Antenna Mast

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Philip Camera:
Not to burst your bubble, but the A3 alone is exceeding Rohn's specifications on boom/element length for that tower.  Putting any more torque generators atop it may be looking for trouble.  The BX towers have riveted joints.  The tower is designed, as all self supporters, to convert torque from the antennas twisting in the wind into twist that is transmitted throughout the tower.  Rohn specifically recommends that antennas with boom lengths less than 10 feet.  Anything more, and they don't really talk about element lengths, and you may start overtwisting the tower and pop the rivets.  Gd luck if you try it but I wouldn't do it.  Phil  KB9CRY

Luther Harlow:
I recently got a Rohn HDBX-48 heavy duty 48 foot self supporting tower, and I currently have a Cushcraft A3S beam and a dual band vertical sitting atop it. I have a few more VHF/UHF beams I would like to put on the tower. I was just wondering what would be the limit for mast height? The rotator I'm using is the Yaesu G-800SA meduim/heavy duty rotor. There is about 3-4 feet of free mast space between the vertical and the beam. Probably not enough room for some VHF/UHF yagis though.

Thanks, 73,
Luther - KG4USK

Ed Grooms:
The "pure" way to determine separation requirements is to model the installation with a good computer modeling program.  Most of us are not that inclined so we turn to each other for experience based advice.

You said there was about 3' between the HF beam and VHF/UHF vertical.  I also have a fully rotatable VHF/UHF vertical atop a mast over an HF beam.  I must say the the rotation of the omni-directional antenna is of value beyond belief!

You should be able to run 20' of mast with that rotor and tower without much concern.  It sounds like you might have about half that with a tower shelf mounted rotor.  If the rotor is above the tower, you have to be verrrrrrrrrrry careful about added height due to moment forces in the horizontal plane (wind).

If you do decide to put in a 20' mast, obviously the HF antenna should be relatively close to the tower since it will have the most turning torque.  Any VHF antenna should be 3 or so feet above that at a minimum and a UHF antenna should be another 3 or so above the VHF.  It is not hard to put HF/VHF/UHF on a 10' mast with about 8' extending above the tower.  The 20' mast will give you additional separation to the vertical (an important factor unless you decide to mount the vertical lower on the tower) and provide for possible 1.2GHz, etc.  All this assumes that you are going to mount the VHF/UHF directional antennas in the horizontal plane for "traditional" weak signal work.  If you are planning a circular antenna or vertical yagi, additional separation will be required since the distance is reduced by the vertical elements.  You will not be able to put a "H-frame" or other array type of arrangement above the HF antenna and maintain integrity within the specifications of the rotor/tower.

You need to match the mast to the requirements.  Thin walled tubing MIGHT work.  Thick walled (or solid!!) mast is unlikely a requirement and would probably exceed the ratings of the tower/rotor.  A happy medium is probably going to be the right choice.  Check the total weight, torque, etc and either run it out with a calculator or ask an engineer friend to run it for you.  I would guess ... GUESS ... that you can put a 21' length of 'common' pipe as a mast with a reasonable expectation of success.  It is extremely important (critical) to remember that the ID must never exceed the OD or it will cease to be useable.

Steve Katz:
Ditto KB9CRY's comments.

Hams do a lot of crazy things, and I'm among them, but the HDBX and HBX series were never intended for rotary HF antennas.  They do specifically have a maximum boom or element length of ten feet, listed right in their published data sheets.  The A3S already exceeds that quite a bit by virtue of its 12' boom and 28' or so reflector element -- not to mention the driven and director elements, which are also both over 20' long.

The issue, as Phil says, is one of popping rivets and turning the tower into a pretzel.  This tower is not designed to be guyed (although hams do that, too), but if it could be guyed using outrigger attachments you could reduce torque stress quite a bit by transmitting that moment into the guys rather than absorbing it all in the tower.  The greatest moment occurs when you *stop* an HF beam from turning, because it stops faster than it starts...especially if you engage the brake quickly and don't use an electronic delay circuit that prevents brake engagement until after all rotation has stopped.


Luther Harlow:
I forgot to mention... When the tower was hot dip galvanized, it ate all the aluminum rivets out, and we replaced them with stainless steel bolts. It has survived 3 hurricanes, including hurricane Isabel. It's a very sturdy tower.



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