Announcement

**mikebg** · 11-30-2009, 12:17 PM

Description of antique "FastCoil"

Description of above figure:
In cross section the coil likes a transformer bobbin: paper sheet - wire sheet. To avoid proximity effect in the horizontal direction, between wire turns there are turns of cotton thread having the same diameter as the wire. Similarly, separation is done and in the vertical direction with paper. Note that the thick of paper layer is as the diameter of the wire. As in 19th century the price of polished [one more term as "lacquered" and "varnished"] wire was significantly greater than the cost [anoter term for "price"] of insulated wire, some incompetent researcher would consider that the thread is used to insulate adjacent turns. This mistake was made about 100 years ago, but that is another story.
The term "FastCoil" was introduced by Joseph Rogowski. It means that in the conservative parameters of the coil: L and C accumulates a minimum of energy during the TX pulse. It is believed that after the end of TX pulse, this small amount of energy would be faster to dissipate as heat in order receiver to take first sample as soon as earlier.
Since the energy accumulated in the capacity C is given by the formula
E = (C.V ^ 2) / 2,
we can consider that "FastCoil" must have a minimum equivalent capacity C. The same can be said of inductance L. The formula for energy stored in the coil is
E = (L.I ^ 2) / 2.
Since L and C determine natural resonance frequency of coil according formula
f = 0,159 / sqrt (L.C),
we can say that the term "FastCoil" used in Time domain, in Frequency domain means "Coil having high natural resonance frequency" (the coil is not connected to a "Slow Network" :-).
To reduce the equivalent capacity C of the coil, we should take the following measures:
1. To place turns apart from each other.
2. To place shield at distance from the "Live sheet" of coil.
3. Area of the shield and "Live sheet" to a minimal surface. [strange mashine translation in English]
In the ancient design are implemented the three measures above mentioned.
It is not desirable to reduce the inductance L of the coil because it is required to have a large number of turns. This is true both for the TX coil, and the RX coil. However, inductance L of a coil depends on the square of the number of turns N and is calculated by the formula:
L = A.N ^ 2,
where the coil constant A depends on the geometric dimensions. As is described by Rogowski, tighten of turns increases inductance. From this follows that if we light spaced turns, constant A decline without reduction of efficiency (Ampere-turns).
The increased distance between turns has third positive effect - reducing the unequal distribution of current in wire cross section known as "proximity effect". Litz wire has proximity effect.
To be continued in next posting with description of mold (cast, last) for coil winder and how to make low ohm winding for TX better than with Litz wire.

**Tinkerer** · 12-02-2009, 08:39 PM

Polypropylene "cotton" spacer for coil

Originally posted by mikebg View Post

Description of above figure:
In cross section the coil likes a transformer bobbin: paper sheet - wire sheet. To avoid proximity effect in the horizontal direction, between wire turns there are turns of cotton thread having the same diameter as the wire. Similarly, separation is done and in the vertical direction with paper. Note that the thick of paper layer is as the diameter of the wire. As in 19th century the price of polished [one more term as "lacquered" and "varnished"] wire was significantly greater than the cost [anoter term for "price"] of insulated wire, some incompetent researcher would consider that the thread is used to insulate adjacent turns. This mistake was made about 100 years ago, but that is another story.
The term "FastCoil" was introduced by Joseph Rogowski. It means that in the conservative parameters of the coil: L and C accumulates a minimum of energy during the TX pulse. It is believed that after the end of TX pulse, this small amount of energy would be faster to dissipate as heat in order receiver to take first sample as soon as earlier.
Since the energy accumulated in the capacity C is given by the formula
E = (C.V ^ 2) / 2,
we can consider that "FastCoil" must have a minimum equivalent capacity C. The same can be said of inductance L. The formula for energy stored in the coil is
E = (L.I ^ 2) / 2.
Since L and C determine natural resonance frequency of coil according formula
f = 0,159 / sqrt (L.C),
we can say that the term "FastCoil" used in Time domain, in Frequency domain means "Coil having high natural resonance frequency" (the coil is not connected to a "Slow Network" :-).
To reduce the equivalent capacity C of the coil, we should take the following measures:
1. To place turns apart from each other.
2. To place shield at distance from the "Live sheet" of coil.
3. Area of the shield and "Live sheet" to a minimal surface. [strange mashine translation in English]
In the ancient design are implemented the three measures above mentioned.
It is not desirable to reduce the inductance L of the coil because it is required to have a large number of turns. This is true both for the TX coil, and the RX coil. However, inductance L of a coil depends on the square of the number of turns N and is calculated by the formula:
L = A.N ^ 2,
where the coil constant A depends on the geometric dimensions. As is described by Rogowski, tighten of turns increases inductance. From this follows that if we light spaced turns, constant A decline without reduction of efficiency (Ampere-turns).
The increased distance between turns has third positive effect - reducing the unequal distribution of current in wire cross section known as "proximity effect". Litz wire has proximity effect.
To be continued in next posting with description of mold (cast, last) for coil winder and how to make low ohm winding for TX better than with Litz wire.

mikebg,

it is very interesting to see that most ideas have been tried a long, long time ago.
Lucky for us, we have modern, better materials available.

I am much interested to repeat some of the old methods with modern parts and materials.
Below is one of my tests, using a Polypropylene "cotton" thread as spacer between the magnet wire.
http://www.geotech1.com/forums/showp...8&postcount=12

What do you think of it?

Tinkerer

**lucifer** · 12-03-2009, 07:07 AM

What exactly is achieved with this cotton thread?

**Tinkerer** · 12-03-2009, 09:15 AM

Originally posted by lucifer View Post

What exactly is achieved with this cotton thread?

The cotton thread is a spacer. More space between the windings reduces the
inter wire capacitance.
Less capacitance means a faster coil.

Tinkerer

**lucifer** · 12-03-2009, 10:23 AM

Originally posted by Tinkerer View Post

The cotton thread is a spacer. More space between the windings reduces the
inter wire capacitance.
Less capacitance means a faster coil.

Tinkerer

I see but this will reduce the inductance as well. Suppose always when we gain something we lose something else.

Announcement

Ancient "FastCoil"

Ancient "FastCoil"

Comment

Comment

Comment

Comment

Comment