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Hi all
These days I read a book on radio receivers from the site :
http://www.mikroe.com/en/books/rrbook/rrbook.htm
During the reading, sometimes I find it difficult to understand some of the points, so I ask you to help me understand. Thank you very much.
Question #001 (from sec: 3.1.4. Other Components)
The writer said:

this is the figure he refered to:

as you see, few pF (3 - 7 pF)for 10 meters antenna and few dozens (30-70 pF) for 2 meter antenna.
note C1 value is In inverse proportion with the antenna length.
(a) Why this inverse proportion?
(b) What would happen if we removed this C1?
[glow=red,2,300]Thanks alot.[/glow]

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I am sorry if continued reading found the answer which is:

Every reception antenna behaves as a voltage generator, having its own internal resistance and capacitance. Antenna's resistance damps the oscillatory circuit and reduces its selectivity (which manifests as the "mixing" of stations) and sensitivity (which exerts as signal strength reduction), and antenna's capacitance reduces the reception bandwidth. More precise, antenna's capacitance reduces the upper bound frequency of the reception bandwith (Pic.3.2), making reception of the stations laying close to this frequency impossible. Both these features are undesirable and manifest themselves as less as the capacitance C1 is smaller. On the other hand, the smaller the capacitance C1, the weaker the signal that goes through it from the antenna, the reception therefore getting weaker. As you can see, the compromise solution is a thing to go for, i.e. one must find the capacitance at which the signals from the antenna won't be much weakened while simultaneously keeping the selectivity and the bandwidth big enough. You can start with C1 being about 30 pF. Then,

using C, tune yourself to some radio stations you can receive. If all the stations that interest you are there, and the strongest one of them still does not jam the reception of other stations all's well. Try then with some bigger capacitance for the capacitor C1. The reception will be getting louder, so do continue increasing C1 as long as it is still possible, by changing C, to receive all the stations of your interest that can be heard in your place, without the interference of some strong or local station. If, however, reception of some nearby station isn't possible, smaller C1 should be tried out. In this manner the biggest capacitance for C1 should be found, that allows optimal reception both regarding selectivity and bandwidth. The simplest solution is using variable capacitor for C1, its capacitance ranging from few picofarads to few dozens pF, adjusting it to obtain optimal reception for each station individually. During this, whenever C1 is being changed, the receiver must be re-tuned to the station using C.

But if you have any useful addition, welcome
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Receiver input design is a balance between losses and selectivity in the tuned circuit. More selectivity requires less loading of the tuned circuit. Less loading results in more signal loss, from the signal coupled to the tuned circuit. Connecting the antenna directly will be like connecting a low value resistance in parallel with the tuned circuit, killing the selectivity. The capacitor increase the series impedance Xc to minimize loading while still passing enough signal.

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Hi AN920
I have read with interest all that came in your last reply, so I read it more than five times, I want to further shed light on the following points :
(1)

{The capacitor increase the series impedance Xc to minimize loading.}

Are u mean the series impedance of the antenna? And if so, What can the antenna be considered from this point?
(2)
{Connecting the antenna directly will be like connecting a low value resistance in parallel with the tuned circuit, killing the selectivity. }

Are u mean that with C1 it is will be like connecting a high value resistance in parallel with the tuned circuit.
(3)
{Connecting a low value resistance in parallel with the tuned circuit, killing the selectivity}

I want to offer my interpretation of this phrase and you should correct me if I was wrong:
Connecting this low value resistance in parallel with the tuned circuit make the total i/p impedance is always low for every signal of any freq so there is no selectivity at all.
(4) Why u consider the antenna as alow resistance? I have heard that it is 50, 75 300 ohm, why antennas are low value resistance?
(5) Why antenna is considered to be // to the tuned circuit, why not inseries?
[glow=red,2,300]NOTE: [/glow] Please reply to questions one after another, not all at once so I understand very well not have to repeat the question in different ways.
[glow=red,2,300]I forgot to tell you that your answer above, consisting of 4 lines better than 2 pages in the book. [/glow] thank u very much
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1) Are u mean the series impedance of the antenna? And if so, What can the antenna be considered from this point?

The capacitor add resistance in series so that the low characteristic impedance of the antenna (300, 75 or 50 Ohm) does not load the tuned circuit too much.

(This will become more clear when you study the plots below)

2)Are u mean that with C1 it is will be like connecting a high value resistance in parallel with the tuned circuit.

Yes, with C1 in circuit the total resistance loading the tuned circuit will be (Z- antenna + Xc) apart from the coupling to the rest of the receiver.

3)Why u consider the antenna as alow resistance? I have heard that it is 50, 75 300 ohm, why antennas are low value resistance?

It has been determined that the various antenna types will have that resistance, and can be assumed to be a load of that resistance when connected to a RF circuit.
You read up on this using Google.

4)Why antenna is considered to be // to the tuned circuit, why not inseries?

The antenna acts as a load between point of connection and ground of the circuit.

I have added the diagrams as help.

First diagram shows tuned circuit with low loading from antenna (sine source 300 Ohm) and rest of receiver circuit (50k). Note the the dB magnitude.

Second diagram shows the same circuit but wil larger coupling capacitors. Note that the selectivity got worse but the dB level to our receiver is better.

Third diagram shows a large coupling capacitor that will in effect directly couple the antenna to the tuned circuit. Note total loss of selectivity and severe loss of signal level.

Last diagram shows the effect of placing a load R2 (300 Ohm) in parallel with the tuned circuit. It is very similar to directly connecting the antenna with even more signal loss.

Last diagram

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I once used this loading effect to design a switchable narrow-wide FM detector in a measuring instrument.

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Thank you AN920, It is very clear
Questions:
(a) After considering all of the above:
Can we consider that C1 play a impedance matching between the antenna and the tuned circuit?

(b) Another point is still unclear to me is : if we consider that the length of one meter of the wire antenna is of 50 Ohm resistance.
So 10 meters have a 500 ohm and 2m have 100 ohm resistance.
Now, with 10 meter (500 ohm) the recommended C1 is about 5pF, please look at the total Z:

note 31.8 Kohm
with 2 meter (100 ohm) the recommended C1 is about 50pF, please look at the total Z

note, 3.18 Kohm
Big difference ten times??!!

© As seen above, the writer said:

{Every reception antenna behaves as a voltage generator, having its own internal resistance and capacitance.}

Why did not say internal inductance also?
(d) Regarding the last point you mentioned about a switchable narrow-wide FM detector in a measuring instrument, I think you mean C16 (100nF) that big value cause a wide BW.
I noteced some strange thing, the collector of Q1 is not connected to any supply VCC.
Thanks
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a) C1 helps to improve the impedance match to the tuned circuit.

b) Antennas of a certain length have a typical impedance at a certain frequency. The explanation given by the writer is too simplistic. It is a bit more complicated than that. I suggest you read up on type of antennas, impedance and matching. It is too much to explain here in detail. Here is a quick start
http://www.borg.com/~warrend/guru.html

c) Antennas can be inductive or capacitive.

d) In the FM detector the turning on of the transistor places the resistor in parallel with the tuned circuit, lowering the Q, allowing a wider bandwidth to be detected. The transistor will have low impedance between collector and emitter when turned on by base-emitter current. The 100nF cap serves as DC isolation to the internal Gilbert cell quad detector.

Another way that is often used in receiver design to obtain wider bandwidth but retaining overall rejection outside the passband it so use cascaded filters like the top-c coupled filter in the diagram. We get wider bandwith but the side skirts remain steep. I added R1,2 to give the inductors a Q value like a real life part.

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