AM Medium Wave Radio Receiver Design.

First of all, I am a hobbyist and stuff related to electronics are not very strange to me but the chapter of Radio and RF frequency , intermediate frequencies , resonance are haunting me since I got myself indulged in this project.

My goal is to design a very simple Medium Wave Radio receiver that should not use any kinds of ICs ,IF Transformers or complex components. Only a few inductors , loopstick (found from an old broken radio) , trimmer , ceramic capacitors , transistors (BC547) and resistors are to be used.

The radio should play all the channels from 500 KHz to 2 MHz (legitimate medium wave broadcasting frequency in my country).

So I browsed through the internet and found some schematics.

Circuit 1

This was the first radio circuit that I tried to work with. But without any productive result. Neither did the creator of this diagram mention the number of turns nor the wire diameter nor the solenoid diameter and nor the coil length.Now lets head to the second schematic.

Circuit 2

This one didn't seem to work either! The creator mentioned number of turns only without mentioning diameter of wire and diameter of solenoid and coil length. Now lets have a look at the last circuit I which tried.

Circuit 3 (YT)

I found the last circuit from a YouTube Channel. Same as the 2nd circuit, the creator forgot to share details about wire diameter and solenoid diameter and coil length. Neither did it appear to be working at all!

Here are the pictures of my loopstick and gang

The loopstick has 2 coils. Yellow one is the longer coil and the Red one is the shorter. I did not make any changes in coil turns or coil diameter while using this loopstick in my DIY circuit, I just cannibalised this loopstick antenna wholly and connected the two terminals of my gang condender to the longer (Yellow) coil accordingly and then I tried to get the input from the shorter (Red) wire by connecting one end to emitter and another end to the base of the first RF amplifying transistor (Circuit No 3).

I connected the 4 individual capacitors in the gang in parallel to get a total capacitance of almost almost 320pf (C1 + C2 + C3 + C4) while the small green wire in the middle of the gang joins the ground of both FM and AM side. That can be seen in the image of gang I referred above with a hyperlink.

Not much adequate information is given on the internet about the loopstick in Radios. In some circuits, they mention the number of turns but they forget to mention the diameter of the wire and the diameter of the solenoid on which it is turned that makes it harder to figure the inductance out.

I am willing to develop a heterodyne radio without using IF transformers. Although IF transformers are completely out of market, I have only two IF transformers which were pulled out of that old radio circuit, some of the legs of those transformers got broken. Still, I would prefer a heterodyne schematic with no IF transformers.

Points to be noted...

  1. I powered all 3 circuits with a 9 volt battery.
  2. While tuning, the speaker is only making random "clicking" sound , that too very timidly. Nothing else.
  3. In circuit 2 and 3, 1N4148 diodes have been used for rectification purposes because I dont have germanium ones right now.

Now, are there any problem those circuits? Or it is my faulty design which is making the resonance weak? The nearest Medium Wave Broadcaster is almost 250 Kilometers (155 miles) away from me.

I am stuck in this project and cant find a way out but still have the wish to complete it desperately! So I would like to hear some tips from the experts in this community for the betterment of my project. A working circuit diagram will even be better for me to understand the things properly.

Thank You!

  • 2
    $\begingroup$ My first impression is the 250 km distance. Those receivers (especially the second one) aren't very sensitive so would need to have a strong signal to work. 250km is a long way to go on MW even with a commercially designed receiver. Also, you'll need to a good (!) ground to go with the antenna. I had a crystal set (similar to the second circuit) as a kid and when I connected the ground to a copper water pipe, the signal jumped 20db at least. $\endgroup$
    – Duston
    Aug 25, 2023 at 14:59
  • $\begingroup$ @Duston good ground? Even with battery? Doesn't connecting to the negative of the battery automatically ground the antenna? And even though the station is 250kms away, the channel is very clearly audible in a commercial radio set. $\endgroup$
    – CoderBittu
    Aug 25, 2023 at 17:06
  • 1
    $\begingroup$ @CoderBittu no, that's not an RF ground plane, that's a ground for DC only. Yeah, I'd agree, my fist diagnosis would be: not sensitive/low noise enough to receive MW over that distance. $\endgroup$ Aug 25, 2023 at 18:03
  • $\begingroup$ I'm not sure what about IF transformers you care (or not care) about, but: if you're willing to go the distance of building a superheterodyne receiver, is there a specific reason you want to receive MW? $\endgroup$ Aug 25, 2023 at 18:05
  • $\begingroup$ @MarcusMüller hmm then how do commercial radios create a ground? I have never seen a separate wire for grounding or earthing the antenna. MW as a startup FM will be the next. $\endgroup$
    – CoderBittu
    Aug 26, 2023 at 2:52

