AM Transmitters for Vintage Radios

[Albert H]

Nope. I lived in several parts of the 'States, and in every instance the domestic 110V supply was 55V either side of earth - supplied from an individual building transformer. We also had 235V supplies for heavier current items (like washer / dryers and cookers) in California and in Texas, which was a Live at 235V, and the Neutral tied to earth at the local sub-station, so the Neutral to Earth voltage would rise the further away from the sub-station you were. The higher voltage supply was one of three phases to each property, with the phases sequenced along the street.

[Albert H]

Here's the whole little MW PLL rig. They're cheap enough to make, and they work well. I've built four of them for friends, and they're all happy with them:

MWSynth100mW.png

PCB to follow!

[MiXiN]

About 5 years ago, I bought one of these kits from this place -
https://www.amateurradioshop.nl/webshop ... uwkit.html

I can't remember much, but I do remember being able to pick it up a few houses away from me.

Is this kit similar to the schematic you posted above, Albert? It looks very similar, but I can't ID the IC's in the image provided.

[Krakatoa]

About 5 years ago, I bought one of these kits from this place -
https://www.amateurradioshop.nl/webshop ... uwkit.html

I can't remember much, but I do remember being able to pick it up a few houses away from me.

Is this kit similar to the schematic you posted above, Albert? It looks very similar, but I can't ID the IC's in the image provided.

This kit is actually quite good. The PLL part is like Albert's one, but the modulator is made with a CD4053 (it sort of chops the audio at the carrier frequency) and it has some selectable filter coils at the output to match a 3m pole antenna.

[Albert H]

The recent circuit I put up is based on a simple circuit we did some years ago for an American kit supplier. It's slightly unusual in that it has "carrier control" which goes some way to mitigating the need for tightly controlled audio levels. We also did a little linear power amplifier that could be added to the end of it, and you'd get about 5 Watts carrier out of a pair of cheap bipolar transistors. The only tricky part was winding the coils, but they were not too difficult. I'll put the circuit details for the linear up here when I locate them (in one of the many hard drives here!).

Unfortunately, it's not easy to set up a completely hum-free AM transmitter (especially when you get to a few Watts), but there are steps you can take (and I'll be happy to explain) to minimise the problem. In the worst cases, I have a circuit that deliberately introduces 50 Hz and 100 Hz into the modulation, and the amount and the phase of the signals can be adjusted, to "null" the troublesome hum.

Back in the days of valved gear particularly sensitive electronics (like guitar amplifiers and tape recorders) often had this kind of hum mitigation technique included. In the case of the Ampex tape recorders, they called the circuit the "humdinger" (!) and when it was correctly adjusted, your recordings would be remarkably hum-free. Studer also had something similar.

The "Amateur Radio Shop" AM "bouwkit" uses a 4053 as a PDM "chopper" to develop a few tens of milliwatts of AM. I came up with a simple improvement to the modulator side of things that gave a peak audio indication by blinking an LED. I used a very similar circuit for a little "car sender" AM transmitter for those enthusiasts who insist on keeping their car radio original (just MW & LW), but to allow them to feed their preferred audio to their car receiver. We came up with a slim plastic box that could be tucked behind the dashboard, and there was enough signal to make reception easy, even without direct connection to the car radio. A friend of mine with a 1964 Triumph Herald has one, connected to a portable MP3 player that's in his glove box. The Motorola car radio never sounded better!

[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

You said the sound is "poor?" What transmitter do you use and how do you modulate the signal?

[fmuser877]

https://pbs.twimg.com/media/GFpYNuCWAAA ... name=large

[Albert H]

Are you processing your audio? It's important with AM to keep the modulation index really high. I've always used a lot of compression to get over the background noise that's prevalent on MW & SW.

[fmuser877]

I don't know I just play over the pc.

