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[top204]

An excellent circuit.

That's just what I am planning as the AM modulation Charlie. An audio amplifier feeding the VCC of the output stage. I like the idea of the 2N7000 MOSFET as the final amplifier, I'll give it a go instead of the NPN transistor.

I can use the microcontroller as the oscillator using one of a few on-board peripherals, and the microcontroller will either control an MP3 player board, or raw WAV files played through 16-bit PWM as the DAC. Combining two 8-bit PWMs to give an excellent 16-bit output, with two resistors and a clever, and fast, bit of code I created. :-)

I was going to use the PWM for the MW oscillator and the AM modulation combined, but it may be difficult to get the bit-rate high enough for the frequencies required for good audio. I could use simple frequency multiplier circuits after the output with a few transistors and some ready made inductors, but I want to keep it as simple as possible, and maybe create a kit from it.

[ken_k]

An excellent circuit.

That's just what I am planning as the AM modulation Charlie. An audio amplifier feeding the VCC of the output stage. I like the idea of the 2N7000 MOSFET as the final amplifier, I'll give it a go instead of the NPN transistor.

I can use the microcontroller as the oscillator using one of a few on-board peripherals, and the microcontroller will either control an MP3 player board, or raw WAV files played through 16-bit PWM as the DAC. Combining two 8-bit PWMs to give an excellent 16-bit output, with two resistors and a clever, and fast, bit of code I created. :-)

I was going to use the PWM for the MW oscillator and the AM modulation combined, but it may be difficult to get the bit-rate high enough for the frequencies required for good audio. I could use simple frequency multiplier circuits after the output with a few transistors and some ready made inductors, but I want to keep it as simple as possible, and maybe create a kit from it.

Les this sounds like a very interesting project. I would like to hear how it goes.

I have a brand new 6CS6 on its way and will build an AM transmitter with it for use with old radios. I will also try using a 1R5, hoping to build a valve transmitter that uses less than a 1/4 watt from the power supply (70mW filament and <150mW B+).
https://www.rfcafe.com/references/qst/practical-design-of-mixer-converter-circuits-feb-1941-qst.htm
David
Regarding RF from rectifier noise, I could see it while doing some pre-compliance testing on a product many years ago, capacitors across the rectifier diodes rectified the problem (pun intended).
Re the 9kHz 10kHz spacing, good point, it won't be an issue with my old tube radios.

[david]

Let me know if you ever need dial cord.  I have a good size spool of top quality Japanese cord - quite fine and very strong.   Left over from the 1990s....

David

[charliecoutas]

I tried generating the carrier frequency, around 750KHz using the PIC's clock (64MHz), with the PIC dividing it down. It worked but there was a disturbing rumble on the audio from the AR88 receivers. Looking at the carrier on a scope showed that there was some sort of random AM low frequency modulation. So I went back to using a crystal and dividing down, as show on the circuit diagram, which came from the Internet.

With a 12V supply to the LM386, I get a swing of about 50V p-p at the aerial. Not being an RF guy, could somebody advise is that the sort of voltage you'd expect? Like I say, there are lots of harmonics and I guess that a better tuned PI filter would reduce these?

Best
Charlie

[david]

I think the PLLs in the micros introduce considerable jitter but I couldn't find a spec in the data sheet.  While this should amount to FM it will get divided down and appear as both phase modulation and some amplitude modulation.   
Hard to comment on your RF level without knowing the antenna impedance.   Obviously you're not going to have a 50ohm, resonant antenna at those frequencies.  Where's John when you need him?

David

[tumbleweed]

I think the PLLs in the micros introduce considerable jitter but I couldn't find a spec in the data sheet.

In more recent devices (18FxxK40 and later) they started including a PLL Frequency Stability (Jitter) spec, and they all seem to have it spec'd at +/-0.25%

Don't know if that applies to earlier devices or not.

[charliecoutas]

David

The aerial is about 5 yards of wiring wire, strewn in the same plane as the receiving aerial on the roof, which is about 10 yards long.

