[TowerTalk] Tower advice

[Steve Jones]

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.

Vy 73,

Steve Jones
N6SJ

----- Original Message ----- 
From: "Rick Kiessig" <kiessig at gmail.com>
To: <towertalk at 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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