I used to make a preamplifier like this. The transistor could be different. In the source I recommend a smaller resistor R2 value of 1 - 2 kOhm. I also did not put resistor R1. This resistor widens the band, but on medium waves this resistor is of little use.
I have rebuilt about five different receivers this way. The sensitivity of the receiver has improved significantly.
I give links in Russian. Google translator to help you!
https://pandia.ru/text/78/586/53144.phpView attachment 244284

Thankyou @Bordodynov I went to the link and bookmarked it for later reference.

 

The best suggestion to improve your portable is probably to try an external wire antenna wrapped with a few turns around the receiver, and then to a ground (e.g. water pipe). If you want to experiment, open up one of the radios, find the ferrite rod and wind a few turns of plastic covered wire around it (away from the main coil), and bring them out of the box. Throw one end over something high, connect the other to a ground, (e.g. water pipe) or just leave a few metres lying on the ground (pipe is better).

Before going to the trouble of building the amp, try just touching the telescopic rod or connecting a longer wire to it, if you don't hear any difference (and I suspect you won't), then it probably isn't connected to the AM receiver and there is no point building an amp.

BTW, I found this on the ABC reception site - if you click on Map, then news radio:

View attachment 244283

Which, if I'm interpreting it correctly, suggests you should get good reception all the way down to kangaroo Island.

The map might be assuming flat ground which it is not! I am right in the middle of southern foothills to the mount lofty ranges. This might explain the bad reception of a weakly transmitted signal. Other ABC stations are okay and they have a greater powered transmission.

 

The LC circuit on the antenna is not an oscillator. It is a band-pass filter.
When centered around the desired radio frequency it attenuates the unwanted signals.

 

It is a good question and getting people to think on a specific aspect is an art.
The weak signal has little energy. In a broad band parallel tank circuit the stronger signal dominates.
In a parallel tank circuit specifically tuned for a specific weak signal the energy is stored efficiently in precise resonance
such that the AC voltage is peak as the oscillating wave resonates between the inductor and capacitor.

The movement of energy to and fro in LC tunes circuit is receiving small ripples as a resonant circuit.
An RC circuit is producing small ripples coordinated by its RC charecteristics and having a time constant.
Since the LC can store this energy an incoming small wave will constructively join and increase that resonant energy.

Some like to refer to this as nudging a water tank at the right moment to slosh the wave back and forth therefore a tank circuit
and some like to describe that as pushing a child on a swing, not sure why there are so many varieties of resonance.
I think the picture of the parallel tank MrChips presented is worth visualizing it's resonant effect and how that energy should be handled to improve a weak signal.

 

It is a good question and getting people to think on a specific aspect is an art.
The weak signal has little energy. In a broad band parallel tank circuit the stronger signal dominates.
In a parallel tank circuit specifically tuned for a specific weak signal the energy is stored efficiently in precise resonance
such that the AC voltage is peak as the oscillating wave resonates between the inductor and capacitor.

The movement of energy to and fro in LC tunes circuit is receiving small ripples as a resonant circuit.
An RC circuit is producing small ripples coordinated by its RC charecteristics and having a time constant.
Since the LC can store this energy an incoming small wave will constructively join and increase that resonant energy.

Some like to refer to this as nudging a water tank at the right moment to slosh the wave back and forth therefore a tank circuit
and some like to describe that as pushing a child on a swing, not sure why there are so many varieties of resonance.
I think the picture of the parallel tank MrChips presented is worth visualizing it's resonant effect and how that energy should be handled to improve a weak signal.

Well explained, thankyou @sparky 1

 

To boost am radio I wind 24 m of ins wire on a frame with am radio tuner across ends, the radio is placed in or near frame and both tuned for maximum reception. Really boosts reception . Think it is known as magnetic antenna.

 

Check out options for AM antennas on Amazon or other electronic sites.
you can add it in parallel with the existing ferrite antenna inside your radio or just replace the small ferrite antennas.

https://www.amazon.com/Sutekus-Ante...m+antenna&qid=1627442244&s=electronics&sr=1-5