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FM transmission distance

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mstechca

New Member
according to http://www.zen22142.zen.co.uk/Analysis/efftxd.htm,

it states that the maximum transmission distance is calculated by:



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.
 

_3iMaJ

New Member
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.
 

audioguru

Well-Known Member
Most Helpful Member
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:
 

mstechca

New Member
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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audioguru

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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:
 

mstechca

New Member
audioguru said:
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?
 

audioguru

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

mstechca

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

I don't want to transmit too far, or neighbours will complain
 

audioguru

Well-Known Member
Most Helpful Member
mstechca said:
I don't want to transmit too far, or neighbours will complain
I know, because I am only about 25km away from you. Our FM dial is full.

Is your super-regen radio sensitive enough to receive all 100 FM stations that are received on most FM tuners in our area?

Will your super-regen pickup only the 15 local stations?

Will it get only the closest station?

Anything? :wink:
 

mstechca

New Member
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.
 

panic mode

Well-Known Member
ahhhh that's it
to be determined = hamilton :)
 

audioguru

Well-Known Member
Most Helpful Member
mstechca said:
I have received all FM stations a normal radio in hamilton (my city) can receive.
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.
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.
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

New Member
so that is 3 of us from hamilton now eh?
 

audioguru

Well-Known Member
Most Helpful Member
mstechca said:
so that is 3 of us from hamilton now eh?
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???
 

panic mode

Well-Known Member
yep, the industrial zone in hamilton is bad thanks to stelco and dofasco. :?
 
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