At 15' separation, I don't think the 80m yagi would "see" the MonstIR, and
as you say the MonstIR elements can be retracted. The MonstIR will probably
be affected by the 80m yagi on 20m or 10m, but I doubt it will be
pathological. If it turns out to be a problem, the antennas can be offset 90
degrees.
I'm more concerned about the windload. It's nominally 42 sq. ft., but
remember that the US Tower windload ratings assume the entire load is at
the apex of the tower. The 17 sq ft of windload that will be up the mast 15
feet above the top of the tower will exert far more load on the tower than
it would if it were at the top of the tower. You need an engineer to
calculate how much, but I'm sure it's a bunch.
Also, it's not clear just how much windload the HDX-689 can take. US Tower
doesn't publish the specs on its website. HRO has some specs, but they don't
correlate with what's on the US Tower website. US Tower says the smaller
HDX-589 can handle 60 sq ft of windload at 50 mph, but only 16 sq ft at 70
MPH. The HRO site says all towers can handle 30 sq ft at 50 MPH and the
HDX-689 can handle 60 sq ft of windload at 50 MPH. They don't say how much
it can handle at 70 MPH, but judging from what they publish about the
HDX-589, I would bet the rating is significantly lower. =
The important thing to bear in mind is that virtually every county in the US
has a maximum wind rating of at least 70 MPH, and in many places it's much
higher. The 50 MPH number is pretty-much meaningless, and if you live in a
windy place, the 70 MPH figure could be meaningless, too. My guess is that
the HDX-689 isn't rated for 42 sq ft at 70 MPH, but I don't know. You'd have
to ask US Tower. But even if it is rated for that much, the actual windload
at the top of the tower will be considerably greater due to the 2-el 80m
beam being 15 feet above the top of the tower (oh, and that 3" mast is going
to add some windload of its own, too.)
I'd get a PE involved to make sure this rig will fly.
I sort of question the premise of putting that much antenna on a crankup.
Seems like too much. Is he going to tilt it over for maintentance? That'd be
scary! IMHO, those antennas really beg for a guyed tower.
BTW, your friend might want to consider the new SteppIR 4-el "Dream Beam 36"
instead of the MonstIR. The windload is about half, it weighs 100 lbs less,
and the forward gain is within 1/4-1/2 dB of the MonstIR on all bands except
6m. F/B is comparable or better, though not as good on 40m (21 dB vs 30 dB,
which is still very good.) It also has an 80m dipole option. If it turns out
the HDX-689 can't handle the windload, a smaller crankup with a single Dream
Beam 36 just above the top plate would give him most of the performance he's
looking for at a fraction of the cost and hassle. No interaction problems,
either.
73, Dick WC1M
> -----Original Message-----
> From: Edward Sylvester [mailto:navydude1962@yahoo.com]
> Sent: Wednesday, December 10, 2008 7:13 PM
> To: towertalk@contesting.com
> Cc: Paul F. Merrill
> Subject: [TowerTalk] Spacing between Antennas
>
> Greetings to the Group!
>
> My friend is going to be installing a dream antenna set up for HF, on
> one tower. Here's the scenario:
>
> 1 ea UST HDX689 89' tower
> 1 ea 2 ele Optibeam 80 yagi (17 sq ft)
> 1 ea MonstIR 6-40m yagi (25 sq ft)
> 1 ea 24' three inch chromally mast
>
> The plan is to mount the 80m antenna towards the top of the mast and
> the Steppir underneath. Obviously the more space between the two, the
> better. Will 15' of separation between the two suffice, especially
> since the Steppir can retract all of its elements?
>
> Also, is the system beefy enough? Your opinions are welcome.....
>
> 73,
> Ed NI6S
>
>
>
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