" bravely wrote to "All" (24 Mar 05 09:41:39)
--- on the heady topic of " Loading Coil Q"

nm From:
nm Xref: aeinews rec.radio.amateur.antenna:27218

nm For industrial use, I have a tower and
nm beam I can drag around...I've drug it to the last four field
nm days...Wonder if I'm gonna make it five..?? It's a lot of
nm work for a two day deal....:/ I'm almost tempted to take a
nm field day vacation for a year...Have to see...MK

Lately I've seen TV media trucks with a type of rather wide pole of
what seems to consist of a large coiled up ribbon. When they get to a
location they unfurl the tube and it raises the microwave antenna.
They seem to be made out of plastic or carbon, not sure. The one I saw
went up about 30 feet. It seems very lightweight in any case. Anyone
here know what it is?

A*s*i*m*o*v

.... "Not a morning person" doesn't even begin to cover it.

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.

Dear "Asimov" w/o a call sign:

If you look closely, you will see that the "coiled up ribbon" is a
transmission line or a pair of transmission lines. By elevating the
antenna, which must be connected to the truck through at least one
transmission line, much improved transmission range is effected.

73 Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:

"Asimov" wrote in message
...
" bravely wrote to "All" (24 Mar 05 09:41:39)
--- on the heady topic of " Loading Coil Q"

snip

Lately I've seen TV media trucks with a type of rather wide pole of
what seems to consist of a large coiled up ribbon. When they get to a
location they unfurl the tube and it raises the microwave antenna.
They seem to be made out of plastic or carbon, not sure. The one I saw
went up about 30 feet. It seems very lightweight in any case. Anyone
here know what it is?

A*s*i*m*o*v

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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"Cecil Moore" wrote in message
...
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.
--
==================================

Cec,

(1) A mobile antenna is NOT a 1/4-wave resonant, base-fed,
ground-mounted loaded vertical which behaves reasonably predictable.

(2) A mobile antenna is a relatively-isolated-from-ground, 1/2-wave
resonant, loaded, off-centre-fed vertical dipole which defies rational
analysis.

(3) It is impossible to separate the many different behaviour modes
and effects, differentiate between them and allocate relative
magnitudes. One has to be careful to control one's imagination when
describing effects.

I've never seen one near to, but I understand "screwdriver" type
mobile antennas are akin to long helicals specially at the lowest
operating frequency.
----
Reg.

Reg Edwards wrote:
I've never seen one near to, but I understand "screwdriver" type
mobile antennas are akin to long helicals specially at the lowest
operating frequency.

All of the screwdrivers with which I am familiar use a
3-6 foot straight bottom section. The 2 foot max coil is
approximately in the center with a 3-6 foot whip. The coil
goes in and out of the 3-6 foot bottom section for tuning.

Do you subscribe to the theory that a one foot long coil
radiates approximately the same RF as a one foot long
piece of wire? With any reasonable diameter of coil, the
resistance in that one foot long coil will be greater
than the resistance in one foot of wire. For instance,
the circumference of a coil may be one meter while the
turn spacing may be one centimeter. What size wire do
you need to compensate for that 100/1 ratio?
--
73, Cecil http://www.qsl.net/w5dxp

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