I think the steppIR specs relate to the antenna wind survival and sq ft
does not change with wind speed.
Wind load is proportional the the square of the velocity, so a new L470
rated 85mph/18 sq ft should be good for (85^2/75^2)*18 = 23 sq ft at 75
mph. Its not that simple in reality since the calcs now use a variable
wind speed from the base to the antenna location, but after 35' in
height or so the height of the tower doesn't matter, but for exposure it
does. (different wind field profile).
You have another choice with the 4L - you can have a single 30/40m
trombone for 11.7 sq ft. That gives 40m as a dipole with a possible(?)
survivable wind loading. Plus, the DB36 at 70' won't yield all the
antenna can deliver for 40m.
My gut feel is still the DB36 is a lot for an old tower with dubious
specs, unless you are sure that wind will never exceed 45 mph or so.
Also, a concern not mentioned so far is the rotator torque loading from
a very heavy 36' boom antenna (mast + antenna + rotator will exceed
200#) - that could be a serious problem. Have you checked to see that
an adequate rotator (eg orion) will fit? Seems dubious. My orion just
barely fit an HDX589 - under 0.1" clearance. My SDX2800 is struggling
with the 4L steppIR (no 40 trombone).
A DB36 may cost more than a new 470!
On 8/3/2012 10:25 AM, Steve Dyer W1SRD wrote:
> Thanks Steve.
> It's El Dorado country. From Champion, it's a 75 MPH windspeed county.
> Current specs for a Tashjian:
> 1. LM470 is 18 sq ft @ 72' for 85 MPH w/ 3 sec gust per TIA-222-G.
> 2. WT51 is 13 sq ft @ 53' for 85 MPH w/ 3 sec gust per TIA-222-G.
> I have some older Tri-Ex documentation that lists the LM-470 at 16 sq ft
> in 60 MPH and the WT51 at 9 sq ft in 50 MPH.
> Current specs for SteppIR:
> 1. DB36 17.5 sq ft for 100 MPH per EIA-222-C
> 2. 4 EL 9.7 sq ft for 100 MPH per EIA-222-C
> Can I assume the antenna wind loading is less at 70/75 MPH?
> Again, looking to be safe, not get a permit.
> Steve W1SRD
>>> Would like to be confident my LM-470 can handle a DB36 (17.5 sq ft
>> 100 MPH EIA-222-C) .
>> I believe the 4 El SteppIR (9.7 sq ft) is fine on the WT51
>>> Usual caveats about crank it down in the wind already noted. Also, the
>> Tashjian specs are for a *new* tower. I don't know the vintage or
>> history of the LM-470 I have. It came with the property.
>> What do the current specs say? In the old days - pre-EIA-222 -
>> crank-ups were rated @ 50MPH. Your tower might be that vintage.
>> You didn't include enough specifics. What is your local windspeed
>> rating? You can find an older EIA-222 windspeed rating for all US
>> counties at http://www.championradio.com/county.windspeed.data.html.
>> San Mateo County, CA, is a 70 MPH windspeed zone. Not a high wind area
>> (70 MPH is the minimum windspeed rating) but crank-ups have to be
>> treated with much care wind-wise.
>> You could probably get away with it but if you need to get a
>> building permit, forget it.
>> Steve K7LXC
>> TOWER TECH -
>> Professional tower services for hams
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