AM Radio receiver - can't get it to work

[dvh]

I'm trying to build an AM receiver after I successfully built a radio (Colpitts oscillator, some electret mic).

I don't seem to be able to pick up anything other than noise. I'm wondering why? Let me explain what I did so far:

In the AoE book, there is a seemingly simple circuit I tried to follow (attached). There is a station at 540KHz where I live, so I calculated the LC to resonate there. Connecting a straight wire (about 5-6m long) to it and using power socket ground as ground, all I can pick up is noise (measured with oscillosope with 15pf probe at points X, Y).

Instead of a varicap, I calculated that 330pF should do it and instead used a wiper on the inductor to tune. The inductor I measured to be 480uH. (image attached) The diode is germanium. 

Any help that would notch me in the right direction would be appreciated. I don't think I know what I don't know, so even questions might help!

[Roehrenonkel]

Hi dvh,
can you verify that your local station (transmitter) is active?
Maybe with a known working AM-radio.
Is your diode good?
Try another one.
Try longer antenna-wire.
Try a "frame-antenna" that you can point to the sender.
Try at night, reception is usually better at night-time.

Best regards and good luck.

[pdenisowski]

The nice thing about AM radio is that most hobbyist function generators can create signals with frequencies in the AM broadcast band (~1000 kHz). 

I've often connected a function generator to an antenna and then generated an AM-modulated signal (often a 1000 Hz tone) to make a quick functional test of an AM (or HF) receiver.

[dvh]

Yes, local station is active. I've verified with a working radio.

I'll look into your other suggestions!

I was thinking... The original circuit does not have a wiper on the inductor and I do. I became uncertain how to wire things up. Should the 330 pF cap still connect between the 2 leads of the inductor or should it connect to between the inductor's terminal connecting to the antenna AND the base of the wire to form the resonant circuit?

[iMo]

You have to have the wiper always connected to the capacitor.
Wire the antenna via a 100pF ceramic capacitor (could be 15pF-100pF, the lower the value the better the selectivity) to the wiper as well.
Do not short two adjacent coil turns with the wiper, of course.
Diode - an RF germanium one (like old Tesla's Ga20x) or the new schottky 1N5711..

BTW your 480uH coil with the 330pF will resonate at around 400kHz..

[dvh]

Thank you!

I'll definitely will try it this way! @iMo, I figured I'd oversize the coil a bit so I wouldn't have to use the edges of it that much to get in the AM frequency range.

What is the result of shorting 2 coil turns?

What does R2 and C3 do in parallel?

[wasedadoc]

1.  The signals at X and Y will be small.  You need a good low noise scope to see them.  Especially if using a probe set to x10.

2.  The RF signal at X will be small, possibly too small for a Silicon diode.  Use a germanium one.

3.  I don't understand how your wiper is contacting the coil wire.  If the wire has an insulating coating the wiper will not make contact.  If the wire is not insulated all the turns touching each other will be shorted runs and not working as the inductor you want.

[CaptDon]

What opamp did you use and how is it wired? You don't show power connections and you show only one opamp input. That is nix. Show us more about the amplifier. The R.F. section should work. It could be that the cathode end of the diode is so far positive reference to the anode end that no audio will be recovered.

[Terry Bites]

The front end if correctly assembled should pose no problems.
The diode needs to be a "point contact type".
How is the diode detector coupled to the amplfier circuit?
Is there a signal at Y, how big is it and will your amp have enough gain to get a useful output?
You can get some improvement by pre-biassing the detector diode.
The circuit values for the pot, bias resistor and bypass cap are not at all critical.

https://www.pveducation.org/pvcdrom/characterisation/measuring-ideality-factor

[iMo]

Thank you!

I'll definitely will try it this way! @iMo, I figured I'd oversize the coil a bit so I wouldn't have to use the edges of it that much to get in the AM frequency range.

1. What is the result of shorting 2 coil turns?

2. What does R2 and C3 do in parallel?

1. You will create a "magnetic short" (because you create a shorted ring on the coil)
2. R1 is the load for the detector and the C2 blocks RF frequencies such you get audio only into your amplifier.
Also I would put a series decoupling capacitor in front of your audio amplifier.
I built myself such stuff as a child, but in that time we had a lot of strong long- and medium- wave stations around us 


PS: Congratulation, you have still a powerful station in your country..
Wiki:

.. broadcasts it with 2 MW power on 540 kHz AM from transmitter Solt (the most powerful medium wave transmitter in the world)[2] and several FM stations, covering Hungary and the neighbouring countries.

[dvh]

What can I say! We are that cool We are the best in an "ancient" tech 😂😂

I've uploaded a complete picture to show you how it is wired. It is based on iMo post. The antenna is the yello and ground is the black cable.

I still dont have luck. I use a GA104 diode. I'm not sure if it is point contact. There seems to be a very thin wire in it. Maybe 🤔?

