Ron,
You're asking the right questions.
I have a U.S. Tower MA-770MDP with the MARB-770 base. In the 11 years I've
owned the tower, I've had three different antennas on it: a Hy-Gain TH-7
tribander at 9.4 sq ft and 75 lbs, a Cushcraft 40-2CD 2-el 40m beam at 6.4
sq ft and 44 lbs, and a Force-12 EF-420 4-el 20m beam at 6.5 sq ft and 50
lbs.
In my opinion, the TH-7, which pushed the tower specs to the limit, was too
much antenna for the tower. It swayed and gyrated violently in winds over
about 30 MPH. It was scary. Even with winds in the 20 MPH range, the tower
swayed and turned in ways that alarmed me. I also had problems with the
pull-down cable pulling off its guide pulleys, until I removed them at U.S.
Tower's recommendation. I still had problems with the pull-down cable
getting caught on the main pulley housings.
Lowering the tower 20 feet made a big difference. The tower was much more
stable in higher winds. I'm sure the windload rating increases dramatically
when you do this. However, I still didn't like the amount of sway with the
TH-7, and still had problems with the pull-down cable. O felt that if the
wind exceeded 50 MPH the tower might be at risk. So, I got in the habit of
keeping the tower fully retracted except when using it for contests or to
chase DX. Having the motorized version makes this feasible. If you have the
manual version, it takes a long, long time to crank up or down, and a lot of
elbow grease. You'll eventually get lazy and not do it, which is when the
big wind will come along and destroy your tower.
My tower has been much happier with the 40-2CD and EF-420. They're both
relatively large antennas, but the windloads and weights are well below the
TH-7 and 3-el SteppIR with the 40/30 option. There's still a bit more sway
than I'd like to see when a stiff breeze comes up, but I'm much more
confident that the tower can take it. I suspect that retracting it by 20
feet would increase the limit far beyond any winds we've had in this region
in the last 100 years. Nonetheless, I retract the tower when not in use (now
I have several guyed towers and antennas I can use instead.)
Note that U.S. Tower has changed the windload ratings on their tubular
towers since I bought mine. They didn't do anything to beef up the towers,
they just calculated the windload differently. It used to be 10 sq ft at 50
MPH for all the tubular towers. Now it's 15.5 @ 50 MPH and 5.5 @ 70 MPH for
my tower. The MA-550 is listed with a somewhat greater windload capacity,
probably due to the much greater moment arm on the base of my tower.
Frankly, I don't trust the U.S. Tower calculations, but that's just me.
Assuming you can't or don't want to do a guyed tower (which I believe is the
best alternative), I would consider one of the following:
1) Adapt the concrete base to one of the U.S. Tower lattice towers that has
a significantly higher windload. If the bolt pattern isn't compatible, which
I'm pretty sure is the case, you can cut off the tubular tower bolts, drill
holes in the concrete and install new bolts with epoxy. That's been
discussed on this reflector numerous times. Make sure you get expert advice.
2) Go ahead with the tubular tower, but plan on retracting it when not in
use. You may be able to get away with bringing it down 20 feet. Actually, I
would try to find out what the windload is with the tower at 40 and 30 feet.
Could be that you only have to go down 10 feet to be safe. Then you could
leave it at that height for most uses, cranking it up to the max for
contests or rare DX. I highly recommend the motorized version if you go this
route.
73, Dick WC1M
> -----Original Message-----
> From: w4lde [mailto:w4lde@numail.org]
> Sent: Sunday, June 22, 2008 11:40 AM
> To: TowerTalk; SteppIR Yahoo Group
> Subject: [TowerTalk] Advice on Tower
>
> I recently have moved to a new home, SW of Atlanta and have a SteppIR
> 3-element with the 30/40 add on sitting in the garage waiting on a new
> tower.
>
> I have approval from the HOA and County to erect a Tower, home deed
> was
> filed with modifications to allow a ham radio tower and antennas.
>
> Although no specific restrictions on height or style are place on me
> by
> the HOA or the deed modifications, when I started the process
> I thought a US Tower MARB-550 would look very neighbor friendly and be
> easy to get buy in's by all.
>
> I already have a base for the tower in-place but I am a little
> concerned
> as to the ratings for the MARB when comparing a TX-455 style.
>
> My Counties wind rating is 70MPH, we have already seen 50MPH gusts
> during a thunderstorm recently.
>
> The tower is rated at 9 sq/ft at 70 MPH and 61#'s weight, the
> specified
> wind load by SteppIR is 8.1 sq/ft. at 58#'s I would elect to use the
> rotating n base thus keeping the rotor at the bottom of the tower and
> reducing the extra weight and little load.
>
> Now the questions?
>
> 1. Since I am almost right on top of the spec's, wind load and
> weight,
> should I abandon this Tower selection.
> a. As it now stands I have a 3'-6" x 3'- 6" x 6" concrete base
> with
> rebar and mounting bolts per the US tower spec's
>
> 2. Is there a way to adapt any type of rotating guy device to this
> type of tower?
>
> 3. I would also like to hear from SteppIR - US Tower users with
> similar
> setups.
>
> Plans are to erect the Tower some time in mid September or early
> October
> depending on deliver time so I need to get the order underway.
>
> Maybe I can improve on the ratings by not keeping the Tower raised to
> the maximum height unless I am chasing some DX expedition or am I
> dreaming?
>
> 73's de
>
> Ron W4LDE
>
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