[TowerTalk] tower height
Roderick M. Fitz-Randolph
w5hvv@aeneas.net
Mon, 28 Apr 1997 03:32:22 +0000
>What is considered optimum height above ground for a tribander? I know it
>is affected by surrounding terrain, but assuming flat level surroundings,
>and that the antenna will see the ground as ground, and not some rooftop,
>whats the best height?
>
> Is there any consensus on this, Ive heard 80 ft , 105 ft, and just plain
>the higher the better.
>
> It seems to me that there must be sone "node" height that has
>demonstrated itself as beeing the most desireable within practicle limits.
>
>
>Dave McPhie K7SX [doing alright with a 50 ft tower, but always dreaming]
_____________________________________________________________________________
Dave, I should know better than to jump in where angels fear to tread but
here goes. The best height depends somewhat on what you want to do, Dave.
For stateside contacts a relatively low height (say 40 feet to 60 feet)
might be best. If your prime goal is to contest or to work DX, I submit
the following. The information may be at odds with some of the veritable
plethora of information that is available from various magazines, books,
and internet sources. It is provided as in input to your prime computer
for digestion and consideration.
Some years back, I ran across an unclassified DoD document that showed a
chart for ANTENNA GAIN - DB BELOW OPTIMUM as the Y component on the chart
and HEIGHT as the X component on the chart. The curves themselves were
essentially parabolic in nature. There was a curve for ARRIVAL ANGLE
OF DOMINANT PROPAGATION MODE for 3, 5, 7.5, and 10 degrees.
The 3 degree arrival angle curve showed the optimum point was at 5 wavelengths.
The 5 degree arrival curve showed the optimum point was at 3 wavelengths.
The 7.5 degree curve showed optimum at 2 wavelengths height.
The 10 degree curve showed optimum at 1.5 wavelengths height.
(These optimum points were as close as I could determine from the chart).
>From these curves, it is relatively easy to extrapolate and estimate the
20 degree curve.
Applying the information from the ARRL Antenna Handbook in regards the
angle above and below which signals arrive 50% of the time (and the angle
below which signals arrive 99% of the time and the angle above which
signals arrive 99% of the time) to the frequencies of 7, 10, 14, 18, 21,
and 28 mHz, it becomes obvious from the curves that 93 feet is that height
(assuming perfect ground I suppose: the information I gathered does not state
that but I think it must be implicit) which affords the greatest advantage
of arrival angle of dominate propagation mode. This is true for 10 through
40 meters. The chart does not indicate the predominate arrival angle of
80 and 160 meters.
The exact height is somewhat forgiving in that the chart indicates that the
optimum height for 20 meters is approximately 1.4 wavelengths but shows
that for a height of 1.75 wavelengths, the dominate propagation mode
arrival angle suffers less than 1 dB loss from optimum.
Perhaps, this would explain why so many contesters and DXers have their
beams around 100 feet.
Chapter 23 of the 17th edition of the ARRL Antenna Book goes into great
detail (perhaps so much so that it tends to become confusing as to "THE"
optimum height) but is certainly worthy of some deep study, Dave).
I had a tribander at 132 feet and one at 66 feet. Only on a few instances
during DX chasing or contesting did the lower antenna beat out the upper
antenna. The upper antenna was at least equal to the lower antenna and
the vast majority of the time the upper antenna was anywhere from zero to
1 S-unit stronger on receive (and one has to assume that it probably was
so on transmit as well, witness the signal reports I got back when switching
back and forth). If I had to do it over again, Dave, I would put my
antennas at 90 feet. (That, incidentally, is the height of WD4NGB's 10
meter antenna and he has confirmed 210 countries as a Tech Plus on 10 meter
SSB!!)
I have this chart of curves printed out on a laserwriter and can copy them
on a xerox for anyone that is interested; please send a no. 10 business sized
SASE for a copy of the chart.
This information is offered to assist David McPhie in his search for the
best height for a tribander. It is not offered to incite flames. I am
too old and weary for those, now. If you feel anger at what I have put
forth here, go kick a tree or something.... Flames give me gas which
inevitably
results in my banishment to the yard and it is not warm enough outside for
that yet. If you have constructive comments, though, I welcome them.
Rod, W5HVV
w5hvv@aeneas.net
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com