FM impedance antenna matching

[AmRadiant]

hello all,

about this project:

http://www.electronics-lab.com/projects/rf/021/index.html

For my objectives only the first stage (oscillator T1) can be enough (100-150 mW at 13.8 Volts): so I must match this stage to antenna (50 or 75 ohm impedance).

What solution do you suggest?

1) using C7 L4 C8 instead C6 (but I think that impedance out from T2 is lower than T1... so I'm not sure)?

2) a socket directly to a coil from L1: I've tried from cold side, but from first wound power decrease significantly...?

Some ideas?

Thanks!

[audioguru]

If you connect an antenna directly to Q1, then the antenna becomes part of the tuned circuit and the frequency will change all over the place if anything gets near the antenna.
On my FM transmitter, I used a buffer transistor to isolate the antenna from the oscillator.
Also on my transmitter, I used a voltage regulator to keep the frequency constant as the battery ran down, and also used pre-emphasis (treble boost like FM radio stations use) so the sound is the way it is supposed to be.

[AmRadiant]

Ok, thanks!

I suppose that for higher output I must simply change Q2 and Q3 2N3904 with -for example- 2N3866, and use lower resistance for R6 and R8...

I use 13.8 Volts stabilized, so I presume I can cut away voltage regulator...

[audioguru]

I suppose that for higher output I must simply change Q2 and Q3 2N3904 with -for example- 2N3866, and use lower resistance for R6 and R8...

I use 13.8 Volts stabilized, so I presume I can cut away voltage regulator...

I haven't seen a T0-39 case on a transistor for many years. It might melt with 13.8V. Digikey don't sell 2N3866 transistors so either it isn't made anymore or nobody is buying it.
The audio preamp transistor in my transmitter is biased for a 5V supply.

[MP]

13.8V will not hurt a 2N3866. Mouser has them as their PN 610-2N3866 @ $1.50 each. They are also sold by Futurlec, Digitron, American Semiconductor, Electronic Surplus, Ocean State Electronics, and American Electronic Resource. Searches can easily find thousands with PartMiner, Global Spec., and other IC search engines, but beware since you can easily find them at prices like $4.00. These are also available in a TO5 package.
Advanced Semiconductor, Inc. still manufactures the 2N3866.
I think the best price can be found from Mouser electronics.

Digikey has a lot of stuff, but I would not use them as a standard by any means.

MP

[AmRadiant]

ok... many thanks!

In my country (italy) 2N3866 is very common... and costs about 1 euro.

But unfortunately 2N3553 is uncommon at all  >:(

for 13.8 Volts supply... well I use that with 2N2219 (or 2N2218, too) without problems (heatsinked), so I presume that works ok with 2N3866...

[audioguru]

Doesn't the high RF power interfere with real FM stations ot TV stations?

[MP]

In my country (italy) 2N3866 is very common... and costs about 1 euro.

1 euro is approx 1.25 USD, so the price from Mouser is pretty close to what you have been paying for this part.

MP

[AmRadiant]

Doesn't the high RF power interfere with real FM stations ot TV stations?

ehm... yes, of course  :D

with a dipole from TV antenna (using correct lenght for FM), the first stage from 2N2219 project seems to work very well about 800-1000 meters open space (13.8 Volts)... my tests.