The biggest issue is detuning of the 20m Yagi by the 80 dipole. This can be
greatly reduced by opening the feedpoint on the 80 when operating on 20. This
is easily done with one of the relays in the 80 band-segment relay box.
Ed - W0YK
Sent from my BlackBerry
-----Original Message-----
From: "Jim Thomson" <Jim.thom@telus.net>
Date: Fri, 8 Jan 2010 01:22:26
To: <towertalk@contesting.com>
Subject: [TowerTalk] 340N + mag 520...how close can you get them ?? / Al mast
gents
F-12 claims you can install another HF yagi, or tribander, etc, including
15m.....within 1 meter [ 39.37"] of the F-12 340N.
Is this true? Seems like very short spacing to me. I'd like to install
the F-12 EF-180B [ 80m rotary dipole] at the top of the mast
then the F-12 mag 520 below it.... then the F-12 340N at the bottom of the
mast. I don't want to use any more mast than required to do the job.
The proposed mast is 2.875" OD 6061-T6 Aluminum [sched 80].. which is
.276" wall thickness. I worked out the section modulus
1.33622 x 40 ksi yield strength = 53,448 inch lbs..... [or 4454 ft lbs] ...
which is plenty strong enough. I have no clue what the actual deflection
will be towards the top of the mast.. in say a 70 mph wind.
Another option was to use 2.875" OD 6061-T6 al mast... [sched 40].. then
stuff some 2.375" sched 80 inside of it. Total wall thickness in that
case is
aprx .405" .. and section modulus is increased to aprx 1.70541 1.70541
x 40 ksi = 71,016 inch lbs [ or 5918 ft lbs] Again ,. I have no clue what
the
actual deflection is, in a 70 mph wind.
I'd like to use something like a 10.5' mast [ out the top of the tower]...
with the 340N, just 1' above the bearing.... then the 20m yagi.. 4' above the
40m,
then the 80m dipole 3' above the 20m yagi. LL truss bar would be another
2.5' above the 80m dipole. = 10.5'
On paper, none of this should interact. Can one realistically install a 520
just 4' above the 40m yagi.. with no interaction on either yagi ???
Plan B for a mast, is to use 2.875" OD 1026 DOM.. with 1/4" wall
thickness. Section modulus = 1.244131..... x 89 ksi = 110,743 in lbs
or 9228 ft lbs. 1026 dom is cost prohibitive right now, esp with shipping
costs, etc. 6061-T6 is readily available here in town. I already have 20' of
2.875" 6061-T6 [ sched 80]
To test the 6061-T6... I proposed to install it to my deck, horizontally.. with
XXX feet sticking out in mid air....then start adding weight at the extreme
end
and and also along it's length .. and measure the deflection. If there is
some on line calc that depicts deflection, that would be even better.
Is my plan feasible.... or am I out to lunch on this one ??
Later........... Jim VE7RF
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