AM Transmitters for Vintage Radios

Everything technical about radio can be discussed here, whether it's transmitting or receiving. Guides, charts, diagrams, etc. are all welcome.
outis
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Re: AM Transmitters for Vintage Radios

Post by outis » Thu Feb 29, 2024 9:05 pm

yellowbeard wrote: Thu Feb 29, 2024 8:11 pm Aye it's a carrier current transmitter, the RF goes over the mains wiring and can be picked up by radios attached to that same mains circuit or close to the cabling. I never encoutered it myself, seems like a nasty thing to do - I have suspicions that it might get into TV/Video/HiFi where it doesn't belong.
https://en.wikipedia.org/wiki/Carrier_current
usually I see more complex ways to couple the transmitter to the mains. Apparently this works too.
Nowadays the TVs produce more MW interference than the MW to TVs.

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Re: AM Transmitters for Vintage Radios

Post by Zozo » Thu Feb 29, 2024 9:08 pm

outis wrote: Thu Feb 29, 2024 9:05 pm
yellowbeard wrote: Thu Feb 29, 2024 8:11 pm Aye it's a carrier current transmitter, the RF goes over the mains wiring and can be picked up by radios attached to that same mains circuit or close to the cabling. I never encoutered it myself, seems like a nasty thing to do - I have suspicions that it might get into TV/Video/HiFi where it doesn't belong.
https://en.wikipedia.org/wiki/Carrier_current
usually I see more complex ways to couple the transmitter to the mains. Apparently this works too.
Nowadays the TVs produce more MW interference than the MW to TVs.
I wouldn’t recommend injecting 2 Watt AM signals into domestic electrical wiring, especially in a residential area. Typically carrier current systems always had an annoying hum, and usually the signal in your mains wiring would be in the µV range, and nowhere near 2 Watts.

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Re: AM Transmitters for Vintage Radios

Post by Albert H » Thu Feb 29, 2024 10:35 pm

I did one carrier current transmission system when I was at University (a very long time ago!). To get reasonable results, we found that we had to run about 20 Watts into the coupler, and even then our signal couldn't be received more than a couple of metres from the mains wiring! It also hummed (not too badly, but enough to be annoying). We used a valved rig in those days, with a couple of 6146s in the final, and used a repurposed guitar amplifier as the modulator!

After a couple of months, we moved to a "leaky feeder" system (as used in tunnels), which used a coaxial cable with slots cut into the braid (which was horribly expensive as I recall). The cable was run in a loop around the campus, with a couple of further loops around the halls of residence. Each feeder had a terminating resistor at the far end. We ran the same valved rig into the leaky feeder, and found that we could receive it almost 2 km from the campus! Obviously the power was reduced, and the coverage on the sites was excellent, and there was no hum.

After a couple of months of operation, it occurred to me that perhaps we should try other frequencies down the cable. The original AM effort was on 1 MHz (later 999 kHz), and a couple of us built a valved VHF FM rig that was on ~94.2 MHz. The VHF rig did around 9 Watts (using a QQE03/10 output bottle). The oscillator was a free-running job (no PLL in those days) and tended to wander a bit in frequency, but that didn't matter in those days, since receivers were also free-running and used "AFC" to lock on to the signal you wanted to receive. In those days, Band II was mostly empty (apart from the three BBC national services, and the Police around 100 MHz), so our little FM signal was easily received in the area. We were delighted to discover that the leaky feeder radiated the VHF signal really well - possibly better than the MW signal!

Periodically, we'd have an inspection of our gear by the Post Office, so when they were coming, we turned the concealed VHF rig off!

A couple of years later, I built my very first stereo coder for that system, and we replaced the mono mixer in the studio with a homebrew stereo one. We discovered that virtually none of our listeners listened on MW, because the VHF FM stereo sounded much nicer! Unfortunately, since we were only licensed for the MW output, we had to keep it going!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: AM Transmitters for Vintage Radios

Post by outis » Fri Mar 01, 2024 1:58 pm

Albert H wrote: Thu Feb 29, 2024 10:35 pm I did one carrier current transmission system when I was at University (a very long time ago!). ...
I am interested in the coupling part, the diagram in the other post shows no real coupling but just connecting antenna to neutral
It's not like the uni carrier current.

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Re: AM Transmitters for Vintage Radios

Post by Albert H » Fri Mar 01, 2024 11:20 pm

As I recall, it needed some big high voltage capacitors, and a transformer affair that looked hand-wound. I'll see if I can find the details.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: AM Transmitters for Vintage Radios

Post by outis » Thu Mar 07, 2024 4:11 pm

Is placing an AM micro-transmitter circuit inside a metal box a good idea?
Does it need to be plastic?

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Re: AM Transmitters for Vintage Radios

Post by Albert H » Sat Mar 09, 2024 12:29 am

Outis - if you're going to use a ferrite rod aerial to radiate your signal over a meter or two, you'll have to use a plastic (or wood) box to allow the RF to radiate. However, if you're building something that is designed to use a wire aerial, it doesn't matter what you house the transmitter in.

I have made a simple little transmitter that produces a few milliwatts of power into a metre of wire. The oscillator is a free-running Colpitts oscillator, but is stable enough for casual use:
Mini-MW.png
If you're going to build this, it's worth getting a 12V gel-cell to power it. If you use the usual cheap mains power supply, it'll be certain to hum!

All the inductors were cheap moulded chokes. In use, mine goes about 10m through a couple of walls, so it's powerful enough for domestic use!
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"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: AM Transmitters for Vintage Radios

Post by stretchyman » Fri Mar 15, 2024 10:09 am

20W carrier, programmable DDS. PWM modulator. Good for about 10 miles into a simple base loaded vertical.

