Does anyone know why the distributed winding capacitance of a loop
antenna, or any inductor, degrades the efficiency?

It would seem that a loop antenna with 100pF of winding capacitance in
parallel with a external capacitor of 200pF would resonate at the
same frequency as a antenna with no winding capacitance and a external
capacitor of 300pF,but apparently that's not the case.

The best explanation I got was that winding capacitance represents
'low Q' and a external tuning capacitor represents ' High Q'

What is the difference between high and low Q, and why should a loop
antenna with no winding capacitance perform any better than one with
50% of the total capacitance in the windings? Where is the energy
loss?

Thanks,

-Bill

On 28 Apr 2007 21:32:18 -0700, Bill Bowden
wrote:

Does anyone know why the distributed winding capacitance of a loop
antenna, or any inductor, degrades the efficiency?

Hi Bill,

For the usual reasons: Resistance (not capacitance).

It would seem that a loop antenna with 100pF of winding capacitance in
parallel with a external capacitor of 200pF would resonate at the
same frequency as a antenna with no winding capacitance and a external
capacitor of 300pF,but apparently that's not the case.

It could be the case, your mileage may vary.

The best explanation I got was that winding capacitance represents
'low Q' and a external tuning capacitor represents ' High Q'

You got bum explanations then.

What is the difference between high and low Q, and why should a loop
antenna with no winding capacitance perform any better than one with
50% of the total capacitance in the windings? Where is the energy
loss?

It seems you may be, instead, writing about Unloaded and Loaded Q.
Loaded Q would be that found in service (in the actual application,
whatever that might be). Unloaded Q would be that found at the bench
with no other attachments. The Loaded Q's lower value is due to the R
of the "load" ...as it stands to reason. That load will be an
antenna's radiation resistance (and any Ohmic loss of the structure).

The energy loss is called radiation - if you do it right.

73's
Richard Clark, KB7QHC

On 29 abr, 06:32, Bill Bowden wrote:
Does anyone know why the distributed winding capacitance of a loop
antenna, or any inductor, degrades the efficiency?

It would seem that a loop antenna with 100pF of winding capacitance in
parallel with a external capacitor of 200pF would resonate at the
same frequency as a antenna with no winding capacitance and a external
capacitor of 300pF,but apparently that's not the case.

The best explanation I got was that winding capacitance represents
'low Q' and a external tuning capacitor represents ' High Q'

What is the difference between high and low Q, and why should a loop
antenna with no winding capacitance perform any better than one with
50% of the total capacitance in the windings? Where is the energy
loss?

Thanks,

-Bill

Hello Bill,

I assume that you mean radiation efficiency (ratio between actual
radiated power and total electrical input power).

I think inter-winding capacitance does not decrease efficiency, it may
only change the radiation pattern when the inter-winding capacitance
is that much, that the current distribution in the coil is affected.
This is almost the case with relative large loops.

When you have a loop close to a halve wave, just the own capacitance
is sufficient to get resonance (as with, for example, a halve wave
dipole).

Radiation efficiency may be reduced by losses in the insulation. When
windings are close together, the Electric Field strength in the
insulation can be that high, that loss becomes significant. This is
mostly the case when windings are touching.

Another thing can be corona discharge (that may in the end destroy
your insulation).

Best regards,

Wim
PA3DJS

"Bill Bowden" wrote in message
oups.com...
Does anyone know why the distributed winding capacitance of a loop
antenna, or any inductor, degrades the efficiency?

-Bill

Hi Bill.
I agree with your assertion that distributed winding capacitance
degrades efficiency.
My thoughts about this are ;
Assume a 10 turn loop, between each turn there is a capacitance,
so, you have a complete circuit, (L,C,R) there is current
flowing through this circuit that is not flowing through the entire 10
turn loop. (this happens in the other 9 turns also)
I think these extra currents flowing that don't make the entire 10
turn circuit increase the losses.

Anyone care to run with that, or explain it more clearly, or shoot it
down.

Mike

Bill Bowden wrote:
It would seem that a loop antenna with 100pF of winding capacitance in
parallel with a external capacitor of 200pF would resonate at the
same frequency as a antenna with no winding capacitance and a external
capacitor of 300pF, but apparently that's not the case.

The "100pF of winding capacitance" is NOT across the
entire coil as is the 200pF external capacitor. When
the operating frequency of a coil is more than ~15% of
the self-resonant frequency, the lumped circuit model
starts to fall apart. In your above example, the operating
frequency is ~60% of the self-resonant frequency so you
need to use the distributed network model (or Maxwell's
equations).

Quoting from an IEEE white paper about RF coils at:

http://www.ttr.com/TELSIKS2001-MASTER-1.pdf

"... lumped element circuit theory does not (and cannot)
accurately embody a world of second order partial
differential equations in space and time."

