At 01:22 PM 4/12/02 -0400, Gary wrote:
>After the fourth iteration I have made the 'final' decisions. The Spring
>(?) antenna/tower project will consist of 70' AN Wireless tower (*massive*
>windloading spec!), TH7 at 5' above top of tower, Cushcraft A3WS w/30m
>add-on at 80', and 40m rotatable dipole at 85'.
I think this spacing may turn out to be too close, particularly between the
A3WS and the TH-7.
>All of this will be
>(hopefully!) turned by a HAM IV rotor using a <fill-in-the-type> 20' mast.
I think this is asking a bit too much of the Ham IV. A Tailtwister would
be a stronger choice.
>(five feet of mast 'nested' in tower) I will be making one 100' run of
>LMR-400 to an Ameritron remote coax switch at the top of the tower. (Two
>'spare' coax runs will be installed on the tower.)
Wouldn't it be better to have the relay box at the bottom of the tower,
more accessible for maintenance.
>After reading the TT mail for about a year, I belive I have adherred to the
>basic tenents and good practices espoused on TT. I have no capability of
>'modeling', so I am asking if anyone sees any major problems with this
>arrangement. I have the *written* authorization/building permit of
>zoning/codes officer to install a tower (et al) to a max height of 85'.
>
>Is there anything I should -- nay, MUST -- change? What type of mast will
>*reliably* keep all this aluminum in the air for the next ten years?
You should do the calculations, but 20 feet of mast with two good sized
yagis and a rotary dipole virtually demands a heavy chrome-moly mast, I
think. I would put the TH-7 as close to the top of the tower as you can --
5 feet is unnecessarily far, and cramps your other choices too much. Your
proposed 5-foot spacing between the two yagis virtually guarantees interaction.
> Is my
>plan to make all booms parallel wise? Or should I make the 40m dipole
>perpendicular to the TH7/A3WS booms. I'm not sure if the spacing between
>the TH7 and the A3WS is 'optimal', but have little latitude because of the
>structure's height restriction.
You might want to seriously consider gamma-matching the boom of the TH-7
for 40m, because this would allow you to put a good deal more space between
the two yagis and reduce the total wind loading, as well as the rotary
inertia of the whole deal. Check the towertalk archive for any number of
messages by N4KG describing how he does this.
I presume that your choice of the TH-7/A3WS combination is dictated by what
you already have. If not, and depending on your budget, the Force 12 XR-5
and 30/40m WARC yagi are alternatives worth considering.
Of course, all this free advice is worth about what you pay for it!
73, Pete N4ZR
Check out the World HF
Contest Station Database at
www.pvrc.org
|