AM Transmitters for Vintage Radios

[outis]

I have a few vintage radios, AM/MW, LW etc and I have built a little AM transmitter - very simple, using just 1 transistor - in order to send audio to them.
Vintage radios can sound great but there is very little content nowadays on Medium Wave.
The idea was to be able to listen on a vintage radio stations on DAB/DAB+ as that's were most content is now. Of course one can feed it from any audio source.
Anyway, the project has been successful. I can listen to DAB/DAB+ on all of my older radios.
Unsurprisingly, perhaps, DAB from the line out of a DAB Radio like the Roberts Gemini 25 (RD25) received on AM through my little - and very unsophisticated - "transmitter" sounded better on my Roberts R900 (which isn't really 'vintage' as it's 1980s) than on the RD25 itself.
So I am happy about how this worked.
But I have two questions:

1. I tried to listen to on modern PLL AM radios and it's very difficult to lock the signal. I understand why this is happening but do you think there is some way to get them to lock on it? Other than getting expensive equipment or by trying to fine-tune by trial and error?

2. I have two radios with a VU-meter, an old Ferguson 3T10 - on which it also sounds great, and a Panasonic RF-B600 which is PLL and has 1KHz tune step. When checking the signal strength the readings on both tuners the place the sound is best it is not where the signal is the strongest and where the signal is the strongest the sound is crap. Any ideas why this might be?

PS. I estimate the output if this very simple AM transmitter to be around 10mW. Used various frequencies from 1188-1530 KHz.
The short (2ft) coiled wire antenna, gives a range of 10ft. I connected it to a car generic "wipe" antenna and got some 30 ft.

[fmuser877]

My AM goes about 15 houses away but surprised its not more.
the sound is pretty poo
I wanted to cover my town playing pod cast and audio books because its ok for that.
But even my fm aerial low down on the pole goes way more like 2 miles. insteed

[outis]

My AM goes about 15 houses away but surprised its not more.
the sound is pretty poo
I wanted to cover my town playing pod cast and audio books because its ok for that.
But even my fm aerial low down on the pole goes way more like 2 miles. insteed

It depends on the antenna and the trasmitting power.
With this same design I use now I had gotten 2 miles in the past, when using an inverted L 1/4λ antenna (45m) on 1620KHz - reception by car radio. An 1W FM from a dipole didn't do any better.
This is running on 8V, 1.6-1.7mA so I guess its transmitting power is in the 10mW area.
To cover a town with such low power you nee an l/4 antenna.
Once I had, at the same time, running 20W on AM (45m antenna) and 20W FM (dipole) - playing from the same source - I could hear both on a car radio 15 miles away but the AM I could hear behind a low mountain too, no FM could reach there.

[shuffy]

I'd need to see your circuit to answer both questions properly but here are my guesses!

do you think there is some way to get them to lock on it?

If your TX is free running, it's going to be way less hassle and cheaper to fix the TX than mod the receivers to lock on your carrier.

2. I have two radios with a VU-meter, an old Ferguson 3T10 - on which it also sounds great, and a Panasonic RF-B600 which is PLL and has 1KHz tune step. When checking the signal strength the readings on both tuners the place the sound is best it is not where the signal is the strongest and where the signal is the strongest the sound is crap. Any ideas why this might be?

You can resolve FM on an AM receiver to some extent, by tuning slightly to one side of the carrier frequency. As the carrier deviates due to the modulation, this looks like AM to the receiver. My guess is that your modulating signal is affecting the carrier frequency and causing some FM / phase modulation. Far more likely with a free running oscillator than a crystal or synthesizer. Again, I'd need to see your circuit to confirm.

[jvok]

1. I tried to listen to on modern PLL AM radios and it's very difficult to lock the signal. I understand why this is happening but do you think there is some way to get them to lock on it? Other than getting expensive equipment or by trying to fine-tune by trial and error?

You could probably tune it by ear easily enough. Set the receiver to the frequency you want then adjust the tuning on the transmitter for best sound (or better yet, highest signal strength if your radio has a meter). Exactly the same as when tuning an analog receiver except your adjusting the tuning on the transmitter not the receiver. That will probably get it close enough for the pll to lock. If not you can get a frequency counter on ebay for <£30 which should be good enough.

Problem is the transmitter will drift over time, so even if you get it tuned perfectly today it might not still be in tune tomorrow. Only way around this is to build a crystal controlled transmitter.

2. I have two radios with a VU-meter, an old Ferguson 3T10 - on which it also sounds great, and a Panasonic RF-B600 which is PLL and has 1KHz tune step. When checking the signal strength the readings on both tuners the place the sound is best it is not where the signal is the strongest and where the signal is the strongest the sound is crap. Any ideas why this might be?

I've found this before while playing with am too. My guess is when you're perfectly tuned the treble is limited by the IF bandwidth of the radio. By tuning off to one side you're shifting the treble part of the signal inside of the IF bandwidth. This gives you more treble which sounds clearer, although probably at higher distortion because you're not getting both sidebands properly anymore.

