Transmitter Pi Tuning and Loading network design

[Y2KEDDIE]

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?

 

[davenn]

Y2KEDDIE,

The "50 Ohm question" comes up from time to time.

I have posted this stuff on here in the past

The standardization of 50 Ohm impedance goes back to developing coax cables for kilowatt radio transmitters in the 1930s. The quick answer is that 50 Ohms is a great compromise between power handling and low loss, for air-dielectric coax.

more from MIcrowaves 101

http://www.microwaves101.com/encyclopedias/why-fifty-ohms

cheers
Dave

 

[duke37]

50Ω is chosen for convenience. Older transmitters used to use higher loading resistances, possibly 400Ω. A half wave dipole will show a centre feed resistance of about 50Ω and if fed with 50Ω cable, this what the transmitter will see.

A valve in a transmitter may need a 10k load and a low pass pi match can do the matching to 50Ω and reduce harmonics. Transistors will need a much lower load.

There are 4 ways of connecting an L match and in theory will match to anything.
The matching has to get the resistance right and also the reactance so two controls are needed. The controls both affect the resistance and reactance so they need to both be adjusted, one affecting the other.

Obviously, the values of the components will need to be set depending on the requirements. Very high capacitances will be needed at low frequencies. The values can be estimated by plotting on a Smith chart, this is not easy understood.

 

[Y2KEDDIE]

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?

 

[davenn]

this one top class one .....

https://www.cst.com/

this is another serious one ....

http://www.awrcorp.com/products/microwave-office


Dave

 

[duke37]

I use 5spice and ltspice for simulation. Both are free but the free version of 5spice is limited in circuit size and will not let you store digital circuits.

 

[duke37]

I have found a book!
The Amateur Radio Handbook. 3rd Edition RSGB 1961

Pi network for a valve transmitter.
Chose a loaded Q of about 12 to give a reasonable compromise between selectivity and power loss.

Y=sqrt(R2/R1)
Xc1=R1/Q*(1+Y)
Xc2=Xc1*Y
Xl=R1/Q*(1+Y)^2

This neglects the effect of the anode choke, the series DC isolation capacitor and the safety load choke.

The capacitors are adjustable to take these factors into acount and to trim a little if frequency and load is changed.

 

[Y2KEDDIE]

Hey Duke,

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

Eddie

 

[duke37]

I am sorry but my printer/scanner is dead. Also It may not be advisable without specific permission.

Wikipedia has a page 'Antenna Tuner' which gives quite a bit of information.

I hope to play with MMANA to calculate the necessary feed to my end fed inverted V. I hope to plot this on to a Smith chart and then to look at a Z match which will also give a plot on the Smith chart and see if they coincide. Unfortunately, I have other things of more importance to do first.

 

[Y2KEDDIE]

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.