Date: Mon, 19 Aug 2013 13:17:44 -0400
From: <RShirbroun@newportlabs.com>
To: <towertalk@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.
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