[outis]

I'd need to see your circuit to answer both questions properly but here are my guesses!

do you think there is some way to get them to lock on it?

If your TX is free running, it's going to be way less hassle and cheaper to fix the TX than mod the receivers to lock on your carrier.

2. I have two radios with a VU-meter, an old Ferguson 3T10 - on which it also sounds great, and a Panasonic RF-B600 which is PLL and has 1KHz tune step. When checking the signal strength the readings on both tuners the place the sound is best it is not where the signal is the strongest and where the signal is the strongest the sound is crap. Any ideas why this might be?

You can resolve FM on an AM receiver to some extent, by tuning slightly to one side of the carrier frequency. As the carrier deviates due to the modulation, this looks like AM to the receiver. My guess is that your modulating signal is affecting the carrier frequency and causing some FM / phase modulation. Far more likely with a free running oscillator than a crystal or synthesizer. Again, I'd need to see your circuit to confirm.

Thank you, here is the diagram - modding the receivers is not something I would consider.

[Albert H]

Sadly, there's not much you can do with something as simple as that.

However, I came up with a simple circuit for a reasonably stable AM low power rig, which just uses one FET (for the oscillator) one cheap bipolar transistor (for the modulated RF amplifier) and a cheap 8-pin audio amp IC for the modulator. I'll put the circuit (and a suggested PCB layout) up here shortly.

If you want to go a little further with your low power AM, and add a simple compressor / limiter to allow you to get a reasonably consistent modulation level, I'll put a simple circuit up here too.

The whole "domestic" AM transmitter system would cost <£15 to build, though it might be even cheaper if you already have some of the parts in your junk box!

[shuffy]

Well it's certainly simple! But you pay the price for that simplicity in terms of frequency stability and some FM component in your modulation. You might not have such a pronounced FM effect at the lower end of the band, because the capacitances in the transistor which cause it are not as significant. However, getting spot on the frequency you need for that PLL receiver to lock will always be tricky and you'll most likely drift off it.

So your frequency source and method of modulation are key. A simple, low power design I used to like is the Wenzel transmitter, a crystal oscillator followed by a modulator based on a long tail pair (diff amp). 4 transistors. I built one years ago and thought it sounded nice on air but you'll need a crystal for the frequency you want. I wondered if anyone had added a PLL to it and just discovered this (which obviously has been around for years!):

http://www.geocities.ws/raiu_harrison/m ... gizmo.html

Be careful where you click as the page was full of dodgy links. OK, it's nowhere near as simple as your single transistor TX or even the classic Wenzel, but I'd say it's one of the simplest designs which solves your problems and lets you program the frequency.

Also while googling single transistor AM transmitters earlier, I discovered this one, which he claims has a lot less FM than other single transistor designs (looking at it, I'm not sure I'd agree), but it's still free running.

https://www.youtube.com/watch?v=2B5bEMpgrLc

As ever, there are LOADS of other options but the Wenzel sprung to mind first. Hope some of this helps.

[Albert H]

MW Zender II+.png

This is the little PCB I came up with for my little MW rig. Using about 5 ft of wire for the aerial, it covers my house and garden, and can be heard a couple of doors away on my car radio.

The Coils I used were ready-made ones from http://www.spectrumcomms.co.uk/Components.htm (their part number is 90µOL) and they sell most of the other parts too.

The output power is tiny, and there's only the slightest attempt at modulation bandwidth limiting, but its range is so small, it's not going to offend anyone!

It's best to use a regulated power supply (or a well-charged gel-cell as I do), as voltage changes can cause the frequency to move a little.

[Albert H]

Another little AM PLL PWM rig I did as a "car broadcaster" is here:

AM-Car Broadcaster.png

This is sure to stay on frequency!

[radium98]

outis can you show a picture of the AM 20W aerial , i would like to see a picture of that . thanks never seen one as a pirate one . all what i saw are very long way antenna and mast pylone.

[outis]

Well it's certainly simple! But you pay the price for that simplicity in terms of frequency stability and some FM component in your modulation. You might not have such a pronounced FM effect at the lower end of the band, because the capacitances in the transistor which cause it are not as significant. However, getting spot on the frequency you need for that PLL receiver to lock will always be tricky and you'll most likely drift off it.

So your frequency source and method of modulation are key. A simple, low power design I used to like is the Wenzel transmitter, a crystal oscillator followed by a modulator based on a long tail pair (diff amp). 4 transistors. I built one years ago and thought it sounded nice on air but you'll need a crystal for the frequency you want. I wondered if anyone had added a PLL to it and just discovered this (which obviously has been around for years!):

http://www.geocities.ws/raiu_harrison/m ... gizmo.html

Be careful where you click as the page was full of dodgy links. OK, it's nowhere near as simple as your single transistor TX or even the classic Wenzel, but I'd say it's one of the simplest designs which solves your problems and lets you program the frequency.