[CatalinaWOW]

Have you checked continuity between your slider and the coil with an ohmmeter?  In your pictures it is not clear how thoroughly the insulation has been sanded from the coil, and an oxide coating on the slider may also be an issue.  Even with some level of continuity, a significant resistance in the contact will reduce the Q of the circuit greatly.

I wouldn't worry too much about shorted turns.  It isn't ideal, but it doesn't seem to totally kill operation.  This method of tuning was widely used in crystal radios of the 1930s to 1950s and didn't seem to cause much trouble.  It certainly didn't harm the example I built in the late 1950s and my slider was much wider than yours. 

From a theory standpoint it removes some number of turns from the coil (changing the inductance by a small percentage, and adds one or more low inductance (single turn) coils magnetically coupled to your main loop in a step down configuration.  These parasitic coils are "tuned" by whatever stray capacitances are around, mostly the coil to coil contribution and probably resonate far higher than your main circuit.  Unless Murphy strikes and this resonance is harmonically related to the frequency you are interested in it seems unlikely to be impactful, again consistent with the experience of thousands of these configurations historically.

As another piece of evidence that it is not a huge issue, I once had access to a US Navy WWII radio receiver that used variable inductors for tuning.  A complicated bit of mechanical gadgetry, the coils were about 3 to 4 inches in diameter (75 to 100 mm), about 8 inches long (200 mm) and were made of some hard silver plated wire, spaced about a tenth of an inch apart.   The mechanical contact with with coil was a slider that spanned two turns.  As the coil was rotated by a gear train the slider moved along the coil, always shorting the turn under the slider.  A lot of engineering obviously went into this radio, and it worked well enough to be produced, so the shorted turn wasn't a deal killer.

[CaptDon]

Unhook the scope, it is killing your resonant circuit. You will probably recover only about .1vpp into the hi impedance input of an amplifier and your slider is totally bullocks, get rid of it and use a tuning capacitor. These things are broad as a barn door anyway. My experience is with a strong local station the tuning when at resonance only provides a marginal increase in recovered audio. In fact, you could probably do away with the tuned circuit all together and place the diode directly across the high impedance input of the phonograph input of an amplifier and throw 15 feet of free wire onto the diode and get recoverable audio. When I lived near a 5KW A.M. station I could get reception on my guitar amplifier just by holding the cable that plugs into my guitar!

[mikerj]

You definitely need to check continuity between the wiper and the coil, the enamel insulation can be surprisingly tough to remove fully.  Adding an earth connection to the other end of the tuned circuit (as shown in the schematic) can greatly boost the output of simple receivers like this, which is effectively just a crystal radio and an audio amplifier.  A physical metal rod driven into soil is ideal, but a water pipe may also work if you have copper piping in your house.

[SuzyC]

In your picture the bottom end wire of the coil is clearly not connected.

[CatalinaWOW]

In your picture the bottom end wire of the coil is clearly not connected.

It shouldn't be.  The wiper and the top are the two connections, makingita variable inductor.

[MathWizard]

I'm building a 5.2MHz AM TX and RX, and with some of the air core inductor's I made, none any bigger diameter than 2-3", 5-8cm, and with the right capacitor, I was getting -50 to -40dBV, 3-10mVrms of a local strong station. So some of the 2 transistor receiver's work great with a signal that strong.

One of the key's to 1 simple 2BJT receivers is to have 1 BJT saturated. Here's a book I just found, I should have a look at it too.
https://www.worldradiohistory.com/BOOKSHELF-ARH/Technology/Author-Groups/A.T.Collins/Simple-Radio-Circuits-Collins.pdf


I making a tuned RF receiver, I have 3-4 RF amp stages with filtering in between each stage, for my weak 5.2MHz signal, so far it's just under 1mVrms at the antenna.

Then I had 1-2 audio stages, and for now the actual audio output is from heaphones from a clock-radio audio board.

But the sound quality was not great, so I'm reworking the audio TX section, and now the latest problem is I'm having a hard time getting rid of FM stations all over the circuit at -50 to -60dBV. All my audio band filtering, doesn't seem it affect the FM range at all, or it's on the power lines, or the breadboard and circuit boards.

But on my RF receiver board, I have a way lower noise floor and no real FM problem. So IDK, what the inductors are really doing at 100MHz.

[MathWizard]

You definitely need to check continuity between the wiper and the coil, the enamel insulation can be surprisingly tough to remove fully.  Adding an earth connection to the other end of the tuned circuit (as shown in the schematic) can greatly boost the output of simple receivers like this, which is effectively just a crystal radio and an audio amplifier.  A physical metal rod driven into soil is ideal, but a water pipe may also work if you have copper piping in your house.

How much worse is just the AC ground wire, from say your oscilloscope ?

On the radio TX RX I'm making, disconnecting the AC GND connections made it all worse, but I've never tried hooking on to the houses panel right over top of the GND stake/rod.

But when I goto finalize the radio, I'll want a AC GND on there, and then maybe try making some balun type thing for the antenna's.