FM transmission distance

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mstechca

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according to **broken link removed**,

it states that the maximum transmission distance is calculated by:

**broken link removed**

The problem is that E = volts per meter of the receiver.

I can't determine volts/meter for my receiver.

Could I calculate the distance some other way?
The thing that is the most common between my transmitter and my receiver is that they both contain an LC tank circuit.

I have heard of inductive coupling (using a transformer between amplifier stages). could I use that to form a distance equation?

when you think about it, the transmitter and the receiver are inductively coupled to each other, wirelessly.

but whatever the case is, I need a simpler equation for maximum transmission distance.

because right now, the way I see it, my 3V transmitter can transmit 2.8KM if my receiver has a 20uV/m sensitivity.

and please answer, because I don't want to jam anyones signal.
 
Its better to use something called FRIIS's equation. That equation doesn't even look right. It should be over D^2. Find a better equation.

P.S. Most equations will just give you distance in freespace.
 
Tony Van Roon's 3V FM transmitter is very low powered and has a range to a very sensitive FM radio of about 100 feet, only to across my street.
My Rev4 FM transmitter uses a 9V supply and an RF output amplifier for much higher output power and its range is more than 2km to a very sensitive FM radio.

If the 3V transmitter uses 10mA then its supply power is 30mW with a brand new battery. My FM transmitter uses 54mA then its supply power is 486mW.
The 3V transmitter is about 40% efficient because it operates in class-A. My transmitter is about 70% efficient because its RF amplifier operates in class-C. Crunching the numbers reveals that my FM transmitter is 28 times more powerful than the little guy.

Good FM radios have an RF amplifier and a very high gain IF amplifier for their high sensitivity of close to 1uV/m. Cheap ones don't have an RF amplifier but the high gain of their IF amplifier still allows them to perform pretty well with a sensitivity of about 10 to 20uV/m.
Your FM receiver doesn't have a high gain IF amplifier. It also doesn't have an FM detector. It might have a sensitivity of only 100 to 200uV/m. Surely it will also reduce the range of your transmissions. 15 feet? :wink:
 
I have uploaded a rather stripped (non-working) version of my superregen detector.

I was wondering if I could calculate the sensitivity of it by the following equation:

Z / (Z + R2 + R1) * V

where Z is the impedance ofthe tank circuit (inductor and capacitor in parallel), and V is the supply voltage.

maybe then I can get volts/meter

Why do I think it works, because I'm dealing with a forward biased diode between base and emitter of the NPN, and I could just ignore the collector which is why R1 and R2 are added together.
 

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Your equation has nothing to do with a radio's sensitivity. The sensitivity is determined by the actual voltage gain of the circuit.
Since your super-regen oscillates instead of amplifying, I don't think there is an equation for its sensitivity.
Just build it and measure its range. 15 feet? :wink:
 

I'd be walking 100's of miles if I did that.

wait a sec, isn't the gain of the transistor equal to: collector current / base current?

but then how do I convert gain to volts / meter?
 
mstechca said:
wait a sec, isn't the gain of the transistor equal to: collector current / base current?
Click to expand...
At DC and low frequencies. Its Hfe is much much less at 100MHz or higher, look at its datasheet.

how do I convert gain to volts / meter?
Click to expand...
A super-regen doesn't have gain that can be calculated since it oscillates instead of amplifying.

Can't you walk 15 feet to measure the range from your very low-power transmitter? :wink:
 
Can't you walk 15 feet to measure the range from your very low-power transmitter?
Click to expand...
My units were farther apart before.

I don't want to transmit too far, or neighbours will complain
 
 
I have received all FM stations a normal radio in hamilton (my city) can receive. as for TV stations, I can receive the local one (CHTV), and I can barely receive Toronto's new station, CKCO.

so maybe I can get an approximate volts/meter on that.

I can't conclude that I can transmit 60 or so km to my receiver, because I don't know the exact power the transmitters are transmitting.
 
mstechca said:
I have received all FM stations a normal radio in hamilton (my city) can receive.
Click to expand...
You are just around the corner fom me and my radios get about 100 FM stations.

as for TV stations, I can receive the local one (CHTV), and I can barely receive Toronto's new station, CKCO.
Click to expand...
CKCO is up the hill in Kitchener. Toronto has about 15 high power TV stations.

I can't conclude that I can transmit 60 or so km to my receiver, because I don't know the exact power the transmitters are transmitting.
Click to expand...
I think you are stretching it a little. 15 feet is more likely.
You have a flea-power transmitter and don't have a "real" radio. :wink:
 
mstechca said:
so that is 3 of us from hamilton now eh?
Click to expand...
No. That is more of us in Mississauga. Hamilton is a half-hour or less drive away. It stinks there because of the steel-making plants. What are they burning???
 
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