[Antennas] 40m Apartment antenna saga continues <g>

[Sam Morgan]

Hello all,

Update on the 40m apartment antenna for my 2nd story, 49'x29', rolled
shingled, flat roof. And some requests for help on the following related
items. 

The landlord has oked a 30" or 36" (mast coil and capactance top hat)
antenna with a 60" stinger above that. So I would like to have opinions of
both configurations ... with a 30" or a 36" bottom part.

For the coil I want to use the #640 (no picture) but the details are:
http://www.texasbugcatcher.com/cata/tbcspec.htm#6inch

<quote>
The #640 coil has a winding length of just over three inches yielding a
slightly over-square form factor. It is designed to give superior
performance on 10 thru 40 meters. The #640 resonates on the 40 meter US
'phone band with a 5-foot whip without the use of a capacity hat.
Addition of a hat will, of course, allow the use of an even shorter whip
in those installations where this is desirable. The #640 will require a
whip at least 72 inches long for full 40 meter band operation.
</quote>

I am unsure of is the best configuration with in the 30"/36"
part. I have a 20" hat from texasbugcatcher I will use for the
capacitance hat.

Reading from the following: 
http://www.texasbugcatcher.com/cata/tbcspec.htm#HATS

<quote>
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. 
</quote>

I have the impression, if I am to use their 6" coil, I need to use 12"
between the top of the coil and the hat. With the 36" (bottom) total
configuration this leaves me with only 18" of mast below the coil. If I
end up limited to 30" total it would leave me with only 12" below the
coil.
------

>From reading here:
http://www.texasbugcatcher.com/cata/tbcspec.htm

<quote>
 The GLA Systems "Texas BugCatcher" antenna can be assembled to meet
almost any mechanical configuration necessary to fit a particular
installation. Our masts are available in stock lengths from 6 inches to 5
feet, with custom lengths of up to 8 feet. However, it is important to
remember that the center loaded antenna has the highest theoretical
radiation efficiency (as much as 150% higher than a similar base loaded
system). With this in mind, we strongly recommend a base mast of 3 to 5
feet and a whip length of 4 to 8 feet. Variations from this range will
generally give excellent results but should be used only when
installation requirements prohibit true center loading. When selecting
mast length, consideration should be given to several factors:

    * A mast should be used that will allow the bottom of the coil to be
at least 6 inches above the roof line of the vehicle.
    * This should be increased to 12 inches if the edge of the roof is
close to the antenna as would be the case with vans, suburbans and other
"sport utility" type vehicles.
    * When the overall length of the antenna becomes a problem, remember
that, in most cases, the length of the whip (or effective length of the
whip-hat combination) is more important than the length of the mast so
long as the ratio of whip length to mast length does not exceed about
two. For example, if an antenna consisting of a 5-foot mast, coil and 7-
foot whip must be shortened by 12 inches it would be better to take the
12 inches off the mast (reducing the radiation resistance by 20%) than to
take it off the whip (requiring an increase in coil inductance and losses
reducing antenna current by as much as 14% and radiation efficiency by
26%) as long as the coil is kept above the roof of the vehicle. A better
approach to this situation would be to add a capacity hat. 
</quote>

So.....
I know this is way to short to perform very well, but it's the best I can
get from the land lord, so I would appreciate any comments, other than to
move, which is not an option. 

Any one have a link to a 4nec2xx view of my pending dummy load? <g>

Sam Morgan 
KA5OAI
mailto:ka5oai at cox.net