Receiver Circuit Question -- What role does this antenna capacitor play?

[The Tortoise-Man]

Without an input there is no point having a receiver.

I'm confused, I not understand the train of thought here. What do you mean by
'Without an input there is no point having a receiver.'?

The input is simply on the left side, it can be eg an antenna, a generator,... Why you assuming
that there could be no input? Could you clarify what you mean by this remark?

A band-pass filter is distinct from a tuned circuit in that the BPF has steep sides with a flatter top. Stagger tuned circuits can be lightly coupled to make a BPF. But here there is only one tuned circuit.

Yes, of course a tuned circuit alone cannot be BPF. My incorrect conjecture was
that is may become a BPF if we consider it together with C1 in series (ie the circuit from #89)
but as you remarked that's wrong as one may check oneself eg by considering a
simulation plotting the output power as a function of frequency.
If circuit from #89 would be a BPF the power output function should have a
"bell-shaped", but that's not the case here.

I think that's might be the most simple way to see that this circuit can never be a BPF for
all the time.

If you were to model the antenna as a tuned circuit, then couple it through C1 to the L//C2 tuned circuit, you might claim it becomes a BPF. But the antenna for MW is under-size, so is a wider-band structure, not a tuned circuit by itself.

Here you probably refer to the observation that if we slightly modify the circuit from #89 by adding a coil in series with appropriate parameter L1, then the following circuit should become a BPF (of cource if C1, L, C2 are choosen in sophisticated way), right?

 

[Baluncore]

Here you probably refer to the observation that if we slightly modify the circuit from #89 by adding a coil in series with appropriate parameter L1, then the following circuit should become a BPF (of cource if C1, L, C2 are choosen in sophisticated way), right?

No, I do not. How would you like to be dragged through a blackberry bush backwards? That is what you seem to be trying to do to me here.

Why you assuming that there could be no input? Could you clarify what you mean by this remark?

For completeness. Can you think of a reason why an input is needed ?
Then how many ways are there to inject that energy, without killing the Q of the front-end ?

 

[The Tortoise-Man]

No, I do not. How would you like to be dragged through a blackberry bush backwards? That is what you seem to be trying to do to me here.

Ok sorry, then I have misinterpreted your idea
where you wrote:

"If you were to model the antenna as a tuned circuit, then couple it through C1 to the L//C2 tuned circuit,
you might claim it becomes a BPF. "I assumed that there you referred to an antenna model of shape
like this:

And since I found in the net (http://www.elektronik-bastler.info/stn/lc_filter.html ;unfortunately in german)
a bandbass filter having the shape of the image in #91, I conjectured
that this L1C1-unit in series was exactly that part which the "bare"
LC2-tank needed to become a honest BPF. Sorry, wrong resonings.

 

[The Tortoise-Man]

For completeness. Can you think of a reason why an input is needed ?
Then how many ways are there to inject that energy, without killing the Q of the front-end ?

I'm not sure on what you're getting at here. By "killing" Q (of the tank) you mean to make it small as possible, right?

 

[berkeman]

Yes, killing the Q of a resonant tank circuit means adding more loss, so it rings less and the resonant signal level is lower. Have you looked up resonant tank circuit links yet? (sorry, I haven't been following this thread for a while)

 

[Baluncore]

Ok sorry, then I have misinterpreted your idea where you wrote:
"If you were to model the antenna as a tuned circuit, then couple it through C1 to the L//C2 tuned circuit, you might claim it becomes a BPF. "

But IF you did that you would be wrong.
The short antenna used prevents there being a second tuned circuit, so there can be no hypothetical BPF. I cannot understand why you insist on returning to discussion of the non-existent, mythical BPF.
You would do better finding out why such a simple regenerative receiver can work so well and how it can be so sensitive and selective.

Have you looked up resonant tank circuit links yet?

That is good advice.

 

[The Tortoise-Man]

Yes, killing the Q of a resonant tank circuit means adding more loss, so it rings less and the resonant signal level is lower. Have you looked up resonant tank circuit links yet? (sorry, I haven't been following this thread for a while)

Which "links" do you mean? Did you provide earlier during this lengthy discussion somewhere some recommendable links about resonant tank circuits? I have to admit that I don't have an overview of all the posts here now, sorry if posted some link before and not found it.

Or do you want to know if I searched about
resonant tank circuits on my own? Well, if you indeed mean the second one then what I found is not really satisfying in light
of the course this discussion. What I found about resonant tank circuits can be found summarized
on wikipedia (https://en.wikipedia.org/wiki/LC_circuit), especially the parts on "Series and Parallel
Circuits", that's what have known before.

