precious volts [FMTX]

[mstechca]

I have made myself an FM transmitter and a super-regen receiver.

Here are the interesting parts:

In my room, reception of long distance signals is very bad without a long antenna. I think I have too much metal in my house. The house is over 30 years old!

I (literally) stuck the back of my breadboard to the table in my room next to my computer. and my FM transmitter is assembled on it. The most interesting part about it is that I am using a CR2032 battery bought from the dollar store! It's probably 1/2 dead already.

Even though the battery may be 1/2 dead, It still isn't dead enough to give me no signal. This is why I titled this as precious volts. I'm using the battery down to the end, and trying to get a long distance reception with just 1.5 - 3 volts.

So anyways, I turn both units on, and I get a decent signal fed into the receiver, until I go outside the door. meanwhile, my receiver begins to get cold, because it is -2 degrees celcius (or something like that). I can get most of the signal until I hit the end of the short driveway.

But you know one thing that made a difference, replacing the 330 ohm resistor with a 1mH inductor :!: I think while the 1mH inductor was used the battery life was dropping drastically.

If somehow I can increase the range, AND increase battery life, then I am set.

It is quite amazing what a 33 cent 3V battery can do for me :lol:

and if anyone wants to mimic my part replacement with a simple FM transmitter, just look for the resistor between the RF oscillator's NPN emitter and ground, and change it.

 

[audioguru]

Where is the transmitter's final schematic???
Didn't you measure the circuit's battery current???

 

[Russlk]

I have tried to make a low power transmitter go some distance and can tell you that it is not easy. Tuning the antenna on the transmitter and receiver will help the most (other than increasing power).

 

[zachtheterrible]

How in god's name did you get a VHF circuit to work on a breadboard!!?!? Low frequency?

 

[audioguru]

How in god's name did you get a VHF circuit to work on a breadboard!!?!?

I think he used 1pF capacitors and the stray capacitance of the wiring provided the rest of the required capacitance.

The transmitter doesn't transmit very far because VHF has a difficult job travelling through the series inductance of the wiring.

 

[Nigel Goodwin]

If somehow I can increase the range, AND increase battery life, then I am set.

Range is directly proportional to the transmitted signal strength, to increase it you can either:

a) Increase the power output of the transmitter.

b) Increase the gain of the transmitting aerial (by making it directional).

c) Increase the sensitivity of the receiver, at VHF frequencies this is limited by the noise factor of the RF preamp used.

If you want to reduce the current consumtion you have to either increase the efficiency of the transmitter, or decrease it's output power (which reduces the range)

 

[mstechca]

How in god's name did you get a VHF circuit to work on a breadboard!!?!? Low frequency?

No, high frequency. Somewhere around the 220Mhz range. :wink:

and how in god's name did I do it?
I take a common transmitter circuit and modify it. and the transistor I used is a PN3563 NPN which has an Ft of 600Mhz!

Range is directly proportional to the transmitted signal strength
....
a) Increase the power output of the transmitter.

That makes sense. and that is what I did when I replaced the resistor with a large value inductor as described above. The only competition I have is the battery :lol: because as the battery weakens, so does the power, so does the range.

b) Increase the gain of the transmitting aerial (by making it directional).

I want to avoid an antenna as much as possible. But I must say, the distance is exceptional considering my antenna is about 1 1/2 feet long, and I'm using less-than-perfect batteries.

c) Increase the sensitivity of the receiver, at VHF frequencies this is limited by the noise factor of the RF preamp used.

Noise is low, and my sensitivity is good since I can pick up TV signals.
My superregen detector also uses a PN3563.

If you want to reduce the current consumtion you have to either increase the efficiency of the transmitter

I guess efficency is what I will have to work on.

I think he used 1pF capacitors and the stray capacitance of the wiring provided the rest of the required capacitance.

LMAO :lol:
transmitter's stray capacitance doesn't matter now. In fact, higher capacitance wouldn't even make much difference in my circuit unless it is connected to the RF oscillator.

The transmitter doesn't transmit very far because VHF has a difficult job travelling through the series inductance of the wiring.

I tried replacing the series inductance with a 0.1uH inductor, and I get no signal
I still think my house is bad for reception.

 

[audioguru]

The transmitter doesn't transmit very far because VHF has a difficult job travelling through the series inductance of the wiring.

I tried replacing the series inductance with a 0.1uH inductor, and I get no signal

Exactly. Wires have series inductance. The value of the inductance depends on the length of the wire. Inductance limits RF current. So the long lengths of wires and conductors on a breadboard reduces or eliminates the ability of a circuit to work at high frequencies.