Transmitter Pi Tuning and Loading network design

I’m trying to get an understanding of how Pi Output tuning circuits are designed.

It appears most radio transmitters are designed for a 50-75 Ohm load. Why is this? I’m thinking when coupled to an antenna, the loading control would be resonating it’s capacitor reactance with the inductance of the antenna, so at resonance Xc =Xl. This would only leave antenna resistance; is this where the 50-75 Ohms appears?

I’ve been looking at Low Pass Pi (L) output circuits for different CW transmitters Some only consist of 365 pF for both Tuning and Loading . Consider a circuit centered around 7.150 MHz, using a 10.4 uHy inductor.

When tuning , the 365pF and 10uHy resonate and the loading capacitor is essentially in series with the parallel resonate circuit. When properly tuned, the loading plates somewhat unmeshed, it’s reactance becomes highest and maximum voltage across it is achieved allowing maximum coupled output.

Looking at similar output circuits (some are multi-band transmitters) I see the loading capacitors range from 0-365pF to 0-1400 pF. Obviously the higher value will give you a greater range. How is that value determined?

Can the loading capacitor of the Pi (L) network also be considered Xc of a parallel resonant circuit, with Xl of the antenna? Using the capacitance of the loading capacitor and desired frequency can the Inductance of the Antenna be calculated using the resonant frequency formula?

Is there a formula/s for calculating all this; can it be broken down?

My next question is for adding another network. Some antenna tuners are capacitive input /output with an inductor, in center, to ground. When a Pi (L) is feed into these tuners, how are the capacitor and inductor values calculated. Are all calculations based on a 50 - 75 Ohm input?

I’ve read and re-read various reference texts, but it’s not sinking in. It needs to be kept simple. Possibly the mathematics for these calculations are beyond my understanding. If possible can you simplify in general, as well; such as: this goes up while this goes down, because of….etc?

 

Is there a software circuit simulator out there that I can build a Pi followed with a T network, plug in values and observe the results?

 

Hey Duke,

Is it possible you could scan me the article from your Amateur Radio Handbook. 3rd Edition RSGB 1961?

Eddie

 

Thanks Duke, The Wikipedia site has a lot of the info I desire.

I happened across a Summer 1965 Elementary Electronics. There is an article on constructing a Z bridge for measuring antenna resistance. Also the text described how one could determine the resonant frequency by using a SWR bridge. A graph is made of the SWR for every 50 KHz. The resonant frequency of the antenna is that of the lowest SWR.