Quite a few in the States.

Maybe a bit OTT for the application?

A few have Tx'd into a leaky feeder...

Str.
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Re: AM Transmitters for Vintage Radios

Post by outis » Fri Mar 15, 2024 4:01 pm

stretchyman wrote: Fri Mar 15, 2024 10:09 am 20W carrier, programmable DDS. PWM modulator. Good for about 10 miles into a simple base loaded vertical.

Quite a few in the States.

Maybe a bit OTT for the application?

A few have Tx'd into a leaky feeder...

Str.
I've got a good one if I want "miles" 0-20W (PEP) 600-1800KHz PLL with SWR protection etc etc But it is over the top for "home broadcasting" to a vintage radio. As I experiment of course I may try other designs like those in previous posts, as well as antenna arrangements.

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Re: AM Transmitters for Vintage Radios

Post by stretchyman » Fri Mar 15, 2024 5:18 pm

MW is very easy. IRFP640 (plastic 640) is good for a 20W carrier. Single ended class E aka LuLu.

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Re: AM Transmitters for Vintage Radios

Post by outis » Tue Mar 19, 2024 8:37 am

stretchyman wrote: Fri Mar 15, 2024 5:18 pm MW is very easy. IRFP640 (plastic 640) is good for a 20W carrier. Single ended class E aka LuLu.
I wonder... what if I was to replace the transistor in my circuit with something like IRFP640?

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Re: AM Transmitters for Vintage Radios

Post by stretchyman » Tue Mar 19, 2024 10:57 am

Not sure what you're expecting?

Unless you X4 the power your essentially not going to notice any difference in transmit range.

I (we) dont know what your design is so simply substituting another FET is unknown.

The IRFP640 is as cheap as they get and doesn't need an insulating kit, however fine for 20W (carrier) MW.

I'm VERY cost conscious!

ANYTHING will work under 2MHz, I'm trying to get efficiencies of >90% and frequencies > 20MHz so GaN is the ONLY option.

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Re: AM Transmitters for Vintage Radios

Post by Albert H » Tue Mar 19, 2024 6:39 pm

GaN FETs are certainly a way to get both plenty of power and high efficiency. My latest SW rigs are using pairs of Class D amplifiers driven by an "outphasing" exciter, and can be fully modulated with 1.95V p-p (+8dBu) of audio. The DC in to RF out efficiency is invariably over 90%, and we've built AM rigs up to 31 MHz using this approach.

The highest power units we've built (for South American clients) deliver 5kW carrier, 20kW peak (these were around 10 MHz), but the more common versions are 250W carrier 1 kW peak. MW rigs at this power level are very popular in some parts of the "third world", since they give substantial coverage, but don't consume a whole lot of electricity. We've even provided some "turnkey" solutions, from microphone to aerial, and have provided installation services in many parts of the world.

We now find that we're competing with a couple of Indonesian companies who are supplying very cheap equipment (of somewhat dubious quality). The equipment appears to be from China. I'm glad that I've retired!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Re: AM Transmitters for Vintage Radios

Post by outis » Tue Mar 19, 2024 7:50 pm

stretchyman wrote: Tue Mar 19, 2024 10:57 am Not sure what you're expecting?

Unless you X4 the power your essentially not going to notice any difference in transmit range.

I (we) dont know what your design is so simply substituting another FET is unknown.

The IRFP640 is as cheap as they get and doesn't need an insulating kit, however fine for 20W (carrier) MW.

I'm VERY cost conscious!

ANYTHING will work under 2MHz, I'm trying to get efficiencies of >90% and frequencies > 20MHz so GaN is the ONLY option.
The schematic is in the first page of this topic. A single transistor AM transmitter.

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Re: AM Transmitters for Vintage Radios

Post by stretchyman » Wed Mar 20, 2024 7:47 am

OH, 'Technology Unleashed'!

Still not sure what youre trying to acheive?

Whilst I embrace simplicity, it does have its limits!

I'd highly recommend building a simple single ended class E thing, aka 'LULU'

It's relatively simple, uses easy to find parts, is very efficient and all you need is a 'scope to get it tuned up.

If you outline your needs I could possible provide more help and advice.

We've all been there as I started (a long time ago) with a 2 transistor transmitter (Osc/Pa) which I piddled around with for years.

Much time wasted, fun however. Years on, discover the 'LULU', what a revelation!

Time to move on up my friend.

Str.

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Re: AM Transmitters for Vintage Radios

Post by stretchyman » Wed Mar 20, 2024 7:52 am

Just to add....

Looks like youve already had plenty of advice and circuits to build that others have offered but I guess you still have your single transistor model.

Why?

Str

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Re: AM Transmitters for Vintage Radios

Post by outis » Wed Mar 20, 2024 8:30 am

stretchyman wrote: Wed Mar 20, 2024 7:52 am Just to add....

Looks like youve already had plenty of advice and circuits to build that others have offered but I guess you still have your single transistor model.

Why?

Str
At the moment that's my project.
I am experimenting to explore its limits and limitations. So, yes, exploring the limits of simplicity if you like. While enjoying some great AM content that I sent to my Vintage radios too!
All ideas are welcome and there will be other projects but at the moment I haven't exhausted the "What IFs?" regarding the current. Besides I am using this as a platform for "proof of concept" and trial of different antenna/grounding configurations and of directionality (I have used ideas and advise from here for those).

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Re: AM Transmitters for Vintage Radios

Post by stretchyman » Wed Mar 20, 2024 6:37 pm

Hmm, ok, Ill leave you to it then.

Good luck on your quest, whatever it is!

Str.

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