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

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 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

Wimpie wrote:
The vertical coil behaves
almost as a quarter wave resonator, just a small top capacitor was
necessary.

Sounds like a 75m mobile bugcatcher antenna. :-)
--
73, Cecil http://www.w5dxp.com

"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

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

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

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

Thanks very much, Jim.
--
73, Cecil http://www.w5dxp.com