The new aspects which I learned here (and I didn't know before and which confused me at fist time)
is that (if I understood Baluncore correctly) then when we are talking about Q of tuned circuit
(ie barely that's the red thing in #68 or the green encircled thing in #1) then
it's Q hardly depends which components are there on the left in the "black box",
ie the Q of the tuned circuit not only depends on R, L, C as suggested
suggested on the wikipedia site. That's what what confused me before since I assumed that
if one is talking about Q factor of tuned circuit, then this Q literally depends only on it's
components R, L, C and not what is connected externally to it.

So for example the Q of the tuned circuit depends on the coupling capacitor and the antenna
itself (in #1). See also my posts #76 & corrections in #82. That's how I understand
the concept behind the Q factor of ther tuned circuit now.

Unfortunally I still haven't found a book/ online script where the concept of Q is taught exactly
in this way, that unsettles me a bit if that's how I think about it is really the correct way.
Could you recommend a source which discusses the concept of Q in this spirit?

 

[Baluncore]

Unfortunally I still haven't found a book/ online script where the concept of Q is taught exactly in this way, that unsettles me a bit if that's how I think about it is really the correct way.

There is no "correct" way, it depends on your viewpoint. Anything you believe today may be changed tomorrow. You will not survive long in the real world if you demand certainty.

Keep an open mind and accept that some words can have several meanings in different fields. Learn to think in many alternative reasonable interpretations.
https://en.wikipedia.org/wiki/Postmodernism

 

[The Tortoise-Man]

But IF you did that you would be wrong.
The short antenna used prevents there being a second tuned circuit, so there can be no hypothetical BPF. I cannot understand why you insist on returning to discussion of the non-existent, mythical BPF.
You would do better finding out why such a simple regenerative receiver can work so well and how it can be so sensitive and selective.

That was just a spontaneously conjecture of mine. Forgive me if it gets the impression that I
"insist" that there "must" be somewhere a BPF. I threw it just in the room because the
shape of the blue encircled components in #84 reminded me a bit of the structure of BPF circuits.
So I was just wondering if there might be a relation... I not intended to insist that
this has an interpretation as a BPF, I was just curious if there might be some relation due to my
association with BPF circuits I saw before.

But as you said seemingsly I was thinking in totally wrong direction.

Nevertheless, in general receiver circuits it's a natural practise to use BPF's as functional component to naturally filter out some undesired frequences, do you argree with me? If yes, that's basically my motivation why I was wondering if there might be some relation to BPF in this part of the depicted receiver.

But is the reason that in THIS concrete regenerative receiver the blue encircled components
in #84 cannot be interpreted as BPF as you said due to special nature of regenerative receiver's
(ie the usage positive feedback) or does this missing the point and you intended to empasise another aspect?

 

[DaveE]

There is no "correct" way, it depends on your viewpoint. Anything you believe today may be changed tomorrow. You will not survive long in the real world if you demand certainty.

Keep an open mind and accept that some words can have several meanings in different fields. Learn to think in many alternative reasonable interpretations.
https://en.wikipedia.org/wiki/Postmodernism

Yes. This.

In the complex world of RF circuits, people can be sloppy when discussing the concept of Q.

It is all based on the simple harmonic oscillator (SHO) and the physical description of energy lost compared to energy stored per cycle. This is equivalent to the simple RLC circuit. We all know that version and are on the same page.

But, it can be useful to model more complex circuits with these simpler tools. It's how people are applying the simple concept as an approximation of complex circuits that is confusing. You must learn the basics first, and then discuss (or decipher) how it is being used in any real world context. There just isn't going to be the simplicity or consistency you seek. In a pedantic sense, when people discuss Q it is either simple and correct, like in my post #80, or they are (intentionally) a bit wrong.

In practice, it is often used to describe how resonant or how lossy a circuit with stored energy is. This can appear in many different guises.

 

[Baluncore]

BPF ... BPF ... BPF ... BPF ... BPF ... BPF ... BPF

You are still not cured. Can you not let it go ?
You need to be vaccinated against your BPF mind virus.

 

[The Tortoise-Man]

You are still not cured. Can you not let it go ?
You need to be vaccinated against your BPF mind virus.

Well doctor, what is wrong with it? Wasn't my concern in two last paragraphs of #99 clear enough? I think that such question arise quite naturally at that point. And I would like to appreciate to understand "what is wrong" in my reasonings concretely. We should talk here about science, not mysticism.

 

[berkeman]

Thread is on time-out...