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Monkeyman88

Increasing the range of Transmitter

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HI i am new to all this but do have some basic knowledge of how radio transmitters/ recievers work. I bought a Belkin TuneCast 2 fm transmitter to play pranks on friends with (taking over their radio) but the signal is too weak. I took it apart to see how it works and was wondering could i just plug the ariel into a guitar amplifier to amplify the signal or some other simple way?

Thanks a lot, i know its strange!  ;D

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Guest Alun

This would be better of at the receiver because the transmitter will probably overload it.

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Hi Alun,
I think that it is worth trying a TV cable amp/splitter at the transmitter, some even have a gain control.
The Belkin FM transmitter has a range of a whopping 3m for a good signal! It runs on only 3V so I don't think it has much output to overloading anything.  ;D

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Hi Alun,
The Belkin transmitter automatically turns off when the input music ends. Then your transmitter amplifier will probably release its majic smoke!  ;D

It would be wise to bias it off, and let the RF from the transmitter turn it on and let it amplify as class-C:

post-1706-14279142200177_thumb.gif

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Guest Alun

So you think thurmal run away might be a problem. The circuit that I posted was the same as the one usen in output stage transmitter, its DC current consumption would be around 18mA assuming the transistor has an beta of 100, giving a power consumption of just 162mW. Do you really think this could cause sufficient heating of the transistor to cause thurmal runaway?

The oscilator I built only outputs 150mVp-p, not high enough to drive the amplifier you posted, what do you reckon the output voltage of the Belkin transmitter is?

I've still yet to build it properly on a PCB and I do hope the output is much higher but I doubt it'll be high enough to drive your circuit.

The circuit you posted would require at least 700mV peak to at least turn the transistor on, to get maximum efficiency you'd need at least 2.1V peak to get it to turn the transistor for anywhere near 50% of the cycle required for it to work in class C.

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Hi Alun,
I'm afraid that you designed a nice little heater. I figured a typical beta of 200, but its max is 300 and will be more when heated.

I don't know how many volts of output the transmitter will have. It must have a peak of about 0.8V (I don't know where you got 2.1V from) or our output amplifiers won't work.  ;D

post-1706-14279142200864_thumb.gif

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Guest Alun

Why was it alright to use it with out transmitter then?
If the output voltage was only 150mVp-p that would only be 75mV peak which is nothing. The 47K bias resistor must've done its job but why didn't it over heat the transistor?

Can't we just increase its value to 200K?

Assumeing a beta of 300 the bias current would be 12.3mA which is far too low to cause thurmal runaway.

With 0.8 V the power would be very low, because the transistor will only turn on for a very short length of time during the peak of the sine wave. To get significant power you need 50% on-off whitch is impossible with a sinewave of any magnitude unless you bias the transistor above 0.7V. I chose 2.1V because the duty cycle would be high enough to get a decent power level.

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Hi Alun,
If the antenna impedance is 75 ohms, the total impedance of the center-tapped coil will be 300 ohms. When saturated, the transistor will have a collector current of about 30mA into the 300 ohms load. If the base bias resistor is 47k, the beta will need to be 170, about right.

I see what you mean about having about a minimum output from the transmitter of 1.6Vp-p to drive the RF amplifier as 50% class-C. If the transmitter doesn't produce this much, then my RF amplifier will need another RF amplifier, but won't smoke without drive like yours.  ;D 

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Guest Alun

Hi Alun,
If the antenna impedance is 75 ohms, the total impedance of the center-tapped coil will be 300 ohms. When saturated, the transistor will have a collector current of about 30mA into the 300 ohms load. If the base bias resistor is 47k, the beta will need to be 170, about right.


Where did you get 300ohms from?


I see what you mean about having about a minimum output from the transmitter of 1.6Vp-p to drive the RF amplifier as 50% class-C. If the transmitter doesn't produce this much, then my RF amplifier will need another RF amplifier, but won't smoke without drive like yours.  ;D 


1.6V?

This is incorrect.

The transistor has a turn on voltage of about 600mV to 700mV, the sine wave will always be bellow 0V for half the cycle and it will be from 0 to 700mV near zero crossing so the peak voltage will have to be significantly above this for the transistor to conduct for long enough to transmitt a significant amount of power to the load. 50% duty cycle is impossible with this circuit because of the 0 to 700mV points in the crossover part of the cycle prevent it, this is why I suggested a voltage that was bigger than 700mV by a factor of 3.

With an input of 1.6V peak:
sin-1(0.7/1.6) = 26o before it turns on.

This occurs at both crosover points in the 180o positive half of the cycle so:
180o - 64*2 = 128 degrees on per cycle.

This gives a maximum duty cycle of:
128/360 = 35.5%

With 2.1V:
sin-1(0.7/2.1) = 19.5o
180o - 39*2 = 141o
141o/360 = 39.2%

Which I admit isn't much bigger so it woudn't make that much difference but this demonstrates that 50% duty cycle is impossible without biasing the transistor. Also these values are some what optimistic as the transistor won't pass much current when its near its turn on voltage.

4.5V peak would only give 45% maximum.

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Hi Alun,
The impedance ratio of a transformer is the square of its turns ratio. So if half the winding is 75 ohms for the antenna, the full winding has twice as many turns therefore is 300 ohms. ;D

I see what you mean that the transistor without bias will conduct for a very short period and never reach 50%. In fact it will be much less than your calculations because the transistor's input coupling cap will charge with the input waves and supply a negative voltage to the base.

As a compromise, your RF amp will be fine with a 100k base bias resistor.  ;D 

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I have tried the TV ariel Amplifier and it worked! Thanks! Gave it an extra 8db so it now goes about 100m on any frequency....  :D Prank Time! btw do u think i could increase it further by adding another amplifier or would that blow the whole circuit?

Thanks!

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Guest Alun

Yes the second amplifier would be overdriven for sure.

You could of couse build your own class C RF power amplifier, but it sounds like the one you have now is already powerfull enough to get you busted.

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In some countries all unlicenced transmissions are illegal. Here in Canada, the allowed power is very low. Someone built and tested my 1/4W FM transmitter and saw the RF cops go by later with their truck with the antennas on top scanning for him. I got busted for my sparking fan.

You might get away with it if nobody complains of interference from your transmitter.
Just think what would happen if your transmitter creates a harmonic at an RF frequency used by airplane communication and guidance. You might cause a disaster!  :o

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