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@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
>
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|