Message: 2
Date: Wed, 27 Jan 2010 05:51:06 -0500
From: Pete Smith <n4zr@contesting.com>
Subject: Re: [TowerTalk] Antenna spacing question
To: "Tower and HF antenna construction topics."
<towertalk@contesting.com>
Message-ID: <4B601A9A.2040305@contesting.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Isn't schedule 80 water pipe? And if so, aren't you inviting a bent
mast by trying to put a 2-element 40m yagi 10 or more feet above the
tower top?
73, Pete N4ZR
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## Not necessarily. The HDX-689 crank up tower uses sched 80 for the legs
of ALL 5 x sections. It's all 46 ksi steel. I agree though, his is
probably
sched 80 water pipe,,.. and probably 1.9" OD x 1.5" ID x .200" wall ...
which is typ just 30-35 ksi. The cheap and dirty way to test a mast is to
clamp it to your deck.. with XXX feet sticking out in mid air, horizontally.
Then place XXX lbs of weight on the very end.. and measure the deflection.
If the yagi is say 7 sq feet... and a 70 mph wind....[20 psf].... = 140 lbs.
7 sq feet and a 100 mph wind..[40 psf]... = 280 lbs etc. Add a safety
factor
I once had a 204BA mounted 11' above the same tower as his....
[ with one straight section added in the middle, and also guyed].... on the
same
1.9" OD sched 80 pipe [1976] It also had climbing steps welded to it !
It took 65 mph winds.... but I would never do it again. Climbing that mast
freaked me out. If he used the bigger 2.275" OD pipe in sched 80... or
even the 2.875" sched 80 pipe, [ .276" wall] he would be ok,
depending on local wind condx. His 40m yagi may well screw up his
15m array.
I added several more #5 straight sections of Delhi.. [ right to the 72' top
of tower]
and 12 x guys wires [ 4 x per level x 3 x levels] and then 2 x HB
monobanders...
+ a prop pitch. 15m-26' boom 20m-36' boom. Prop pitch was way down inside
the tower.
The top 2 x sections had a slight twist in em, after 68 mph gusts! Delhi's
are very
limited as to what you can do with them. I gave up on the delhi.. and went to
30" wide
commercial L+R tower [ 350 lbs per 20' section] ... and never looked back.
The Delhi's were originally made for deep fringe TV reception. [late 50's]
One large yagi, just above the top bearing.. and perhaps some smaller stuff...
4-6'
above that.... and it's maxed out. We had a local fellow here, who overloaded
a 64' delhi [ no guys] and it folded right in half, during a windstorm !
The 64' Delhi is rated for 3 sq feet.... NOT 10-15 sq ft.
later... Jim VE7RF
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