[TowerTalk] Rohn HDBX 40 - mast length

[Jim Thomson]

Date: Mon, 19 Aug 2013 13:17:44 -0400
From: <RShirbroun at newportlabs.com>
To: <towertalk at contesting.com>
Subject: [TowerTalk] Rohn HDBX 40 - mast length


##  with 13 feet above the top of the tower...and only 2 feet into the tower.....the leverage effect is massive!      The top 2 feet of the tower will fold over, like snapping a tooth pick.    What will work is to move the 
rotor plate DOWN into the tower  another 4 feet.    IE:   6 feet of mast into the tower..and  9 feet  above the tower.   Make a new rotor plate from .375 inch thick  aluminum plate...or  .25 inch thick steel plate. 

##  9 feet of mast above the top of the tower is plenty  to space out  your tribander, then the 6m yagi above that....and the 40m rotary dipole above that.   Orient the 40m rotary dipole so it is in line with the 6m boom and tribander boom below it.   IE:  with both yagis pointed due north..the 40m dipole is oriented N-S...and radiates  E-W.   Done that way, there is no possibility of the 40m yagi screwing up the 15m portion of your tribander. 

##  ARRL has free mast software that is very accurate.  You can input your various ants  at any height above the top of the tower.   You can also input any mast  OD  + wall thickness you want.   Ditto with yield strength  of mast material.   You can also beef up the top 6 feet of the tower with some extra bracing.... like that giant cue –rack they make for adding guy wires etc.   I would also suggest replacing the oem top plate  with a real one.  The oem top plate + rotor plate is junk...the tabs that bolt to the tower legs  will begin to shear from  both sides of all 3 tabs. 

##  as far as torque goes.... ELIMINATE any TQ  by installing  a TQ compensating plate  at the REF end of the tribander boom.   I used the K7NV  yagi-stress  software to design several  TQ comp plates over the years.   Its just a small rectangular plate...mounted vertically... via a pair of U bolts to the boom.     Route the coax such that it goes down the SIDE of the boom..and not below or above the boom.   That way the wind does no see the coax.    If you mount the coax above or below the boom, the wind thinks the boom is bigger diam than it actually is..and just increases the windload.   Once you get to the feedpoint, then the coax can spiral around to the feed point-balun etc.  With the TQ comp plate scheme, the boom is mounted to the mast  at the center of gravity of the yagi.   You then end up with typ  LESS boom on the REF end..and MORE boom  on the DIR end.   The DIR end  with the more boom is what causes the TQ imbalance.   The  TQ compensating plate  down at the REF end of the boom just balances out the widload..so its equal on both sides of the mast. 

## The only other way to TQ compensate  the yagi is to  mount the boom to the mast at the exact center of the boom.  The yagi will now be TQ compensated,  BUT the REF end will now be heavier.  A counterweight is then firmly attached to the lighter end of the boom, the DIR end. 

## done correctly, and with rotor removed..and yagi  free to spin  360 degs...... you will see that the yagi  stays put, regardless of where its pointed.   The yagi is TQ balanced..when you have an equal  boom windload on each side of the mast.     Each individual  ele is already TQ balanced since  each ele is symetrical in nature.... IE: equal amount of ele on each side of the boom.  

##  If you want a gross example..... install a 20 foot boom, with 10 ft on either side of the mast.   Install  JUST the REF only, no other eles.   The ant is TQ balanced...and it will not windmill .   Been there, done that.
I am surprised more folks..and ant manufacturers   don’t include the TQ comp plate.    A local ham was trashing rotors left and right.  The TQ comp plate was installed..end of problem. 

##  For you folks with F12  yagis.... an easy way to add a TQ comp plate is to install a spare F12  ele to boom .... wrap around  bracket mount.   Except orient it so it is vertical...and just b4 the REF.   In many cases, that alone will provide the required TQ comp.   If not, and the software sez to use XXX size plate... then mount the XXX plate to the F12  bracket mount.   The F12  brackets mount to the boom with 8 x .1875 inch rivets.   The lighter duty  .125 inch thick  F12 mounts as used on their 15 + 10m yagis  is plenty good enough for tq comp service.    Then it will never slip on the boom...and it also eliminates the weight of a pair of U bolts + saddles. 

##  Baffles me why folks trash rotors, tower tops, and boom to mast clamp assemblies, etc, etc...when there is clearly an easy solution. 

later........... Jim   VE7RF 






I have just erected a new HDBX 40 in place of my 30 year old HBX 48, sacrificing 8' of height to gain some strength and load capacity (and peace of mind!).  The stock rotator plate allows 2' of mast below the top plate and I will be using the heavy duty Yaesu thrust bearing (along with the Yaesu heavy duty 2800 rotator plus the absorber plate).   I have added steel angle braces to reinforce the rotator plate.

I'm looking at using a 15' chrome/steel 2" mast (in place of my previous 9' mast), so 13' of the mast would extend above the tower.  The mast would support a TX38 tribander just above the top plate, a 6 m beam half-way up, and a 40m rotatable dipole near the top, 12' above the top tower plate and the thrust bearing.  The mast, of course, is very heavy, weighing around 75lbs.  The tribander weighs 40lbs and with 5 ft2 surface area; the 6 m. beam weighs 10lbs with 1.5 ft2; the dipole weighs 10lbs and is 0.5 ft2.

Is this too much mast for this tower?   (BTW - I'm aware the boom length exceeds the 10' maximum for this tower, but the HBX 48 handled a similar tribander for 30 years, with occasional severe ice-loading, without any problems.)

Thanks and 73,
Randy, ND0C

##  with 13 feet above the top of the tower...and only 2 feet into the tower.....the leverage effect is massive.   The top 2 feet of the tower will fold over, like snapping a tooth pick.