I'm not an engineer, but just going by the specs I'd say the Rohn mast isn't
appropriate for the windload you're planning. With a 100 MPH wind rating for
your county, you'd be advised to go with a small guyed tower. Yeah, it's
much more expensive and much more of a project to erect, but it would be
appropriate for the antenna and rotor. One of the heavy-duty crankups may be
able to handle the windload, but they're even more expensive.
The other alternative I would advise is try to find a surplus AB-577 crankup
mast. It should be able to handle the windload, though I don't know if it
would stand up to a full 100 MPH wind. I've used them here for years and
have never had a problem, even with an antenna that had a windload of 10 sq
ft and weighted well over 100 lbs. That said, the max wind probably never
got above 60 MPH, and that only in a microburst. The best source I know for
AB-577s is Ontario Surplus: http://www.ontariosurplus.com/masts.htm. There
are some good photos on the website that'll give you an idea of what the
AB-577 is all about. There may be other sources closer to you.
73, Dick WC1M
From: Wayne Willenberg [mailto:firstname.lastname@example.org]
Sent: Wednesday, August 24, 2011 10:19 AM
Subject: [TowerTalk] Basic Help Needed
I have just returned to ham radio now that I am retired.
My first project is to get a tribander (10, 15, and 20m) up and on the air
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 4
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!
So, my question is: should I abandon this project or can I make changes in
the support of the mast so my tribander can stay up even in the 100mph area
max wind speed estimated for my county?
What I have thought of changing is: 1. Not using the ROHN recommended 18GA
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).
Can I make this mast stand up? What are the weak areas and how can I make
them stronger? Would you direct me to books that discuss tower supports?
Thank you for your help. I really need it.
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