[Antennas] Homebrew 40m vertical questions

Sat Apr 30 23:23:43 EDT 2005

Howdy all,
Back again with part 2 of this mess....
I'd like to just cover the physical aspects of the antenna in this thread
if we may. Meaning after we get this all figured out... then I will start
another one related to care anf feeding of the beastie we settle on.
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First here are the surroundings for the antenna (s) I'm considering.
Location I am working with is as follows:
Building is a 2 story (4 apartment unit) 2 upstairs and 2 downstairs.
Shack is on the bottom floor / outside / corner wall of the building.
bottom floor (up to 8') is brick siding 
2nd floors siding is 9'8" vertically of sheetmetal siding 
The 2nd story flat roof is @ a height of 19' above ground.
The roof is totally flat and made of rolled shingle. 
The size of the roof is 29'x49'.
Antenna base will be placed on a metal plate about 2.5'x2.5' 
This will be located in the center of the roof.
Straddling the top plates of both interior walls of the 2 top floor
ajoining apartments for strength as well as convenience. 
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A self supporting 8'-12' vertical with at 4 (wire) radials and NO guy
wires is the structure I am limited to. This will be mounted on the
2.5'x2.5' 1/4" aluminum base plate (to which the radials will attach)
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For the bottom mast stick either:
5' or 6' of aluminum
http://www.dxengineering.com/products.asp?ID=88
60'' long, heavy duty 1/2'' thick O.D., 6061 Aluminum mast 
72'' long, heavy duty 1/2'' thick O.D., 6061 Aluminum mast
or a 
60" long 3/4 inch O.D., stainless steel tubing
http://www.texasbugcatcher.com/cata/tbcord.htm
I like the larger diameter myself...?
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next comes the coil ...
I believe the #640 for 40 thru 10 meters would be my best bet
Coil is just over 6-1/4 inches in diameter and are wound with 12
gauge wire at 4 turns-per-inch 
http://www.texasbugcatcher.com/photo/680-089.jpg
the above is the # 680 80m model 
I don't have the #640 40m models pic
or...????? 
I'm open for suggestions here, anyone?
prefer it to already have the threaded ends on it...
I'm more concerned with efficiency than bandwidth, my thinking being the
hi Q will mean more to me with the 25w or less power I will be using.
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now comes some of the questions I have as to which way to go:
I already have one of the 20" aluminum capacitance hats on hand.
http://www.texasbugcatcher.com/tag/k5vv-01.gif
I also have eight 23" (1/8" diameter) stingers I had previously attached
to that hat extending the "center to tip" length of the hat to 32"
or looking at it from "tip to tip" across the hat entire hat for a total
of 64".
This hat is ususally placed at the top of the coil as in this picture:
http://www.texasbugcatcher.com/one/tbc-sketch.htm#TEXT
however there is a 12" mast available to use between the coil and hat.
there is this comment to consider:
"Radiation efficiency is greatest when the capacity hat is located
outside the field of the coil, ideally at the top of the whip! In
practice, we have found that the signal loss which results from placing a
capacity hat about one coil diameter above the coil is not more than 2dB
from that "top-of-the-whip" ideal nor more than 1dB from the best
practical separation of two coil diameters." found here:
http://www.texasbugcatcher.com/cata/tbcspec.htm#HATS
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then a top stinger (I have a 102" whip laying around)
or------- 
coming out of the top of the coil 
using a 3' 4' or 5' 1/2" O.D. vertical element
http://www.dxengineering.com/products.asp?ID=88 
DXE-MT-36 / DXE-MT-48 / DXE-MT-60
36'' long, heavy duty 1/2'' thick O.D., 6061 Aluminum mast 
48'' long, heavy duty 1/2'' thick O.D., 6061 Aluminum mast 
60'' long, heavy duty 1/2'' thick O.D., 6061 Aluminum mast 
then placing the top hat I discussed above on the top of that, with no
stinger above this. I would be up to an over all height of 8' to 10' like
this, with the 20" highly visable spoked hat on top. hmm.....
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I guess some questions at this point would be if I had the option of these
different configurations (above the coil) which one(s) and why/why-not
1) 20" capacitance hat with 102" stinger
2) 64" capacitance hat with 102" stinger
3) 12" mast with 20" or 64" capacitance hat with 96" stinger
4) 36" mast with 20" or 64" capacitance hat and no stinger
5) ?????
 
Sam Morgan
KA5OAI
mailto:ka5oai at qsl.net