[TowerTalk] W6NL 40m Moxon

[Jim Thomson]

Date: Wed, 13 Apr 2016 08:55:43 -0500
From: Stan Stockton <wa5rtg at gmail.com>
To: "Joe Subich, W4TV" <lists at subich.com>
Cc: towertalk at contesting.com
Subject: Re: [TowerTalk] W6NL 40m Moxon

If calculating wind load for the purpose of deciding which rotator to use, I would think the boom length would factor in.  Surely a square plate 4'x4' mounted to the mast would put less stress on a rotator than a 50 foot boom antenna with the same 16 SF calculation?

At any rate if the difference in the various ways of calculating the wind load mentioned makes the difference in what you believe will survive, I would think you are cutting it too close.

Stan, K5GO

##  Boom length has nothing to do with it....provided  you have an equal amount of boom on either side of the mast.  If one end of the boom is heavier than the other, then  the fix is a counterweight is used on the lighter  ( LAST  DIR)  end.
IF instead you mount at the center of gravity, then you will have a gross tq  imbalance.  Fix for that is a tq compensator plate  down at the REF end of the boom.

##   I designed a TQ comp plate for a fellow here in town.   We tested it by installing the yagi on a short 30 ft tall   test tower, with just 2 x bearings..and no coax and no rotor.  It was free to spin 360++ degs.
Buddy climbs up there in a  35 mph windstorm..and could easily turn the boom in any direction he wanted..and it stayed put.   It wont weather vane.   Remove the tq comp plate..and all hell breaks loose.
It will weather vane.  It also destroyed 2 of his  ham-4 rotors in the process....( b4 the tq comp plate was added), the previous 2  winters.   In actual usage you also have to factor in the coax + balun heading towards the REF end of the
boom..terminating on the DE.   Running the coax down the side of the boom will  minimize the  windload on the boom.   We factored all that in in the latest version of the tq comp plate. 

##  Done right, the end result is zero tq..with boom broadside to the wind.  With boom pointed into the wind, you will still  have a slight tq imbalance..due to the fact that the boom is mounted to the SIDE of the mast.   A  2 inch OD
boom mounted to a 2 inch OD mast is  not an issue.   A  3 inch od boom, mounted to a 3 inch OD mast creates slightly more imbalance.    If more than one yagi on the same mast, then alternate sides of the mast = almost near zero tq.

##  Of course you require enough rotor tq to overcome the inertia from the mass /weight of the heavy yagi.   No different from my buddy turning his yagi on the 30 ft temp tower..by hand. 

##   If the yagi is not tq balanced, then you require a huge amount of tq to get it to turn..esp in high winds.   Not tq compensating the array is like  using more HP on a car....since you have deployed a chute. 

##  if you think your tq imbalance is bad at  70 mph, its worse at 100 mph.  The issue is easily fixed.  Dunno why ant makers don’t tq balance their products, its very simple.

Jim   VE7RF