In a message dated 4/18/2007 10:56:00 A.M. Pacific Daylight Time,
towertalk-request@contesting.com writes:
> I am planning to replace a cushcraft A3WS that has a failed trap this
spring. It is mounted 10 feet above a 50 foot 25g tower along with an A3S
just
above the tower top. As I got to thinking about it, since I would be doing
some antenna work, might as well replace the A3S with something without traps
and perhaps a bit more punch while there so I don't have to do another
replacement in the future. Well, that then led to the thought of maybe I
should go
up to 70 feet with an added set of top guys and the guy bracket in addition
to the two HBUTVRO brackets which are at about 10 and 30 feet to get a little
higher off the ground. The lower house brackets is bolted to a floor plate,
the other is through bolted to a wall with lumber on the inside spanning
several studs.
> So, the current thinking is a F12 WARC 7 at 8 - 10 feet over the tower
top with a Skyhawk just over the tower top at 70 feet. That's 13.4 sq ft of
wind load plus mast. Since the added set of guys is non-standard for a
bracketed tower, I see nothing in the Rohn book addressing it.
The guyed specs show a capacity of 16.2 sq.ft. @ 70 MPH with 2 sets of guys.
> Seems to me I would have the equivalent of a 70 foot 25g tower
"effectively" guyed in three spots with the two house brackets and a top set
of guys.
I am wondering if this would withstand the wind loads and twisting or I would
need to go with something bigger like 45g? I am in a 75mph wind zone from
what I can tell and the installation is surrounded on all sides by 60 to 70
foot trees that shelter it from the wind a bit.
The maximum distance between guy levels on 25G is around 32 feet. Your
proposed guys should be at the top of the tower (you didn't say which top
section you've got) and then around 30 feet below at the 40-foot level, that
would give you 10-feet between the bottom set of guys and the top
housebracket.
Below that the housebrackets are plenty to support the tower. The maximum
stress is at the top of the tower and the top set of guys so everything below
is
subject to less torque (practically none) so as far as I'm concerned, you're
good to go. You don't need the extra strength 45G.
> Secondarily, am I likely to see much improvement in performance for the
extra 20 feet of antenna height, particularly on 20m?
And I quote, "Higher is better". You'll see some improvement but it's a
small percentage of wavelength which is the key to take-off and arrival angles
so
it won't be real dramatic - good but not real dramatic. To see marked
improvement, you'd have to go to 100' which on 20M is a pretty good sweet-spot
which is hard to beat.
I'd recommend going with a big tribander or SteppIR. Many of them will
make as much difference as increasing the height. Check out our tribander
comparison report (_www.championradio.com_ (http://www.championradio.com) )
before
you spend your money. Then you can spend it on the right antenna.
Cheers & GL!
Steve K7LXC
Champion Radio Products
and
TOWER TECH -
Professional tower services for hams
Cell: 206-890-4188
************************************** See what's free at http://www.aol.com.
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