Hi All,
Please look at this in fixed font.
I'm looking for understanding of a series to parallel conversion for
antenna matching.
I'm pretty sure I'm missing something, so point it out to me.
This is in regard to a crystal radio, so the match is for a low impedance
antenna to a high impedance tank circuit.
The antenna: R=58 ohms C=1072 ohms at 1Mhz.
The tank: L=240uh C=106pf Q = 1000
Tank Z =~1.5 Mohms
Here's my understanding of what I think I'm reading.
I put a matching capacitor in series with the antenna.
Antenna-- -----R-----C--------Match cap-----tank------ground.
and this is supposed to transform the circuit to this.
( Maybe better said, equivalent to this)

l------------l
l l
Antenna--- R C LC---Tank
l l
l------------l
^
Ground

I calculate an 18.5pf cap for the match, making the antenna look like 58R
and 17pf.

So this; Antenna-- -----58R-----270pf--------Match
cap18.5pf-----1.5Mohms------ground.
This converts to;
l-----------------l
l l
Antenna---58R 17pf 1.5M---LC Tank at
l l Resonance
l-----------------l
^
Ground

And I now have a 1.5 Mohms source feeding a 1.5 Mohm load.
The purpose of which is to cause minimal loading of the tank by the antenna.
I don't understand how adding a series capacitor makes a parallel
conversion.
What do I misunderstand or do just need to believe the numbers.
Thanks, Mikek

Ahh....1 should have said Series to Parallel Conversion.
Mikek

On Thu, 13 Jan 2011 11:05:38 -0600, "amdx" wrote:

This is in regard to a crystal radio, so the match is for a low impedance
antenna to a high impedance tank circuit.
The antenna: R=58 ohms C=1072 ohms at 1Mhz.

Hi Mike,

Is this a fantasy antenna?

For the 58 Ohm resistive value, it would have to be about 300 feet
tall - not the size of operation one usually comes to expect for a
Xtal radio aficionado.

If it is that tall, it would exhibit 200 Ohms Inductive reactance (one
fifth of what you report, and the opposite sign).

Something doesn't wash here.

I calculate an 18.5pf cap for the match, making the antenna look like 58R
and 17pf.

58±j17 Ohms is still a complex impedance, and says nothing of match
which can only be expressed in terms of the expected load R.

And I now have a 1.5 Mohms source feeding a 1.5 Mohm load.

How that is arrived at is something of a mystery. By the numbers, you
describe a 26000:1 mismatch.

The purpose of which is to cause minimal loading of the tank by the antenna.

Well, what you have described is sufficient mismatch to insure that.
The English reading of your sentence also is instructive: the tank is
isolated from the antenna, i.e. no signal is passed to it. This seems
to be counterproductive in regards to detection.

I don't understand how adding a series capacitor makes a parallel
conversion.

Haven't we been down this road some months ago?

73's
Richard Clark, KB7QHC

Richard Clark wrote:

For the 58 Ohm resistive value, it would have to be about 300 feet
tall - not the size of operation one usually comes to expect for a
Xtal radio aficionado.

If it is that tall, it would exhibit 200 Ohms Inductive reactance (one
fifth of what you report, and the opposite sign).

Something doesn't wash here.

Perhaps it's a longwire antenna, of crappy wire, parallel to
the ground?

On Thu, 13 Jan 2011 13:30:52 -0500, "Greg Neill"
wrote:

Richard Clark wrote:

For the 58 Ohm resistive value, it would have to be about 300 feet
tall - not the size of operation one usually comes to expect for a
Xtal radio aficionado.

If it is that tall, it would exhibit 200 Ohms Inductive reactance (one
fifth of what you report, and the opposite sign).

Something doesn't wash here.

Perhaps it's a longwire antenna, of crappy wire, parallel to
the ground?


Hi Greg,

No, that would more likely result in a characteristic Z of 600 Ohms.
If by being close to ground you mean the 58 Ohms finds itself invested
in the dirt, well, yes that might be the case.

The long and short of it means that to investigate the problems of an
antenna "loading" a tank coil means that you really must understand
the antenna - not a simple thing as this thread will undoubtedly
reveal.

73's
Richard Clark, KB7QHC

Richard Clark wrote:

If by being close to ground you mean the 58 Ohms finds itself invested
in the dirt, well, yes that might be the case.


:-)

"Greg Neill" wrote in message
m...
Richard Clark wrote:

For the 58 Ohm resistive value, it would have to be about 300 feet
tall - not the size of operation one usually comes to expect for a
Xtal radio aficionado.

If it is that tall, it would exhibit 200 Ohms Inductive reactance (one
fifth of what you report, and the opposite sign).

Something doesn't wash here.

Perhaps it's a longwire antenna, of crappy wire, parallel to
the ground?


