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# a very simple FM Tx

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Hi Walid,
I was talking about the parallel resonant LC at the collector of the RF amplifier transistor on my FM transmitter. Its impedance is infinity at its resonant frequency without a load. But it is a terrible match to a 50 ohm antenna.

The article you are looking at shows a pi (CLC) lowpass filter/matching circuit that has a high input impedance from a transmitter and a low output impedance to an antenna. It is a good matching circuit because it has low loss. It also reduces harmonics that cause interference.

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Hi guru

I was talking about the parallel resonant LC at the collector of the RF amplifier transistor on my FM transmitter. Its impedance is infinity at its resonant frequency without a load. But it is a terrible match to a 50 ohm antenna.

I can't understand or accept that the LC impedance is infinity at its resonant frequency. Here is your formula:

At resonance XL = Xc in magnitude and if they cancle each other because of phase.................... Yes Yes its impedance is infinity at its resonant frequency.

But still some point obscure:
If U GURU, for example, want to replace your C14 (=22p) in your FM Tx

with a Pi C-L-C network as in that site: http://hem.passagen.se/communication/antenimp.html
you have to know the Zout of the RF amplifier transistor (Q3) to match it with the 50 ohm antenna using the calculated values of caps and coil.

May I worgly understood what the above link want to say, may be he want to tell me that the Pi C-L-C network match any Tx at 50MHz.

so tell me what you have to do if u decide to use that network.

thank u guru

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Hi Walid,
I never have used an RF pi network.

The output impedance of the Tuned RF Amplifier depends on its DC current and on its drive level if it operates in class-B or class-C. With a high DC current and high drive level then the resonance of the tuned circuit causes the output voltage to exceed the power supply voltage. Then its output has high power into a fairly high impedance.

I think that a load line must be used to determine the optimum matching impedance of the pi network.

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Hi guru

two weeks ago, i discussed and write an article about a very simple FM transmitter, the one whose circuit is appear at the top of this topic- in an arab community.
my discussion was depending

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A few things affect  why his transmitters have poor range:
1) His battery is not new?
2) His radio has poor sensitivity?
3) His circuit layout is not compact?
4) His transistors are cheap Chinese copies?
5) His transmitting and receiving antennas are pointing in different directions?

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