On 8/5/12 9:03 AM, Grant Saviers wrote:
> 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.
The problem is that the "total load" on the tower is the antenna load
PLUS the load of the tower. The tower's cross sectional area doesn't
change, so you can't just scale the antenna area by the square of the
wind speed. There are towers that have some capacity available at, say,
75 mi/hr and zero at 85.
Vice versa, you probably actually pick up more "rated area" at lower speeds.
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).
>
That does have an effect, but I think the bigger issue is that the tower
cross sectional area is quite large compared to the antenna. A 1 foot
wide tower that is 45 feet tall is 45 square feet.
Another complication is that without looking at the analysis, you don't
know where the critical load is. Is it at the base or somewhere up the
tower. For uniform cross section towers it's easy.. the load is biggest
at the base, but for crankups and tapered towers (BX, for instance)
that's not necessarily the case. (and that's where the wind profile
starts to get really important)
> 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!
>
> Grant KZ1W
>
>
> 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.
>>
>> 73,
>> Steve W1SRD
>>
>>>> Would like to be confident my LM-470 can handle a DB36 (17.5 sq ft
>>> using
>>> 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.
>>> Cheers,
>>> Steve K7LXC
>>> TOWER TECH -
>>> Professional tower services for hams
>> _______________________________________________
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