The author said it was an antenna in his attic.
That's all I know.
Mikek

"Richard Clark" wrote in message
...
On Thu, 13 Jan 2011 11:05:38 -0600, "amdx" wrote:

This is in regard to a crystal radio, so the match is for a low impedance
antenna to a high impedance tank circuit.
The antenna: R=58 ohms C=1072 ohms at 1Mhz.

Hi Mike,

Is this a fantasy antenna?

It's an example from an article, I don't like the number either, seems like
maybe
2 to 12 ohms would be more realistic. I think the capacitance is ok.

For the 58 Ohm resistive value, it would have to be about 300 feet
tall - not the size of operation one usually comes to expect for a
Xtal radio aficionado.

If it is that tall, it would exhibit 200 Ohms Inductive reactance (one
fifth of what you report, and the opposite sign).

Something doesn't wash here.

Ok, how about just working with the concept.

I calculate an 18.5pf cap for the match, making the antenna look like 58R
and 17pf.

58±j17 Ohms is still a complex impedance, and says nothing of match
which can only be expressed in terms of the expected load R.

Sorry, I missed a math step, the R was converted to 1.5M.
see formula below.

And I now have a 1.5 Mohms source feeding a 1.5 Mohm load.

How that is arrived at is something of a mystery. By the numbers, you
describe a 26000:1 mismatch.


Here's what is stated in the article:
" The concept is that at any given frequency, a parallel RC network has an
equivalent
series RC network, and vise versa. We can use this property to transform the
real component
of an impedance to a much higher or lower value.
As long as Xc series R series.
Xc (parallel) = Xc (series)
R (parallel) = XC^2 (series) / R (series)
The Xc of a 17pf at 1Mhz is 9368 Ohms.
To rewrite Rp= 9368^2 / R = 1.513 Mohms

The article then goes on to say,
The utility of this equivalence can be seen by choosing a sufficiently small
value of C series
(a large Xc series) A "small" resistance can then be transformed into a
"large" value.

The purpose of which is to cause minimal loading of the tank by the
antenna.

Mikek

On Thu, 13 Jan 2011 12:42:01 -0600, "amdx" wrote:


"Richard Clark" wrote in message
.. .
On Thu, 13 Jan 2011 11:05:38 -0600, "amdx" wrote:

This is in regard to a crystal radio, so the match is for a low impedance
antenna to a high impedance tank circuit.
The antenna: R=58 ohms C=1072 ohms at 1Mhz.

Hi Mike,

Is this a fantasy antenna?

It's an example from an article, I don't like the number either, seems like
maybe
2 to 12 ohms would be more realistic. I think the capacitance is ok.

Hi Mike,

Then your intuition is firing on all cylinders, great.

Here's what is stated in the article:
" The concept is that at any given frequency, a parallel RC network has an
equivalent
series RC network, and vise versa.

This is classically true. The frill of "at any given frequency" can
be discarded.

We can use this property to transform the
real component
of an impedance to a much higher or lower value.

This concept is not a "property," however; more a transformation (as
should be apparent in the original statement). In other words
concept
equivalent
property
transform
use four words to describe one thing - transform (plain and simple).

As long as Xc series R series.
Xc (parallel) = Xc (series)
R (parallel) = XC^2 (series) / R (series)
The Xc of a 17pf at 1Mhz is 9368 Ohms.
To rewrite Rp= 9368^2 / R = 1.513 Mohms

Well, what looks like hand-waving is probably close to the numbers one
could expect.

The article then goes on to say,
The utility of this equivalence can be seen by choosing a sufficiently small
value of C series
(a large Xc series) A "small" resistance can then be transformed into a
"large" value.

The reason why I say hand-waving (and this is probably your gut
reaction as to "why") is that the five lines of operations you quote
starts with a presumed requirement and then proves it has been met.

What happens if Xc series R series?
What happens if Xc series R series?
What happens if Xc series = R series?
What happens if Xc series R series?

This somewhat clouds the mystery for you of understanding Parallel to
Series conversion. I wonder too. Are we dropping in a new component
and stepping back with a wave and Voila! to find the Parallel circuit
has suddenly been transformed? Yeah, that WOULD be a mystery.

And what is this Xc(parallel) and Xc(series) stuff?

Xc is stricty a function of pi, capacitance, and frequency.

And what is this R(parallel) and R(series) stuff?

R is a function of its, well, resistance. No variables to be found.

No doubt there is more to be extracted from this article.

73's
Richard Clark, KB7QHC

On Thu, 13 Jan 2011 12:41:53 -0800, Richard Clark
wrote:

No doubt there is more to be extracted from this article.

Upon review of other correspondents to your plight, ditch the article
and pick up an EE sophomore book on circuit analysis. The coverage
should be encompassed in the section (from my own copy) called
"Transform Driving Point Impedance and Admittance (Immittance)."

73's
Richard Clark, KB7QHC