In a message dated 8/24/2011 7:49:28 A.M. Pacific Daylight Time,
> My first project is to get a tribander (10, 15, and 20m) up and on the
as quickly as possible. I have selected a ROHN 9H50 telescoping mast that
extends to 50? and has guy mounts at 4 locations up the mast. ROHN
recommends each set of guy wires start at 30? from the base of the mast. I
have bought a pair of Chicago Grips for tensioning the guys and a LOOS
tension meter for setting the guy wire tension to 10% of its maximum
> I thought this would be an easy project (even though I don?t have any
experience in erecting towers). But, it hasn?t turned out to be so easy
when I realized the projected area of the tribander I have chosen is about
sq ft of projected area (EPA), but the footnotes for the mast say the
antenna load should not exceed 2 EPA! When I add 1 sq ft for the rotator
and mast to support the antenna, my EPA is over 5 sq ft!
Hiya, Wayne --
Yes, it's inadequate for your application. And yes, it's not easy to
> So, my question is: should I abandon this project or can I make
the support of the mast so my tribander can stay up even in the 100mph area
max wind speed estimated for my county?
Ouch. 100 MPH is significant and only a tower will handle what you're
proposing. And not a crank-up.
> What I have thought of changing is: 1. Not using the ROHN recommended
wire as the guy wire but going to ROHN 3/16 EHS, 2. Using the software
called MARC (Antenna & Rotator Calculator) to calculate the required steel
alloy and thickness of the antenna mast, 3. Limiting the antenna mast and
rotator to only 2 ft above the top of the telescoping tower mast, and using
4 guys instead of 3 (for a total of 16 guy wires).
Changing guy wire strength won't change anything. The limiting factor in a
tower is leg strength, and since you basically have one leg in your mast -
the leg strength is the same regardless of what you're using for guy wire.
(Using a heavier guy wire will also add more compressive forces to your
mast, thus lowering the capacity even more.)
> Can I make this mast stand up?
Sure, but only until the first big wind.
> What are the weak areas and how can I make them stronger?
You'd have to re-engineer the whole thing which would entail beefing
it up. Is it worth it? Probably not.
> Would you direct me to books that discuss tower supports?
In my tower book - UP THE TOWER, The Complete Guide To Tower Construction
- I have a whole chapter on antenna support selection. (available from
_www.championradio.com_ (http://www.championradio.com) )
Your mast won't work with your big winds. A good choice might be a
self-supporting roof tower (e.g. Glen Martin Engineering) but when you're
working with potential 100 MPH winds, it'll probably just rip the roof off.
Your best bet is a real tower designed for your application and parameters.
TOWER TECH -
Professional tower services for hams
Champion Radio Products
PS - If you want the REAL skinny on tribander performance, I'd also suggest
our tribander comparison test also available from _www.championradio.com_
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