[TowerTalk] Aluminum tubing slip joints seized - my final answer

[Robert Chudek - K0RC]

This antenna might have originated for a non-amateur user that needed 
coverage below 7 MHz. That might be a reason the smaller tube was shoved 
so far inside the larger tube. I'll probably never know for certain as 
the antenna is 40+ years old.

I have a schematic/diagram of the ham version (Telrex Model 40M-BFD) and 
it starts out with 1.5" center tubing. This one goes two sizes larger, 
2" and 2.5". Maybe this one was a 40M-Really BFD ???

73 de Bob - K0RC in MN
------------------------------------------------------------------------

On 9/7/2011 7:19 PM, Gene Fuller wrote:
> There is no "simple " answer. IT ALL DEPENDS - on bending moments, 
> restraining moments, strength  of materials, loading, etc. If the 
> larger dia tube is getting near it's limit you might want to extend 
> the smaller tube further into the larger to accumulate restraining 
> moment. With my LP 46 foot elements I have four layers of tubing near 
> the boom. That way I keep the OD down to 1 5/8" but should be safe up 
> 100 mph with 1/4 " of ice and a good safety factor. So far it's been 
> fine up to 85 mph...............
>
> ----- Original Message ----- From: "Robert Chudek - K0RC" 
> <robert.chudek at gmail.com>
> To: <TowerTalk at contesting.com>
> Sent: Wednesday, September 07, 2011 7:16 PM
> Subject: Re: [TowerTalk] Aluminum tubing slip joints seized - my final 
> answer
>
>
>> The issue was a little more "complex" than I first thought.
>>
>> I discovered there wasn't a simple 4" to 6" of overlap, but about 48"
>> instead. It's apparent this antenna was not assembled to manufactures
>> specifications. The extra tubing did not add to the mechanical strength.
>> It only added unnecessary weight. The excess tubing was at the tip end
>> of the tube, not the tower end.
>>
>> What did work to some extent was inserting another (long) tube from the
>> opposite direction and driving the inside tube toward the tip. This was
>> only partially successful and I had to resort to the hacksaw. Actually,
>> I used a tubing cutter for a clean cut. Then I drove the remaining
>> inside piece back toward the tower end. I had to use this method on both
>> halves.
>>
>> Pulling a few pieces from my aluminum pile I was able to reassemble the
>> antenna. At the same time I added stiffening to the junctions by adding
>> internal sleeves where needed.
>>
>> The remaining task is to model this (again) and see how to make a 62.5
>> foot element resonate at 7.1 MHz. I'm thinking capacity hats somewhere
>> out near the ends, like the Cushcraft 2 element 40m yagi.
>>
>> 73 de Bob - K0RC in MN
>> ------------------------------------------------------------------------
>>
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>