hey whats 'ANT' in the diagram?

 

An ant is a bug.
Aren't you making a radio bug?

ANT is the antenna. It should be about 80cm long for the FM broadcast band.

 

i have used the same circuit with Bf194 transistor .
with a loaded antenna it gives range of 70meters in open areas.

how to make ANTenna?
take 1 solid core wires twist wrap around 2.5 inch aluminium stick[use pieces from shirt hanger]. put the antenna in a black ballpen! to make it look professionnal.

after antenna is over ....connect the solid core wire one end to the ANT point in the circuit.

 

Ithe details & theory behind the project

 

Ithe details & theory behind the project

The transistor is a simple common-base Colpitts oscillator. Look it up in Google.
The microphone is powered from the resistor R1 and its voltage changes increase and decrease the transistor's current which causes its capacitance to increase and decrease which creates FM. The increase and decrease in the transistor's current causes a little AM.

 

hey
i am trying to build this spy fm transmitter. i am following the exact specified values as in the ckt diag but i cannt catch any signal on fm radio. plz help me. suggest if i shld change the coil or var cap. i need help immediately plz plz suggest any changes so that i can make it work.

 

Hello,

Like Audioguru already posted earlier. The varable capacitor should have a larger range.

Quote:
Originally Posted by Roen Hayden
VC6# (Trimmer) 6pF to 8pF

^ does that stand for Variable capacitor? and what does i mean by (trimmer)?

Yes, VC stands for "variable capacitor".

A trimmer capacitor is small and is adjusted with a plastic screwdriver. A metal screwdriver will add stray capacitance and change the frequency of the tuned circuit.

6pF to 8pF is hardly any change. A trimmer capacitor will be rated for 5pF to 30pF or more.

Greetings,
Bertus

 

I got the idea, tell me whether hand made inductor is going to work properly....?

Or,....is there any methode to calculate inductance acurately.

 

Hello,

Like Audioguru already posted earlier. The varable capacitor should have a larger range.



Greetings,
Bertus

hi,can u be able to estimate the exact frequency of this transmitter?(assuming that it's btw 88-108)
Or is there a way to calculate?

 

Hello,

To calculate the value of the coil, take a look at this page about aircoils:
http://info.ee.surrey.ac.uk/Workshop/advice/coils/air_coils.html

To calculate the resonance frequency take a look at this page of the AAC eBook.
http://www.allaboutcircuits.com/vol_2/chpt_6/5.html

I used the calculator from this page for the value of the aircoil:
http://www.mantaro.com/resources/impedance_calculator.htm

I have done some little calculations:
L = 100.440 nH (when dia = 6 mm and lenght = 10 mm and 6 turns)

Lower frequency ( C = 30 pF) = 91.6 Mhz
Higher frequency ( C = 5 pF) = 224.6 Mhz
Middle frequency ( C = 15 pF) = 129.7 Mhz

Keep in mind that the calculated values are the ideal values.
Stray and parasitic capacities will let the frequency go down a bit.

Greetings,
Bertus

 

Hi,

CB's do use FM in Europe

 

Hello,

FM CB use was very populair 20 years ago in holland, now it is hardly used anymore.

Greetings,
Bertus

 

Hi,

FM CB use was very populair 20 years ago in holland, now it is hardly used anymore.

I didn't say a thing about its popularity, just that CB's do run FM in europe these days.
In Denmark we switched to FM somewhere in the eighties (IIRC) after a number of years with both AM and FM (using the same channels for both) and I think it is still legal to use AM if you've got one of the old rigs, but you're not allowed to make any changes at all inside your equipment.
PMR is FM as well.

Admiddetly, CB is a faint shadow of what it was back in the seventies, where it was hard to find a reasonably silent channel on an ordinary wednesday, but that doesn't change the fact that in Europe, you cannot buy a new walkie talkie using AM these days - guess my old AM and AM/FM rigs are close to antique (read "land fill candidates"), but back in the days, they were in near 24/7 use and provided lots of enjoyment.

 

In the USA it is alive and well. We have family FM frequencies around 400Mhz, but the 27Mhz CB band is still very much AM and SSB (much the same thing). I don't think FM 27Mhz would even be legal.

None of this applies in Mexico from what I hear, they won't confiscate your radios if it is installed in your car, but you are warned not to use them, as it is a violation of their federal law.

Not sure what Canada's rules are.

AM is MUCH simplier to modulate and demodulate IMO, so I like the way it is. I built a lot of simple transmitters and recievers back in the day (can you say crystal radio, CB style?).

 

You can vary the number of turns in the coil to make it transmit in the Ham radio band. Less turns the higher the freq and vise-versa. I made one that is 1/2 inch by 1/2 inch that transmits over 300 meters at 1.2 GHz on 1.5 volts DC and a one and a half inch antenna. It has spurious emenations (harmonics) down to 700MHz (The coil is one turn air core 1.8 inch diameter) It uses only nine components.

 

Boy, if you guys had SMT parts you could really make these things tiny! Talk about a bug!

Does anyone have a simple FM transmitter that covers the whole FM band in addition to quite a bit of RF power?

 

Boy, if you guys had SMT parts you could really make these things tiny! Talk about a bug!

Does anyone have a simple FM transmitter that covers the whole FM band in addition to quite a bit of RF power?

That last sentence violates the rules here. We try to keep it legal. Not saying I didn't violate some FCC rules when I was a kid, but it wasn't on purpose.

 

That last sentence violates the rules here. We try to keep it legal. Not saying I didn't violate some FCC rules when I was a kid, but it wasn't on purpose.

Of course Bill but what I meant to say is maximum RF power that the FCC allows. Thanks for the correction.

 

thanks keep it up.

 

thnx ..... m gonna try to build dis one ..