[outis]

An update about the project:

- With the BC109 and 1.5V supply, and 1ft antenna and no ground, I don't get more than 2 feet of "range" at 850KHz - which isn't terrible if I want to send a signal to an vintage radio. I just put the TX and the whatever digital source next to the radio (or behind the radio and no one can see the tx!) But if one wants a greater range to cover the whole room, or the whole house, one can do the following:
1. Supply volt rise, 2. Antenna extension, 3. Grounding, 4. Wavelength (frequency,) 5. On what radio is the listening done.
Increasing the volts in the above example increases the range a bit, at 8-9V the range with 1ft antenna becomes 3ft and at 12V it becomes 6ft. But it is a good start for extending the range.
2. The next step is to increase the length of the antenna. With an 1ft cm antenna there is no substantial increase. If the antenna increases to 3-6 ft, the range goes to 12-15ft at least. But what can "take off" the range is
3. ... "grounding!" The Earth! A well-grounded medium transmitter sounds loud, even 30ft away.
4. The frequency also plays a role in connection to the length of the antenna: For the short antenna lengths we are talking about, the higher the frequency in the Medium Wave, the more the range increases. With the same antenna that gives 10ft at 850KHz at 1500 you will get 12.
With the BC109 and 7.5V supply, and a 5ft antenna, grounded on a metal pipe from an old gas connection (no longer connected to the gas network but going well into the earth) the range - at 1395 KHz - is now 30-40ft indoors through walls and floors, while without obstacles and interference from computers and televisions it exceeds 45ft.
Finally, when some parameters are now stable, for example the supply is at 9-12V, the grounding is sufficiently good and the frequency set, the only thing that can be done for range is to increase the antenna: as long as possible. In the usual set-up I have (BC109/9V/ground) 20ft meters antenna gives more than 300 yards range, while a full l/4 antenna, with an antenna loading coil (150ft) sends the signal 2 miles away (listening with a car radio, because the radio still plays a role...)
5. Finally: the radio. I calculate the range when it is inside the house using different radios. On small handheld transistor radios the range is shorter than on good desktop radios. Outside the house I use a car radio.

So - depending on one's needs and antenna length capability - there is no need for a different better circuit, for as long as this meets the needs.
Nowadays "home broadcasting" covers the need (or desire) to listen to digital content (mp3, YouTube, etc.) on a good vintage analog radio. This circuit covers it. If one wants to listen to the signal on a modern PLL radio, things are not so easy because PLL radios do not "lock" easily on the signal of such a - slightly - unstable micro-transmitter from design like this. It can be done but with some effort, because the PLLs have 9 KHz wide channels and if the transmitter is not tuned exactly on the frequency it will not lock well. I have tricks for that but that's another matter.

[radium98]

How much Range is expected in a disaster way or lets say low profile antenna an AM transmitter of 25w power will go far ?

[Albert H]

The useful range of an AM transmitter is determined (mostly) by the antenna and the earth. 1 Watt into a properly resonant aerial with a good earth can travel much further than 50 or 100 Watts into a poor antenna! 25 Watts into an efficient aerial system can give you enormous daytime coverage - I used less power than this to cover much of the Benelux!

Practically speaking, I use a little 1 Watt AM rig to tune aerials. It's a simple design that gives 95% maximum modulation, and a peak power of just under 4 Watts. It's matched accurately to 50Ω, and is extremely useful for proving aerials and earths, because it's completely "burst-proof" - it will easily handle working into a bad match, an open circuit or even a short! The output FETs are capable of 20 Watts each, so at an aggregate power of 1 Watt, they run completely cold. The rig also includes an audio signal generator which can be tuned from 20 Hz up to 10 kHz, and this proves very useful for testing aerial bandwidths (which can be limited in the case of "shortened" aerials). The whole little package is in a diecast box and includes a 13.8 V gel cell to power it.

The other tool I use for setting up MW (and lower) aerials is an Antenna Noise Bridge. This is a simple device (I'll put a circuit up here for anyone who wants to build one), and using a receiver with a signal strength meter, it's a great way of getting an antenna about right before firing RF into it!

You'll find that most cheap SWR bridges only work down to the top end of the MW band (at best), so it's well worth re-engineering a cheap one for lower frequency use. I'll put up details shortly of how to modify a cheap CB one for MW use.

[radium98]

Thank you , valuable as i see . Yes a noise bridge is good to know the impedance ? or tuned well . Yes that it good if you can post one , i saw too many before ...

[outis]

How much Range is expected in a disaster way or lets say low profile antenna an AM transmitter of 25w power will go far ?

