[TowerTalk] Mosley Antenna Question

[Jim Thomson]

Date: Thu, 26 Feb 2015 14:49:24 -0800
From: "Don " <w7wll at arrl.net>
To: <towertalk at contesting.com>
Subject: Re: [TowerTalk] Mosley Antenna Question

I have a TH7DXS at 72 and and a A3WS at 78 feet. With 80 MPH gusts and 
sustained winds often in the 50's  and 60's during the winter here on the 
ocean the elements move around a lot - have not had one break or bend yet in 
the 13 years they've been up.

Don W7WLL

##  Pretty simple matter to design any of these yagi configurations  to handle a 
60-80-100-120-140 mph sustained wind, gusts,  ice loading,  ice + wind etc. 

##  Both dx eng and also K7NV  ( yagi stress), and a few more programs  that
can model the wind survival for you,  including the boom.    They will also spit out 
ele sag, boom sag, ele deflection,uplift effects and anything else you want to know. 

##  f12, JK ants, M2, inovate  use the same mechanical software.  If you spec
one of their ants for a 100 mph sustained wind, with gusts  to 120 mph, no sweat,
they  will build it for you.   Same deal with  hb yagis, or beefing up existing commercial
designs.   With the software you can see at a glance exactly where the weak spots are 
on the eles, or the boom.   Its fairly simple to beef up these weak areas  by adding an 
inner liner, or in some cases, 2 of them.  Or using thicker wall tubing, or changing the 
tubing schedule altogether. 

##  Dx engineering now sells boom material, 6061-T8, that has a .120 inch wall, instead
of the usual .125 wall.  This is great news, since the sizes simply telescope together, and 
no swaging involved.   IE: 2 inch slides into 2.25  which slides into 2.5  which slides into 
2.75 then into 3 inch.   This not only makes for a nice tapered boom, you can also
make the boom all the same diam, and use short 1 ft pieces of the next size up as outside 
sleeves, to splice them together.  

##  My F12  6 el  15m yagi uses all 2 inch material for the boom, but is .125 wall in the 
middle 2 x sections, then drops down to .058 wall..then down to .049 wall.   36 ft long
and all 6 sections are spliced with 2.25 od outer sleeves.   This makes for a light weight
boom.  YS sez its good for 125 mph.... IF the boom is mounted to the mast at the center
of the boom.   When maxed out, the max stress is where the boom meets the mast..dead center
in the middle of the boom.   40 ksi.   Meanwhile the very end sections of boom are only loaded
to 10.4 ksi.    So no point in going to real heavy wall boom end sections in this case. 

##  It really is no big deal to build or modify yagi els to handle extreme windloads, without
having to resort to huge od tubing.   My f12  44 ft long 40m eles are only 1.25 inch OD
in the middle, but consist of .082 wall tubing...with a .058 inch wall liner inside...and a .035
wall liner inside that.  So its triple wall in the middle 12 ft, and   .175 wall thick.   Next
sections outboard of that are double wall.  After the insulator that is half way out each ele
half, its then all single wall.   Weighs 12.4 lbs, 1.5 sq ft..and good for 100 mph wind. 

##  the software will allow for a lightweight design, with minimal windload.... if designing 
an ele from scratch, ..... or  beefing up an existing yagi.    new tubing is pretty cheap
at dx engineering and other vendors.   Available in both 3 ft and 6 ft lengths. 
Dx engineering used to sell tubing with the slots already on one end only.  If u
don’t want to use the slots, u simply  flip the tube 180 degs.   Good news is, 
you can now buy the same tubing with no slots at all, and price drops quite a bit. 

##  IE:  a 3 ft length of .375 OD  x .058 wall  tubing is just $1.48 !  The price goes up
as the OD starts to increase in .125 increments, like .5 od, .625, .750   1.0  etc, etc....
all the way up to over 2.0 inch.   Check it out. 

Jim   VE7RF