[TowerTalk] Rohn HDBX and Mosely

Mon May 14 14:42:40 EDT 2007

I'm assuming that your Mosley CL-36 wasn't the antenna that was installed on the Rohn HDBX48 tower. I also have an HDBX48 tower, and if I remember correctly without digging out my Rohn catalog, the antenna installed on an HDBX is limited to a maximum boom length of 10 feet, and maybe something like 10 sq. feet of wind load. 

To get a tower permit for for my HDBX48, I had to be sure not to exceed the published antenna limits of the tower. That meant that even antennas like a typical 3-element triband yagi (TA-33, Hy-Gain TH-3 series, etc.) wouldn't work. Only antennas like a 2-element TA-32, TH-2 series, or a 2-element Quad would qualify with the boom length limitations.

I went with a 2-element 6-band quad. The tower was up for about 15 years and never flinched even in the big wind storms.

LJ

-----Original Message-----
>From: Nick Pair <daweezil2003 at yahoo.com>
>Sent: May 14, 2007 11:06 AM
>To: towertalk at contesting.com
>Subject: [TowerTalk] Rohn HDBX and Mosely
>
>I would just like to relate a story on the HDBX 48 that I have had up for about 20 years. The tower was OK for the first 15 years or so, with a little rust showing up around the 10Th year. The bolts that join the sections together were the first to rust, with a little rust showing up on the edges of the X's and the vertical rails(all of which were sheared and formed after galvanizing). At the 15th year I noticed in that winter that the tower top was moving in the heavy winds a lot more than I remembered. Next big wind storm I b raves the storm to go out and watch the tower react to the gusts. I discovered that the tower was lose between sections 2 and 3. I had installed all bolts according to Rohn's directions and even used a torque wrench to verify tightness. After wind died down I was able to climb tower and found out that the bolt holes on two sides had elongated by almost a quarter inch and that the third side had the section of the leg below the second had completely
> broken off from the midpoint of the bolt down. So all I could think of to do was to place a guy wire (using a cable with a hook on the end of it so I didn't have to climb beyond the break) on the top of the next section and apply enough force to keep the tower from rocking and doing more damage. Now I have a useless tower that I can't get down with a gin pole and I can't think of a way to get it down besides just cutting it loose and toppling down. That would destroy antenna and with my luck the rotor too. I might be able to erect a 25g tower with temporary guys to use as a tower crane. Access with a real crane truck is difficult and not within my ham budget at this time. As to the aluminum rivets I was very surprised when I got the tower here and found dissimilar metals on it. I am still surprised to find that they have not been a source of rust over the almost 20 years I have had the tower. They must be of a alloy that I have not heard about, or they might be mixing
> aluminum with the zink they galvanize with.
>   Now about the Mosley antennas, I have not had the TA versions but have had the CL-36's for almost thirty years and other than needing a boom truss system added for my higher than normal wind location and a little more drupe on the elements than other brands, they have preformed well and been mechanically secure all the while. The other fellow that mentioned that the driver seemed longer that the reflector is due to the feed system they use. Their patented balanced feed is a version of the T match that uses a made up series capacitor to feed each side of the insulated split driven element. The capacitor consists of a length of 15 kv rated wire inside of the element halves tied to the center of the feed connector. Its this series capacitor that necessitates a longer than quarter wave element halves to get back to resonance.(forming a electrical 1/4 length) While this capacitor value is a compromise for triband use, it sure makes a clean looking installation with all the
> components inside the element away from the weather. I'm not trying to say this antenna is better than monobanders of the same length, but it is a good balance of the compromises one has to make with a tribander.(and maybe a better balance that they were able to achieve with the more modern designs they are selling now!)(all those duel drivers and the phasing lines are not enough improvement to justify their complexity and weather survivability. 
>    Nick
>    WB7PEK
>    Dodging lightning somewhere on a hilltop in SW WA
>
>
>       
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