# Impedance matching in Transmitter Circuit

Consider the FM transmitter circuit from here (https://eeeproject.com/fm-transmitter-circuit/) with circuit diagram

In this question (Antenna in Transmission & Receiver circuits) I was going to discuss in general how an antenna in such transmitter (or receiver) circuits may be realized (roughly: does it simply suffice to attatch there, where the antenna is drawn in the picture, any metal rod and that's all; or even if it's really neccessary to attatch there "something") and here I would discuss a further facet of my trial journey in the realm of antenna physics.

As far as a learned the motivation behind the practice of Impedance matching is just to improve the efficiency of transmission/receiver circuits by choosing the "optimal" parameters for the antenna in a given circuit in order to reduce the energy loss.

Assume, we dealing with with our "toy example" from above and I want to attach now a matched antenna to it (so an antenna with optimal parameters as far as I understand the priciple of "impedance mathcing" correctly) , instead of taking an "arbitrary" metal rod. If what I wrote before is correct, then the only relevant parameter we can optimize is the length of the antenna rod.

How can I explicietly determine/ calculate the neccassary parameters for my antenna (in this case the only relevant parameter is the length of rod; so I have to determine the "optimal" length) from the parameters of the other components of the circuit, in order to archive the optimal matching for the antenna?

• Make the single rod antenna a 1/4 wavelength long for enough output power to make the transmitter illegal. The capacitor values are much too high for the fM band. Anything moving towards or away from the antenna will change the capacitance of the LC which will change the radio frequency. The frequency will also change as the battery voltage runs down. The simple circuit is missing pre-emphasis so sounds received will be muffled with no high audio frequencies. Oct 10, 2021 at 19:50
• This circuit is so simple that the difference between a proper match and almost anything else will hardly be noticable. If you want a "proper" FM transmitter, you will need a much more complex circuit. There's no point in trying to do impedance matching if the circuit itself is has poor performance, which is the case here. Oct 10, 2021 at 20:39
• @Audioguru: What do you mean by "Make the single rod antenna a 1/4 wavelength long for enough output power to make the transmitter illegal."? especially I not understand what you by "making the transmitter illegal" Oct 10, 2021 at 20:44
• @Bimpelrekkie: Yes, this circuit is simple, but from conceptional view, what should one do I one have (maybe a more complicated) receiver/transmission circuit, and one wan't to find the "matching" antenna. How one should start and and what ideas & calculations are involved? Oct 10, 2021 at 20:47
• I not understand what you by "making the transmitter illegal" In many countries you're not allowed to transmit anything in the FM band as that can disturb the reception on other receivers. Oct 10, 2021 at 20:50