Greetings, fellow towerophiles --
In the course of a tower construction project involving Rohn 45G, I
made a discovery that I think has gone un-noticed by amateur tower owners and
builders. In their catalog, Rohn provides drawings for 70, 90 and 110 MPH
along with different tower heights. At the top of each sub-drawing is a
little rounded box that has the "allowable projected area" in square feet.
The round member number on the left is the one that everyone uses for
planning and construction. In the general notes with each drawing, Rohn
builds in (deducts) an amount equal to a couple of feedlines; one 1/2 inch
and one 7/8 inch cable for 45G. So far so good.
Upon further reading of the notes, it states that "tower designs include
three side arms, symmetrically placed having a total effective projected area
equal to 8.0 square feet". For commercial antenna mounting, this is normal
and Rohn has allowed for it. In the case of amateur installations, the side
arms are not used. In other words, Rohn has deducted 8.0 square feet already
from the tower loading figure and the number that is on each drawing is the
NET tower load. For amateur loading, you can ADD the 8.0 square feet onto
the posted figure to come up with a more appropriate wind load number. This
same situation applies to 55G as well.
One note of caution; Rohn states that the side arms are "symmetrically
placed" meaning the load is distributed over the whole tower, not just on the
top. While it may not be appropriate to add the whole 8.0 square feet to the
top of the tower, it does show that there is additional tower capacity
available that is not obvious from the number in the box. Maybe you do learn
something everyday.
Comments?
73, Steve K7LXC
TOWER TECH -- professional tower supplies and services for amateurs
>From fisher@hp-and2.an.hp.com (Tony Brock-Fisher) Wed Aug 14 13:27:01 1996
From: fisher@hp-and2.an.hp.com (Tony Brock-Fisher) (Tony Brock-Fisher)
Subject: Radio Shack Headphones
Message-ID: <9608141227.AA26062@hp-and2.an.hp.com>
Robert, KA5WSS writes:
>I'd like to spend less and I'm considering that Radio Shack
>boom mike and headphones.
I wrote a review of the Radio Shack Pro MX50 headphones with boom mike,
which appeared in NCJ about a year ago. As it is lengthy, I'll send
the complete text to Robert separately. The bottom line was:
1. Ear pads are small.
2. Stock mike sucks.
3. Heil HC-4 CAN be installed after delicate surgery.
4. Price is right.
Since writing the article, in the course of contesting, the headset has
broken twice. Once when an operator fell asleep leaning on his hand; once
mysteriously at Field Day. Radio Shack sells replacement parts.
At $39 bucks, if you're careful with it, it's a good deal...
-Tony, K1KP, fisher@hp-and2.an.hp.com
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