Also while googling single transistor AM transmitters earlier, I discovered this one, which he claims has a lot less FM than other single transistor designs (looking at it, I'm not sure I'd agree), but it's still free running.

https://www.youtube.com/watch?v=2B5bEMpgrLc

As ever, there are LOADS of other options but the Wenzel sprung to mind first. Hope some of this helps.

Thank you. Yes, I hadn't thought of the FM effect. Will go lower.
My objective was to be able to listen to DAB on vintage radios, and enjoy the AM sound of older radios, experimenting the same time. And that is achieved.
I will see if I can reduce the FM now.
I do have a PLL, 0-20W AM PLL, good sound too, but a bit of an overkill to use such a rig, to send a signal 10ft away to a vintage radio.

[outis]

Sadly, there's not much you can do with something as simple as that.

However, I came up with a simple circuit for a reasonably stable AM low power rig, which just uses one FET (for the oscillator) one cheap bipolar transistor (for the modulated RF amplifier) and a cheap 8-pin audio amp IC for the modulator. I'll put the circuit (and a suggested PCB layout) up here shortly.

If you want to go a little further with your low power AM, and add a simple compressor / limiter to allow you to get a reasonably consistent modulation level, I'll put a simple circuit up here too.

The whole "domestic" AM transmitter system would cost <£15 to build, though it might be even cheaper if you already have some of the parts in your junk box!

Thank you, for the circuits too. I may try one. They brought in memory the Maplin shops... I prefered walking into the shop and getting the components.
Gel-cell batteries are my choice of power source but threw away last year some that I had for years.
Part of my current experiment are the 9V batteries I use - non-rechargeable but very much re-chargeable in reality
This simple transmitter so far is not working well with a power supply. I'm currently trying out different to see which works best. As you can see it doesn't look elegant, but does the job. I will see how to tidy it up.
I use the PLL radios tune as close as possible to an "official" channel at the moment. and trying different ways to cover the house and the garden without going beyond.

[outis]

outis can you show a picture of the AM 20W aerial , i would like to see a picture of that . thanks never seen one as a pirate one . all what i saw are very long way antenna and mast pylone.

That antenna no longer exist. I never took a photo of it although it may be in a photo accidentally, If I find any I will post it.
But here is an image with a set up looking quite similar: an inverted L antenna. Hanging between low buildings.

[Albert H]

My "garden MW aerial" was very similar to the one in the post above, but I made it slope upwards as steeply as possible. My tuning coil was similar to yours, but was wound around a large jam jar, so it had a glass former! The trick was (I found) to get the radiator as vertical as possible.

[shuffy]

My tuning coil was similar to yours, but was wound around a large jam jar, so it had a glass former!

Mine used a piece of white soil pipe and had maybe 80 turns with random taps all over the place. I remember back in the day it took me all night to make! I've still got it somewhere. Used it to load all kinds of wire antennas - I'd put a torch bulb and croc clip in series with the antenna and go through the taps until I got the brightest light - then attach the wire directly and crank it up!

[outis]

My "garden MW aerial" was very similar to the one in the post above, but I made it slope upwards as steeply as possible. My tuning coil was similar to yours, but was wound around a large jam jar, so it had a glass former! The trick was (I found) to get the radiator as vertical as possible.

I guess i didn't need to optimise things
My set up was almost horizontal. One end at about 10m (top of my house) the other over a garden to a derelict house, probably 6m up. So there was a slopping a little.
At both ends there was about 1m washing line nylon wire at each side, then standard porcelain insulators. And between the insulators about 40m wire.
Another wire 5m or so would go down to a loading coil for tuning the antenna and from there to an RF connector.
The grounding wasn't going up to the antenna but to a metal heating pipe, next to the TX.
Tried different set ups, an 807 valve based - the most powerful I had, some 80W - that's why I said no need to optimise anything. It could go tens of miles (car radio) - this needed a strong amplified to modulate and at some point my amplifier fried. I didn't have any other strong enough to have a decent modulation.
So fell back to the PLL 0-20W - for tests, it would still go more than 20miles on a car radio - or used earlier "reincarnations" of this simple AM transmitter I use now (the only notable difference with the present one is the length of the coil's ferrite rod, now I use an 8mm, the one in the earlier was 14mm. Giving that it was working with 9-12V (12V from a gel-cell) 5-6mA it wasn't never giving more than 50mW yet with that antenna it would go 2-3 miles. More than enough.

[Nige from Brum]

Some interesting dutch designs on this site.

Here's a few aerial ideas to try.

http://www.pa2e.nl/jackdonio/radiomorni ... tenne.html

http://lpam.nl/antennes.html

You can use your browsers settings to translate to english.

[fmuser877]

odd the sites are blocked for me

[outis]

Project finished and now the transmitter is much more tidy

Some interesting dutch designs on this site.

Here's a few aerial ideas to try.

http://www.pa2e.nl/jackdonio/radiomorni ... tenne.html

http://lpam.nl/antennes.html

You can use your browsers settings to translate to english.

Thank you - The AM pipe antenna is something I would like to try.