1 Answer 1

  • Circuit #1 AM Radio Receiver Circuit

...is a regenerative-type receiver. It could be adapted to use a "loopstick" (ferrite rod). Regenerative receivers can be made very sensitive, but are often very difficult for beginners to properly adjust, especially when components are substituted.

Adjustment of the variable resistor is quite critical - at one point, the circuit should start oscillating. There is a small adjustment range near this point where the circuit is most sensitive to AM-modulated signals. When oscillating, you can still hear the station, but oscillations are annoying.
The audio driver for an 8-ohm loudspeaker is quite weak in this circuit - it won't be very loud when working properly.

  • Circuit #2 Simple AM Receiver Circuit

A simple resonant antenna circuit fed by antenna and ground, followed by an audio amplifier.

Like circuit #1, getting the antenna, ground, and LC resonant circuit working properly gives builders much grief. Driving headphones is a better idea than trying to drive a low-impedance loudspeaker, but the DC current that flows through headphones likely creates a very loud "pop" that could be ear-damaging.
Most common headphones are quite low-impedance (32 ohms). This circuit works best with high-impedance headphones (2000 ohms or greater) which are difficult to source.

  • Circuit #3 (Hand-drawn)

Most appropriate for OP's ferrite rod, with two windings. The winding with fewer turn should drive the transistor RF amplifier's base (red wires?). The winding with more turns (yellow wires?) is tuned by a parallel variable capacitor. Stripping the thin, transparent insulation on these wires requires care - many builders don't do this properly and don't end up with a tuned circuit when the capacitor is added.
The variable capacitor described has multiple parts. Since the ferrite rod has unknown inductance, it is difficult to suggest which sections of the variable capacitor should be used. I would suggest trying it two ways:

  • Use one gang (C1: 5pf - 135pf)
  • Use two gangs, in parallel (C1+C2: 10pf - 270 pf)

No ground, and no external antenna should be needed. These ferrite rods are designed to require neither - but should not be mounted in a shielded box. A nearby power transformer can annoyingly add 50/60 Hz modulation that masks a radio signal.
As antennas, these ferrite rods are directional - if you point the rod's long axis toward the radio station, you get a weak signal. Face its broad side toward the station.

Another potential problem involves feedback - with two stages of RF amplification, too much gain can cause the whole shebang to oscillate. This makes breadboarding quite risky. But having a big signal to drive the diode detector is good.
Once again, driving a loudspeaker directly with DC is not a great idea, giving feeble audio. A step-down audio transformer is proper here (1000:8 ohm) but these are difficult to source.

Heterodyne receiver?
An ambitious project without test equipment like signal generator, oscilloscope. If you do wish to attempt a multiplying-type mixer, try a direct-conversion receiver first, where the RF signal is directly converted to audio. You must put up with the howling carrier, (or tune carefully so its frequency tends toward zero-Hz).
Once you can get a mixer working properly providing decent audio, the next step in complexity might involve the mixer driving an intermediate frequency amplifier to a diode detector.

  • $\begingroup$ It must be noted that the nearest radio station is 250km or 155 miles away from me. So, from what you said I understood that the circuit 1 and 3 are worth retrying again? $\endgroup$
    – CoderBittu
    Aug 27, 2023 at 7:58
  • 1
    $\begingroup$ Circuit #1 has low parts count, but is technically complex, and frustrating for most beginners (especially where stations are weak), but when properly built, is excellent. VK3YE's site: vk3ye.com/gateway/nonov99.htm shows how to add an antenna to a ferrite rod regen, and includes a proper audio headphone driver...well written. I've built a few regens, but have little experience with anything like circuit #3, so am reluctant to guess what difficulty you may experience. I'd suggest tackling RF and audio separately: get an audio amplifier working first. $\endgroup$
    – glen_geek
    Aug 27, 2023 at 13:26

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