Charlie

[John Lawton]

I listen to the old radio comedies while I am working in the workshop, and swap the cycle of them every few months. They are such gentle comedies with no modern PC (Political Correctness) crap in them, so they are actually funny and are allowed to "take the piss" or use "double entendres" . I was born in the wrong era. LOL

My favourite old comedy is Round The Horne, their use of innuendo was masterful.
https://ia-petabox.archive.org/details/completeroundthehorne

[david]

David

The aerial is about 5 yards of wiring wire, strewn in the same plane as the receiving aerial on the roof, which is about 10 yards long.

Charlie

A (very) fractional wavelength antenna is going to be very high impedance so you would need to swing some volts in to it.  This is an area that John would be able to offer some excellent advice in.
Isn't this what they use Bluetooth audio for - getting program material from A to B?  No fun in that.

David

[John Lawton]

If you want to cover a building, then a loop aerial might be a good solution, i.e. a low impedance loop around the circumference of the whole building. It generates a field mostly inside the building but radiates much less outside. There is also a leaky feeder system, using coax but the experts here would know more.

[charliecoutas]

Ha! Round The Horne, The Navy Lark and The Goon Show were all favourites of mine.

I listened to Journey Into Space which was serialised on Radio4 Extra a while ago. Good stuff.

[JonW]

You could use a transformer to couple into a higher Z antenna or use a compact resonant loop as you are only looking at transmitting tens of feet. 

A ferrite antenna would be small and cheap.  Paper HERE
Here is a complete Radio with a ferrite antenna/tuning caps, use the front end parts and add the Valve back end.   https://quasarelectronics.co.uk/Item/3063-one-chip-am-radio-kit

[John Lawton]

Wow, that paper is scholarly, seems strangely old fashioned these days. Nice to see Quasar Electronics is still going. My circuit could be driven by a 5V DAC if the gain is increased to x20 to then give 0-100V output range.

[charliecoutas]

Yes, what a wonderful paper about ferrite rods. As you say John, it seems very unusual to have something so detailed these days.

A friend at the museum was wondering if ferrite rods could be used as a radiator for transmitting. I asked ChatGPT about this. Here is the reply:

Ferrite rods are commonly used as receiving elements in antennas, especially in applications like AM radio receivers. They are known for their ability to concentrate magnetic flux and are often employed to enhance the inductive coupling in these types of antennas.

However, using ferrite rods as transmitting elements is not common and generally not recommended. Ferrite materials are more suitable for receiving applications due to their ability to enhance inductive coupling in the presence of magnetic fields. When used for transmitting, the power levels involved could lead to issues such as saturation of the ferrite material, which may result in distortion of the signal or damage to the ferrite rod.

For transmitting applications, other materials like copper, aluminum, or various alloys are more commonly used for antenna elements. These materials can handle the higher power levels associated with transmitting signals and are better suited for the electric fields involved in transmitting antennas.

If you are designing an antenna for transmission, it's advisable to choose materials and designs specifically intended for that purpose to ensure efficient and reliable operation.

Charlie

[John Drew]

G'day all,
Just found this topic. Interesting experiments going on.

It's surprising how far a little sniff of RF can travel with a half decent antenna. 100mW will travel to the other side of the earth with modern digital reception.
Anyway that's a digression.

The antennas being discussed are very small and would have a radiation resistance of about an ohm and a capacitive reactance of some thousands of ohms.
 You could improve things with a loading coil to add inductive reactance, and there are formulae to calculate the size but if end fed will you be able to provide an effective earth?
A pi coupler at 1MHz will need some large capacitors but would assist with harmonic reduction at least and may contribute to matching the impedance of the transmitter, which will be low, to the antenna. You'll end up with a large coupler and a tiny transmitter.

Personally I'd buy one of those 9850 PLL programmable oscillators that provide a sine wave, run it in a short wave band which improves the efficiency of the antenna and connect a short wire (say 5m of wire) to act as antenna. Put up with all the losses but you'll still hear it for 100m or so.
You'll need a FET amp for the PLL oscillator to drive which can be used as the final which is modulated by the lm386. It won't be illegal as the effective radiated power will be tiny.
Just some thoughts when I should be hitting the sack at midnight.
Merry Christmas all.
John