"The concept of coil 'self-capacitance' is an attempt
to circumvent transmission line effects on small coils
when the current distribution begins to depart from
its DC behavior. The notion has been developed by
starting with Maxwell's equations and using only the
first two terms in the Taylor series expansion for
the distributed current to obtain an expression for
the self-impedance of a generalized closed circuit.
Upon extracting Neumann's formula for the self inductance,
the remaining negative component of the reactance permits
an expression for the coil self-capacitance. These formulae
are valid for a PARALLEL combination of an inductance and
a capacitance when the operating frequency is well below
1/SQRT(L*CL). They permit a coil with a SLIGHTLY
nonuniform current distribution to be treated AS THOUGH THE
CURRENT WERE UNIFORM and the coil was shunted with a lumped
element capacitance."

The author shows how to estimate the VF and Z0 of a coil
that is operated at more than 15% of its self-resonant
frequency. It can thus be modeled as a transmission line.

The same author shows in his class notes at:

http://www.ttr.com/corum/index.htm

that the calculated self-resonant frequency of a particular
coil based on the measured self-capacitance was in error
by 65.2% when the "lumped-element assumption" was used.

The calculated self-resonant frequency based on the
transmission line distributed network model was within
5% of the measured self-resonant frequency.
--
73, Cecil http://www.w5dxp.com

amdx wrote:
Assume a 10 turn loop, between each turn there is a capacitance,
so, you have a complete circuit, (L,C,R) there is current
flowing through this circuit that is not flowing through the entire 10
turn loop. (this happens in the other 9 turns also)

Reminds me of a transmission line distributed network
for which a velocity factor can be calculated.

Anyone care to run with that, or explain it more clearly, or shoot it
down.

Please see my other reply where an IEEE white paper
agrees with you.
--
73, Cecil http://www.w5dxp.com

Wimpie wrote:
I think inter-winding capacitance does not decrease efficiency, it may
only change the radiation pattern when the inter-winding capacitance
is that much, that the current distribution in the coil is affected.
This is almost the case with relative large loops.

This is almost *always* the case with relatively
large loops?
--
73, Cecil http://www.w5dxp.com

On Sun, 29 Apr 2007 06:52:17 -0500, "amdx" wrote:

I agree with your assertion that distributed winding capacitance
degrades efficiency.
My thoughts about this are ;
Assume a 10 turn loop, between each turn there is a capacitance,
so, you have a complete circuit, (L,C,R) there is current
flowing through this circuit that is not flowing through the entire 10
turn loop. (this happens in the other 9 turns also)
I think these extra currents flowing that don't make the entire 10
turn circuit increase the losses.

Hi Mike,

Capacitance does not bring loss. Loss ALWAYS resides in Resistance
and nothing else.

Between you and Bill, there appears to be a fixation on the loopS
(emphasis on there being more than one). If you are going to blame
them (that emphasis on there being more than one), and try to tie it
to loss (that emphasis being naturally in Resistance, not
Capacitance); then it follows it is in the natural increase in
conductor Resistance that occurs when wires are spaced closer than 3
or 4 wire diameters to each other. When wires (or loops in this case)
are in close proximity, the magnetic field of the near wire (or loop
in this case, and each loop in proximity to the next) FORCES the
current in that loop to the surface of the wire - INCREASING that
conductor's Skin Resistance. Loss thus increases by proximity.
Capacitance does too, but that is merely a correlating factor.
Remember (and this is good advice, especially suited to Newsgroup
rumors you may pick up): Correlation is NOT causality.

73's
Richard Clark, KB7QHC

On 29 abr, 15:50, Cecil Moore wrote:
Wimpie wrote:
I think inter-winding capacitance does not decrease efficiency, it may
only change the radiation pattern when the inter-winding capacitance
is that much, that the current distribution in the coil is affected.
This is almost the case with relative large loops.

This is almost *always* the case with relatively
large loops?
--
73, Cecil http://www.w5dxp.com


Hello, Cecil,

Yes you are right, as soon as electric flux is leaking via inter
winding capacitance, the current distribution is no longer uniform.

Maybe Bill can find more info when searching for Tesla coil inductors.
I made a small one myself (H-bridge, running at about 700 kHz, [yes, I
know it is in the AM broadcast band]). The vertical coil behaves
almost as a quarter wave resonator, just a small top capacitor was
necessary.

Best regards and thanks for the correction.


Wim
PA3DJS

On Apr 29, 6:47 am, Cecil Moore wrote:
amdx wrote:
Assume a 10 turn loop, between each turn there is a capacitance,
so, you have a complete circuit, (L,C,R) there is current
flowing through this circuit that is not flowing through the entire 10
turn loop. (this happens in the other 9 turns also)

Reminds me of a transmission line distributed network
for which a velocity factor can be calculated.

Cecil -

I think this will interest you:
http://www.rhombus-ind.com/dlcat/app1_pas.pdf

73, ac6xg