[TowerTalk] Ground level rotator mounting

[Roger (K8RI) on TT]

On 9/7/2013 1:09 PM, Don Moman VE6JY wrote:
> Taking the mast to the ground is especially appropriate for the Rohn
> BX/Delhi series of towers as torque up at the top of the tower isn't
> helpful to the longevity of the rivets... and it's all downhill from there.
>
> I have seen and done numerous installs in Rohn and bigger. Oscillation, to
> me anyway, is so remote a possibility that I don't even consider it.  I use
> anywhere from 1.5 to 3.5 IPS S40 water pipe and weld on flanges for
> coupling.  This reduces the inherent elongation of using simple pinning
> bolts thru the mast and allows an unobstructed  path for the coax to also
> go down the inside of the pipe and thru the rotor.

I think the welded flanges are a good idea .  It sounds like you also 
mount the rotator independent of the tower which removes all shearing 
stress which is essentual on a tower like the HB series. I'd think this 
would also remove the 10' boom limitation and you'd only have to deal 
with the wind load.

73

Roger (K8RI)
>   
>
> I am in the process of finishing one as we speak, 2.5 inch ips x 110 feet
> for a large HF log periodic.  Sure it will twist somewhat but given the
> beamwidth it is inconsequential.   If I get around to it, I would put some
> treated 2x6's around the mast to keep it roughly centered. There is no need
> for precision in this application. I always let the weight be supported by
> the top bearing.
>
> Don
> VE6JY
>
>
> On Sat, Sep 7, 2013 at 4:38 PM, <john at kk9a.com> wrote:
>
>> That is a good point, Roger.  Search Tacoma Narrows Bridge for a visual of
>> harmonic oscillation destruction.
>>
>> John KK9A
>>
>> To:towertalk at contesting.com
>> Subject:Re: [TowerTalk] Ground level rotator mounting
>> From:"Roger (K8RI) on TT" <K8RI-on-TowerTalk at tm.net>
>> Date:Fri, 06 Sep 2013 22:20:28 -0400
>> List-post:<towertalk at contesting.com">mailto:towertalk at contesting.com>
>>
>> On 9/6/2013 9:38 PM, Doug Wetzel wrote:
>>
>> When I put up my HDBX-48 in the spring, I'm thinking of mounting the
>> rotator at ground level.  Has anyone on the list done something similar?
>> What did you use for mast all the way up the tower, and did you use
>>
>>
>> Be careful about mechanical resonances. A long mast is in effect a torsion
>> bar that with its flex, the inherent slight give in the rotator, plus the
>> mass of the antenna May result in a low frequency oscillation.
>>
>> You may or may not end up with a resonant condition.. If you do, the
>> condition may or may not be destructive.
>>
>> IOW you are now in unexplored territory unless you can find someone who has
>> use the same rotator, mast, and antenna combination.
>>
>> Many years ago I built a simple, laminated, wood mast. It worked quite well
>> for a while, but one windy day I noticed the mast was oscillating. You
>> could
>> easily see the node and nulls.
>>
>> As the wind increased the nodes became larger and the speed was so fast
>> that
>> the nodes looked like a solid mas several feet across. IIRC the nodes
>> appeared to be nearly 3 feet across before it let go. The longest piece
>> left
>> was about 3 feet long.
>>
>> Now this was a different material and a different mode of oscillation.. If
>> a
>> mast went into severe oscillation I'd expect the destruction to be confined
>> to either the the rotator, or the antenna. Possibly both.
>>
>> I've had relatively minor oscillations of a relatively low frequency with a
>> 40 foot mast. The antennas were swinging about 2 feet in azimuth. It did
>> destroy a Ham-IV and an HDR300 before I installed the PST-61
>>
>> multiple thrust bearings to align the mast?
>>
>>
>> Typically they only keep the mast aligned and do not support weight except
>> for one.
>> 73
>>
>> Roger (K8RI)
>>
>>
>> Thanks in advance.
>>
>> Doug
>> K7IP
>>
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