and 45 degrees, even for long DX, and is probably
nearer the top end of that range more times than many would imagine.  A
dipole at 100 feet, over alluvial ground, is only down about 3-5 db at 40
degrees, if I'm reading the charts correctly.  My 100' inverted vee, over
sloping terrain, probably was only down that much as low as 35 degrees in
favored
directions.  This means that for take-off angles in the 40 degree region,
dipoles and verts should be close in performance.  I really believe that
this is what I am seeing with regard to Top Band propagation.  Most of the
time the useful angles, because of absorption at lower angles, are high
enough to equalize the playing field between verticals and horizontals at
reasonable heights (100' range)
Since, for many people, it's easier to get a horizontal at, say 125 feet,
than to put up an efficient vertical with extensive radials at the same
height, they may find that they achieve comparable results with the simpler
antenna.  Having experimented with my vertical for a season now, I'm
probably going to try to get the old inverted vee a bit higher and
experiment some with that next season.  I may try to retain the vertical but
am concerned about interaction effects spoiling the test, but I would like
to be able to run them both.
In conclusion, if we discard the notion that very low angles of take-off are
needed for Top Band, and concentrate upon putting as much energy as possible
into useful angles, say between 30 and 45 degrees for DX and somewhat higher
for North America, then it doesn't matter too much how we get that energy to
"couple" into the electromagnetic field of the planet.  Verticals pack more
energy into lower angles, sacrificing the higher angles needed for regional
communications.  Horizontals pack more energy into higher angles,
sacrificing the lowest angles needed for occasional DX paths.  But both
antennas put significant energy into the truly most useful range of angles
needed for most DX on Top Band, which is probably centered somewhere around
40 degrees for most conditions.  If it's easier to
build verticals, then build them, but guys like me in the mountains may do
as well with our dipoles.

----- Original Message -----
From: "Bill Tippett" <btippett@alum.mit.edu>
To: <topband@contesting.com>
Sent: Thursday, May 09, 2002 9:39 PM
Subject: Re: Topband: More on 160 dipoles


>         Thanks to Bob for that great info.  This may answer a question
> I've wondered about for a long time which is "Why do high dipoles
> work well on 80 but not on 160?"
>