Buck wrote:
I have aluminum poles of three diameters, 1.5, 1, and .5 inches. I
have enough of the first two to create a pole of about 15 feet each
and about 8 to 9 feet with the 1/5 inch poles. The poles are no more
than 4 feet long each, but they are able to tuck inside each other,
at
least for the same diameter.

What I want to do is to make up a 'kit' with these that I can take to
the field to setup whatever antenna I want to use at the time. For
example, I might set it up as a 20 meter vertical in one
configuration, but in the evening, I might reconfigure it to 75
meters
or 15, or 30 or whatever I desire.

One consideration is to create a high-q coil for it. I am thinking
of
using 1/4 inch copper tubing to make a ten to twelve inch diameter
coil so I can match it to the lower bands.

That thickness of coil is overkill....

I have been reading that
base loaded coils have to be matched to the antenna, but center
loaded
verticals are more closely matched to 50 ohms.

The loading coil is used to tune out the capacitive reactance.
The input impedance will vary to length of the vertical, coil loss,
ground loss, etc..Just because a center load is used, doesn't always
mean the input impedance will end up 50 ohms..

from what I have been
reading, the center loaded coil requires more turns and a top loaded
coil even more.

Yes.

A capacity hat is also a viable option.

It's the best option. And top loading is best using capacitive
loading, where as if using a coil for top loading, the coil losses
overshadow any increase in efficiency from the improved current
distribution, etc.. Top loading using wires is the best system you
could use, if you can swing it.

I am wondering how much I am helping or hurting myself with the
larger
coil or if I would be better off with a smaller coil.

It's not that critical...

One thought is
to connect x number of poles, then the coil, then another set of
poles
or a whip to make the final adjustment for the match or to match the
antenna by tapping the coil.

I am open to all suggestions, but I am interested in knowing the best
location for the coil and the best size for it.

The *best* location for a loading coil is appx 2/3 to 3/4 up the
radiator. But even half way up is fine. Base loading is the least
efficient, and has the worst current distribution. But it uses the
least number of coil turns, and would have the least coil loss.
Being low, it's the easiest to adjust also...
Don't forget that the ground system for a short loaded vertical,
becomes even more critical than for a full 1/4 wave vertical.
It's for this reason that I usually avoid verticals for portable
use, unless the ground issue can be dealt with. IE: at the beach
is a good place for verticals...But in the boonies, on lossy, rocky,
ground, they may not do so hot if you have no radials. In that
case, I would take the vertical "sticks", and hang a dipole from
them...:/ My mobile antenna is my usual "portable vertical"...MK

" bravely wrote to "All" (23 Mar 05 15:13:22)
--- on the heady topic of " Loading Coil Q"

nm It's the best option. And top loading is best using capacitive
nm loading, where as if using a coil for top loading, the coil losses
nm overshadow any increase in efficiency from the improved current
nm distribution, etc.. Top loading using wires is the best system you
nm could use, if you can swing it.

How about distributed loading?

A*s*i*m*o*v

.... May you find the light and walk the mountain tops.

On Wednesday, 23 Mar 2005 22:30:32 -500, "Asimov"
wrote:

How about distributed loading?

Hi OM,

The wrong way, and it is called air cooled resistance. If you simply
did a Googles group search using that very query, you would find a
trove of hits.

73's
Richard Clark, KB7QHC

Asimov wrote:
How about distributed loading?

Helical antennas are not as efficient as
other forms of loading.
--
73, Cecil http://www.qsl.net/w5dxp


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"Cecil Moore" bravely wrote to "All" (24 Mar 05 09:19:15)
--- on the heady topic of " Loading Coil Q"

CM From: Cecil Moore
CM Xref: aeinews rec.radio.amateur.antenna:27211

CM Helical antennas are not as efficient as
CM other forms of loading.

Considering that sometimes efficiency isn't quite as important, like
for example reception, are helical antennas less used simply because
the math is a little harder or not discussed enough? I recall seeing
this type being popularized during the old CB craze in the mid 70's.

A*s*i*m*o*v

.... "Ol' Frothinslosh": The pale, stale ale with the foam on the bottom.

Asimov wrote:
"Cecil Moore" wrote:
CM Helical antennas are not as efficient as
CM other forms of loading.

Considering that sometimes efficiency isn't quite as important, like
for example reception, are helical antennas less used simply because
the math is a little harder or not discussed enough?

Probably a collection of reasons. A high-Q loading coil in
the center of a vertical is much more efficient than spreading
the loading out over the entire antenna. Helical antennas are
hard to wind and are generally wound on PVC pipe which also
reduces efficiency and increases wind load. Even more efficiency
is lost without a good ground plane. If one doesn't compromise
on the ground plane, why compromise on the antenna? Most of us
old timers have tried helicals and given up on them - lots of
effort to achieve a poor performance.
--
73, Cecil http://www.qsl.net/w5dxp

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"Cecil Moore" wrote
A high-Q loading coil in
the center of a vertical is much more efficient than spreading
the loading out over the entire antenna.

============================

Wrong! Spreading a multi-turn coil allows a MUCH thicker wire
diameter to be used with spaced turns. Also the coil diameter can be
increased to minimise the number of turns.

Result : higher coil Q, lower coil loss, greater efficiency.
----
Reg.

Reg Edwards wrote:

"Cecil Moore" wrote

A high-Q loading coil in
the center of a vertical is much more efficient than spreading
the loading out over the entire antenna.

Wrong! Spreading a multi-turn coil allows a MUCH thicker wire
diameter to be used with spaced turns. Also the coil diameter can be
increased to minimise the number of turns.

Result : higher coil Q, lower coil loss, greater efficiency.

Got to disagree with you on that one, Reg. Mobile shootout
field strength measurements put all the helicals, no matter
what wire diameters were used, considerably down from the
well-designed bugcatchers and screwdrivers. That meter of
wire in each turn of the helical has more resistance than
the centimeter of radiating bottom section that it replaces
at the feedpoint.

It is well known and accepted that moving the mobile loading
coil from the center of the antenna to the base will reduce
the efficiency even though the inductance required for loading
is decreased. With a helical, part of the loading coil is
at the base and that's simply a bad idea when efficiency
is important.

One mobile, in particular, should have performed well. It
was made from 1/4 inch copper tubing with a large diameter
and proper spacing between turns but it was about equal to
a Hustler and considerably down from the top performer which
was top-loaded.

What wins the mobile shootouts is the longest possible straight
bottom section under the coil where the highest current occurs.
That maximum current occurs all up and down that straight bottom
section when a good top hat is added to the antenna. I once won
the shootout competition by putting all the loading (coil+top-hat)
at the top of the antenna using cheap stuff from my junk box.

If one wants to win a mobile shootout, one cannot afford to
install a helical coil at the maximum current section.
--
73, Cecil http://www.qsl.net/w5dxp


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