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Re: [TowerTalk] Rohn HDBX and Mosely

Subject: Re: [TowerTalk] Rohn HDBX and Mosely
From: "" <>
Reply-to: "" <>
Date: Mon, 14 May 2007 11:42:40 -0700 (GMT-07:00)
List-post: <>
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.


-----Original Message-----
>From: Nick Pair <>
>Sent: May 14, 2007 11:06 AM
>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
> GPS? Comic books? 
>Check out fitting gifts for grads at Yahoo! Search
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