RF Transistor Selection
I am constructing FM transmitter to work in radius of 100 feet. For the frequency multiplier section I used BF494 with voltage divider bias (R1=15k, R2 = 4.7k). The colector has LC tank circuit with 4.7pF capaitor between collector and emitter. Varactor diode is used for frequecy modulation which is coupled to the tank circuit using 22pF trimmmer. The value of emitter resistor Re=68R. A 75 cm antenna is coupled to collecter of BF494 using 2.2pF capacitor. Frequency = 100MHz. Vcc = 9V.
My problem is that I am hardly getting 40 feet range using above configuration.
I tried replacing BF494 with 2N3904 but there is not significant improvement in the range. How do I increase the output power? Which transistor should I use.
For FM transmitters go to h**p://www.talkingelectronics.com/
In u r case, the short range may be due to low quality transistor. Cheap transistors from uknown manufacturers does not have the specifications of the "original" ones.
Have also in mind that high frequencies need short connections. Copper tracks and component leads must be kept as short as possible.
Try using another transistor which FT is at least 10 times the working frequency, i.e. BFW92 or BFR 91.
Also, if possible, increase the supply voltage to increase the output power.
Third try to couple better the antenna to the oscillator, usually a short antenna has low radiaton resistance, so try to couple it with a capacitive divider not to load the oscillator.
Mandi
HI Mandi,
Do you mean to couple the antenna as shown in schematic below? If yes how do I calculate the value of capacitors for best results?
Hello Cmos.
It think the circuit is small.
Please, Post the whole schematic diagram so that we can help you.
S.