Suppose I have a device A with RF transmitter and device B with RF receiver with range 10 meters.
Is there a way a A gets notified when B is out of range? Do we have to keep checking from A at regular interval that B is accepting signals or not?
How to find whether RF transmitter and receiver are not in range
RF
Related Solutions
The antenna will work identically for transmitting and receiving, so there is no limitation from the basic physics. The problem is purely one of overdriving a sensitive analog input, which is the receiver input. If the transmitter output is limited to the same power supply that the receiver runs on, this might not matter. It may temporarily make the receiver deaf for a short time after the transmission while its AGC recovers.
Look at the datasheet for the receiver carefully and see what the voltage limits are for the RF input pin. This is actually one of those rare cases where you may be able to use the absolute maximum limits instead of the operating limits if due care is used with all parameters. Your firmware knows when it is transmitting and can know to ignore anything from the receiver during that time and a short time afterward. During this time you only care that the receiver not be damaged.
A better all around solution would be to use a transceiver chip. These have the transmitter and receiver built into the same chip, with a single antenna connection. Whatever protection the receiver might need to not be hurt by transmitter RF levels has already been built in.
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.
Best Answer
If you just have a transmitter at one end and a receiver at the other, there is no way for the transmitter to know that the receiver is out of range (or turned off).
You would need two-way communication between the points for the originator to know that the recipient received the transmission.