- Turning on your TV or radio does not cost the broadcaster more money, because broadcast transmissions would otherwise be dissipated by some other object.
- some energy is drawn in by the receiver circuit of the radio.
Both statements are true. Do you think one contradicts the other?
I think of it as analogous to the big decorative water sprinkler at the local park.
(I agree that "dissipated in the air" seems unlikely).
- Letting your dog drink water from the sprinkler does not cost the park more money, because the same amount of water comes out of the sprinkler, whether some of the droplets land on the dog's tongue or whether they all falls on the ground.
- Some water is drawn in by the dog.
Often several dogs catch water drops on their tongues at the same time at the same fountain, and still the vast majority of the water is "wasted" landing on the ground.
Likewise you can have thousands of people tuning in to the same TV station, and still the vast majority of the photons pouring out of the transmission antennas never hits a receiver antenna, but instead is "wasted" hitting trees or mountains or escaping to outer space.
There is no way to tell from looking at the park's water meter whether dozens of dogs drink water from this fountain, or no dogs at all -- the same water comes out the sprinkler either way.
There is no way to tell from looking at the broadcaster's electic meter whether thousands of people are tuned in, or no one is tuned in -- the same electromagnetic power comes out of the transmission tower either way.
This is very different from the way energy flows "through the air" in a air-core concentric-coil transformer, or "through the air" in an air dielectric capacitor,
or the way mains powered devices "draw in" only the amount of current and power they need.
- Do radio receivers use any power from a transmitter?
A few crystal radios have no batteries or mains connection -- all the power they have comes from the radio transmitter, and the radio uses power from the transmitter to drive the earphone.
You could argue that most radios extract only the signal from the station; all the power from the antenna ends up warming the BE junction of the first transistor in the pre-amplifier, and 100% of the power "used" by the radio in later stages and to drive the speakers comes from batteries or mains power or a clockwork spring.
How much would make a difference?
Well, if we packed enough radios and their antennas all around the broadcast antenna, eventually we would form a Faraday cage -- those radios would absorb all the broadcast energy, and other radios outside a Faraday cage cannot hear any transmissions from inside it.
There are a few things this analogy does not capture perfectly.
Although it is tempting to think of the antenna as a "bucket", since the bigger it is the more photons it catches,
a tuned antenna can catch far more energy than one might expect from its size and the local energy density -- 'Energy-sucking' Radio Antennas.
If a puppy is catching the droplets on his tongue, and then a German Shepherd steps over him and catches the droplets first, then nothing reaches the puppy -- unless the puppy moves a bit to the side to get out of the shadow of the big dog.
Likewise, if you put one radio antenna close to and immediately "behind" another radio antenna (as seen from the transmission tower), the upstream radio one will receive perfectly, as if the downstream radio is not even there, and the downstream radio will hear nothing -- until the downstream radio moves a bit to the side to get out of the shadow of the big dog.
However, if you move the downstream radio antenna further away from (and yet still behind) the upstream antenna, it will also start to hear the station -- the power from the station "curves around" the upstream radio.
A typical FM broadcast tower puts out 100 kW ERP (+80 dBm) and 300 m high.
FM radio receivers are expected to work down to a signal strength of 0.5mV/m.
A typical radio reciever has a sensitivity of -90 dBm with an antenna roughly 1 m long.
It intentionally DOES oscillate.
It goes beyond a regenerative receiver and is called a super-regenerative receiver, invented by Edwin Armstrong in 1922.
The values of R3 and C3 are arranged so that after oscillation starts the voltage builds up on the emitter due to rectification of the oscillations. After some period (10's of microseconds typically) the bias will be such that the oscillations stop. The voltage will then decay with the time constant of R3/C3 until oscillations start again. This typically occurs at a frequency in the range of 10's to hundreds of kilohertz.
Normally in an oscillator the oscillations build up from noise however if any signal at the oscillation frequency is present the oscillations will build up more quickly. This is the signal being received. The average supply current will depend upon how quickly the oscillation starts which depends upon the signal present. Thus the supply current is dependent on the received signal, the output from R1 depends on the supply current.
Usually the transmitter is amplitude modulated at an audio frequency and this can be recovered from C7 with suitable filtering and amplification.
A super-regenerative receiver can get extremely high gain out of a single stage but when no signal is present the output will be very noisy. Because the build up of oscillations is exponential it usually has a logarithmic response.
They are used less these days because they tend to radiate and also have a very broad bandwidth. A common application is for garage door openers.
kevin
Best Answer
It's a bit like asking how to build an internal combustion engine from a gearbox. It's just easier to get a receiver circuit and build it without reference to a transmitter circuit. AM or FM Receivers are usually more complex than an AM or FM transmitter. My advice is get a circuit by googling and disregard any commonalities with the transmitter you have built.
Why is it capable of being received on a whole load of frequencies even when you radically change tank capacitance and inductance?
Answer: because it's a badly designed transmitter with lots of harmonics being generated. You said you received it on 95MHz and 108MHz - it may well be transmitting on a couple of spaces in between that a digitally tuned FM receiver isn't able to perfectly tune to. FM receivers in the UK tune every 200kHz and conceivably there could be a couple of spots that they jump over.
You put a monstruous 1000uF cap across the coil - that cap at the sort of frequencies being transmitted probably looked like an inductor in series with several ohms and maybe moved things about a bit but, due to so many harmonics being generated you were still bound to find that an FM receiver could still pick-up the transmission. Here's a graph showing the impedance of an electrolytic capacitor changing to inductance above a certain frequency: -
How long is a piece of string or how much inductance has a piece of wire - wire has inductance and getting something like 0.1uH is trivial - 10 cm of wire has around 100nH = 0.1uH.