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
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
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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