Counter-balanced "Tower" Design
I have been looking at a number of options for a fairly light-weight,
free-standing (unguyed) tower (mast) with a counter-balanced tilt-over.
The antenna(s) I am considering are simple and light-weight, either
1) a 20 meter wire Moxon, or
2) a 20 meter - (or 5 band) Hex-beam.
The mast height would be between 25 and 30 feet.
My QTH is on a high bluff about salt-water. The mast base would be at
about 100 ft elev. The ground then slopes very gently to the edge of
the bluff, which then drops almost vertically about 80 ft to the beach.
The distance from mast base to bluff is about 60-70 ft. The antenna
would be facing salt-water from approximately 300 degrees True through
North to about 120 degrees True. The shortest distance across
salt-water to the nearest land (Whidbey Island) is about 18,500 ft; the
longest in excess of 75,000 feet.
Japan is about Heading 300; Europe 030; Caribbean/Brazil 120 degrees True.
Modelling I have done using HFTA and my terrain profile indicate the
following gains using a 2 element Yagi and an 8 degree take-off angle:
My QTH - 25 ft antenna elev = gain 11 dbi
Flat - 25 ft = 0.9 dbi
Flat - 35 ft = 3.5
Flat - 45 ft = 5.8
Flat - 55 ft = 7.3
Flat - 65 ft = 8.3
Flat - 75 ft = 9.2
Flat - 85 ft = 10.3
Flat - 95 ft = 10.8
Flat - 105 ft = 11.0
"Flat" means over flat terrain.
The higher antennas would only equal the 25 ft yagi height at much
higher take-off angles - typ 15-25 degrees, or MUCH higher antenna heights.
If I'm interpreting the data correctly, and if the model is accurate, I
am reaping the benefit of the bluff height without needing a higher
tower or mast. That's exclusive of any kick I get from the salt-water.
It would take a 105 ft high yagi over flat terrain to equal my 25 ft
yagi with an adjusted height of 125 ft (MSL.) (I am uncertain what the
lower physical height would mean; that is the actual near-field height,
which would be over average ground, versus the far-field height, which
appears to the antenna to be much higher...)
I am currently running a Steppir vertical on my metal roof house and it
is performing very well, and is apparently loving that salt-water far field.
For various reasons - esthetic and neighbors, and installation and
maintenance, I am looking at a low elevation, low visual impact antenna
with a tilt-over capability. Not a crank up. Not a "traditional"
looking lattice tower. I don't want to climb. I want to lay the tower
over for maintenance and wind (it can blow a lot here!) I have
considered the MA-40. I think it is over-kill for my situation. I also
don't like the tilt-over scheme - cable and winch.
So, I considered a simple tilt-over. I have a DX Engineering vertical
tilt-over. It tilts from the base. The combined antenna/mast/modest
rotator would weigh about 25-30 lbs. Plus the weight of the mast. I
could "walk" it up or down without too much trouble. But, it seems to
me a counter-balanced scheme would be superior.
I made up a simple drawing - you can find it here:
http://www.rbogash.com/Tower%20design.jpg
After I designed this, I noticed a similar installation in a near-by
park. There, they have a number of wooden flagpoles. I've measured
them as 5 inch butt diam, and 32.5 ft length. They are hinged at the 5
ft level. They don't use counter-balances. The supports are steel
U-channels. I already have several big ones in my garage. All manner
of sophisticated bearings, collars, bungees and shock absorber schemes
have crossed my mind (being a dumb retired engineer), but for the
purposes of this exercise, the above simple layout should be fine.
For the mast, I have considered wood poles like at the park, aluminum
section push-up masts, aluminum and fiberglass flagpoles, and steel
parking lot light standards.
Typical 25 - 30 ft flagpoles can be found here:
http://flagpoleco.com/25taonepifl.html
Light standards, 4 or 5 inches square, can be found here:
http://www.affordablelighting.com/25ft_light_pole_5x511.html
For weights, I would use a set of cement or cast-iron weights from a
body-building free weight set. I could adjust the weights any way I
wanted, right down to allowing the antenna to be walked up and down with
one finger. A likely counter-balance weight would be 125-175 lbs.
The mast would normally be locked in the vertical position and lowered
for maintenance and wind events. However, I have also contemplated the
option of leaving it unlocked and "floating." In the event of a wind
storm, with say sustained winds of 50 and gusts to 75, the antenna/mast
might float to an angle - say 20 degrees off vertical in the sustained
wind, and, in effect, become "unloaded." When a 75 mph gust struck, the
mast would then really be seeing only a 25 mph delta increment.
Any and all comments and critiques of the above scheme are solicited.
73s
Bob Bogash
W7DDD
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