On 6/8/2012 5:55 PM, Steve Jones wrote:
> Rick-
>
> I think you're asking for trouble if you don't have a thrust bearing above
> the rotor. The wind will either crack the rotor housing or break the bolts
> holding it to the plate. This sounds like a pre-fab cellular tower. A
> friend of mine built a sturdy open-sided metal box to put on the top of his
> tower. The box enclosed the rotor at the bottom and a thrust bearing on
> the top. This assembly protects the rotor and the mast. You might want to
> consider such an accessory.
Some time in the last year of two we had a discussion on here about this
and some one posted links to a cage that had been built to go on top of
tubular towers. It held the rotator and provided support for a sleeve,
or thrust bearing above the rotator. I haven't looked for it, but it's
likely to be in the archives.
It just forms a 3 or 4 sided box with the bottom and top tapered like
the top of a 25 or 45G tapered top. It had (in this case) the top of the
mast extending into the cage with the rotator mounted on that. They
could have easily made a rotator, or accessory shelf to which they could
bolt the rotator.
73
Roger (K8RI)
>
> Vy 73,
>
> Steve Jones
> N6SJ
>
> ----- Original Message -----
> From: "Rick Kiessig" <kiessig@gmail.com>
> To: <towertalk@contesting.com>
> Sent: Thursday, June 07, 2012 11:20 PM
> Subject: [TowerTalk] Tower advice
>
>
>> After a multi-year battle with the city, I am finally in the process of
>> drawing up plans for a tower that have a decent chance of being approved.
>>
>>
>>
>> The tower height will be about 9m above immediate ground level (but I'm
>> near
>> the top of a 100m-high hill, so I have some hope of getting a decent
>> signal). The tower will be octagonal galvanized steel, 290mm diameter at
>> the
>> base, 140mm at the top, in 4 sections. I'm planning to use a PST-61
>> rotator
>> with either an Ultrabeam UB50 or a SteppIR UB-18 (the rotator is
>> intentionally oversized, since I'm hoping to increase the size of both the
>> tower and the antenna in a few years when the city's rules are changed to
>> reflect a recent court decision in favor of hams). The tower was not
>> originally designed for use as an antenna mast, but the manufacturer is
>> willing to do a little customization to make it work, such as adding a
>> plate
>> at the top of the tower with bolt holes to match the mounting plate on the
>> PST-61.
>>
>>
>>
>> Would it be OK to run RF and control cabling on the inside of the tower
>> without supports for strain relief other than one at the top? (coax will
>> probably be LMR-400 UF in conduit, not direct burial; the total length is
>> pretty short (roughly 20m) so the expense and hassle of using LMR-600 or
>> Heliax may not be worth it).
>>
>>
>>
>> The manufacturer has suggested three 20mm grommeted exit holes close to
>> the
>> top of the tower, one for each of the three cables that will be needed
>> (slightly below the mounting plate for the rotator). Sound OK?
>>
>>
>>
>> The tower will have an access plate at the base, which I can use to tie
>> the
>> coax shield to a ground rod that would be present in that space. The base
>> of
>> the tower would also be tied to ground rods. Is there anything else I
>> should
>> allow for in or near the base?
>>
>>
>>
>> I was thinking of running one conduit for the RF coax and a separate one
>> for
>> the antenna and rotator control cables. Is that overkill?
>>
>>
>>
>> This design would not have a thrust bearing or an interior shelf for the
>> rotator. The rotator would need to carry the full load of the antenna (5.8
>> x
>> 6.6m, 44kg, 10.1 sq ft wind load). Am I asking for trouble, even with the
>> oversized PST-61?
>>
>>
>>
>> What's a reasonable maximum length for the mast, going from the rotator to
>> the antenna? I'm thinking of using 50mm galvanized water pipe. A longer
>> mast
>> would mean a slightly shorter tower and a little more flexibility with
>> total
>> height (very tightly restricted here), but would also present a larger
>> load
>> on the rotator, right?
>>
>>
>>
>> 73, Rick ZL2HAM
>>
>>
>>
>> _______________________________________________
>>
>>
>>
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>
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