[TowerTalk] Cut off extra 7 ft of mast or leave it?

[Jim Thomson]

Date: Sun, 22 Nov 2020 18:39:24 -0800
From: David Winarsky <david at winarsky.com>
To: TowerTalk at contesting.com
Subject: [TowerTalk] Cut off extra 7 ft of mast or leave it?

<Im putting up my first tower, a LM-354HDSP with a Steppir DB11 and a JK 40 meter horizontal dipole, both in the same mast. I have a 20 ft 2 in OD mast from Tashjian and am wondering whether I should cut off the unused length or leave it?  

<Karl at Tashjian suggested I install the rotator plate 3 ft below the top of the tower and the DB11 2 ft above the top, with the JK40 8 ft above that (10 ft from the top of the tower). That will leave 7 ft of the mast unused. 

<Do you folks recommend I leave the extra 7ft of the mast in place or cut it off?

<Appreciate the input and any other comments or suggestions on my plan. 

<73,
<David K6ZD

##  what kind of mast ?   what is the OD ?   wall  thickness ?   KSI  strength of mast.   aluminum or steel.  is it  chromolly  or  dom, or plane jane  35 ksi ? 
Knowing that, simple software calcs to provide  bending  moment where  mast enters the tower.
I would put the rotor plate down six to  eight  feet.   That reduces the stress  on the top of the tower... by a bunch.   Leaving 14 foot  above the top of the tower.  Ok, then put the 40m rotary dipole at the top, or close to the  top.
Align the  40m rotary dipole  so its  inline with the steppir  boom.  Then just do mental math..and  either  add or subtract  90 degs  from your  rotor indicator dial. 
Depending on snow and ice, and length of the 40m  rotary dipole,  you might require a non conductive overhead truss  line on the 40m dipole, hence mounting the
40m dipole 3-4 feet below top of mast.  Remember to cap off the  top of the mast.  DXE sells  caps.  Then no water  running down the inside of the mast. 

Jim   VE7RF