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Author Topic: Simple Dipole design - water and hurricane proof  (Read 1329 times)
EI4HQ
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Posts: 50


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« on: July 12, 2009, 02:23:43 AM »

Hi,

I'm currently finalising details for my new tower. I've a significant interest in utility listening and the 'icing on the cake' will be 4 RX only antennas right at very top of the tower, above the HF beam. The 4 antennas will be :

1. Marine VHF (156-162 MHz)
2. Marine AIS (circa 162 Mhz)
3. ADS-B (2 GHz)
4. Airband (118-136 MHz)

1,3 and 4 will be simple vertically polarised dipoles - gain is not an issue. 3 is already taken care of with a commercial grade antenna.

My question is this: does anyone have a proven 'bombproof' homebrew VHF dipole design that will remain watertight and survive 90 knot winds for extended periods?

These antennas will be quite inaccessible and I want a design that will last many years without maintenance...

All suggestions welcome...

73
Cormac, EI4HQ
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NA4IT
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Posts: 876


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« Reply #1 on: July 12, 2009, 04:59:23 AM »

I would take a look at some of the discone antennas. They are really designed more for wide band listening.

de NA4IT
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WB6BYU
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Posts: 13288




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« Reply #2 on: July 12, 2009, 08:39:48 AM »

The old ARRL Antenna Book had a design for a folded dipole
array for 2m that could be adapted to this.  It used angle
stock for the element, fed slightly off center for a
better impedance match.  The whole antenna including the
mounting stub was built from a single continuous length
of stock.  Heavy-duty commercial antennas use a similar
design, often with the radiator bent from round stock
and passing though holes in a mounting stub where it is
welded.

Commercial antennas that don't use a folded element usually
use large diameter material:  I have one that uses solid
1cm rod.  One approach I was considering is to get
aluminium tubing that fits into a "T" connector for
plastic water pipe.  The ends of the tubing can be sealed
with pipe caps and the coax attached inside the T fitting
to keep it out of the weather.  A similar structure
can be built as a coaxial dipole with the feedline
dropping out the bottom of the lower element, though
this does require a bit more work to mount as the
mounting post really should be inside the lower element
with the coax inside of it.


Overall, supporting the dipole in the center with the
element grounded probably allows for the most robust
construction.  The element can then be fed through a
gamma, T, Clemmons, or delta match, or as a folded
dipole.  The impedance of a folded dipole may be too
high (depending on the spacing from the tower) but it
can be brought down using a 4 : 1 balun or with a pair
of quarter wave coax cables as a parallel transmission
line, all of which can be put inside the support arm
for protection from the weather.  (My favorite approach
is to use a 4 : 1 half-wave coax balun to a delta match
made by wrapping insulated wire around the driven
element either side of the center to give a series
capacitance.  This doesn't work as well with large
diameter elements because there aren't enough wraps of
wire around it by the time I've reached the desired
capacitance.)
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AC4RD
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Posts: 1235




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« Reply #3 on: July 12, 2009, 11:10:32 AM »

I'm sure you'll find a good dipole design for your tower, OM ... but I can't resist commenting that NOTHING is "hurricane-proof."  Take a look at the history of the American cities of Biloxi and Galveston and you'll see what I mean.
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N4JTE
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Posts: 1156




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« Reply #4 on: July 12, 2009, 03:01:18 PM »

Have to agree about hurricane proof antennas, after surviving hurricane Andrew in Fl. where 30in. precast concrete poles were snapped in half by wind and reinforced concrete buildings were decimated, I figure the best hurricane antenna would be your mobile rig and battery power, ain't nothing else left!
Bob
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