As mentioned it is on the antenna and the grounding.
A nearby 1W local station (RSL) is heard more than 10miles away.
Myself experimenting with a 20W, fairly good low profile antenna (inverted L l/4) and some quick and dirty grounding was heard 55555 at 25 miles away in a car radio so I guess the range was at least 40 miles and behind a mountain on 1566kHz.
Using the same antenna and grounding but low power, just 10mW, was getting 2-3 miles. While going up to 80W (still same antenna and grounding) more than 40 miles. I had no way to get a reception report further. All above during daylight.

[outis]

BTW
Last week, on the 13th of the month it was World Radio Day (in French: Le jour mondial de la radio)
That is an international day celebrated on 13 February each year (decided by UNESCO on November 3, 2011)
To celebrate the date, I used this little low power transmitter to broadcast for three days (13-15 February) from a place just outside Paris, France. Of course the range was some hundred years at best.

[radionortheast]

I have a mw transmitter with the atu it is possible to use the neutral, the capacitance can sit on a knife edge can detune everytime you turn it off, it is possible to get it so it dosen’t detune everytime you turn it off, selecting the right value of high voltage capacitors, the right inductance setting, using afew high voltage capacitors into a transmitter the match won’t be great.

While admit it dose work better than any wire you could put up round the house, avoids having messy wires, you won’t attract the attention of the neighbours having long wires outside digging up the garden, it doesn’t work like a proper outdoor antenna would, if you have a bluetooth receiver connected up to it, you have something that plugs in, convenient. I think your not concerned about a good match, you could try some high voltage capacitors, maybe connect to wirelessly over bluetooth just incase don’t want to be blowing up your laptop.

[Zozo]

The biggest problems with power line radiation are twofold -

• It's horribly inefficient: we had to run over 8 Watts into the coupler on one University system I did back in the 70s, and we still didn't cover the whole campus reliably.
• The coupler to get the RF onto the mains cabling is tricky to make and requires some expensive components if it's going to be safe!

Remember - the Yanks have a mains system that's just 55V either side of earth, whereas ours is much less forgiving!

Yes of course. But I only want to have signal from a low power transmitter to the house which is much smaller than a campus. Even a feet from the outlets will do.
As for the coupling, here is the thing. Powerline networking uses quite a simple couplers, and similar power. It cannot be that much different.
If you look at the links in the previous post you'll see "Coupling was usually done with a complicated network of inductors and capacitors. This was a difficult task when you think about it. A typical AC line at 60 hz was basically zero ohms or ground. At 650 khz, it was just high impedance enough that we could modulate it with low power transmitters. The problem was tuning the coupling. Since the power load changed all day as people turned off and on equipment, it was impossible to use really good inductive coupling. Therefore, a simple capacitive coupler was easier and worked just as well. You take a ten watt transmitter and hook it to the power line with a .1 1kv capicitor and that was it!"
Or in the pics.
My idea has been to connect as in the pic, ground to the socket's ground and the antenna to the neutral. Perhaps use an .1 1kv capacitor as above.
Or perhaps what the diagram shows here: https://darkliferadio.proboards.com/thr ... ng-circuit

https://web.archive.org/web/20161021033 ... MITTER.HTM

I thought I'd give this approach to "Series Cathode Modulation" a try on my mini AM TX I built a few years ago before going over to "Heising Modulation" I mentioned I was going to try. Basically just the cathode follower from the RF valve to the anode of the modulation valve was all I was interested in. Regarding the rest of the design there's a better ways to do things, especially the output network. Also my TX is built around the 6BQ5 / EL84 rather than the 6AQ5 / EL90 as shown in this schematic below.

carrierTX-2.png

All-in-all it seems it works rather well, and the modulation sounds good. Sadly the recording is not a direct feed like I've done in the past, but the fidility is excellent in person.

[outis]

I thought I'd give this approach to "Series Cathode Modulation" a try on my mini AM TX I built a few years ago before going over to "Heising Modulation" I mentioned I was going to try. Basically just the cathode follower from the RF valve to the anode of the modulation valve was all I was interested in. Regarding the rest of the design there's a better ways to do things, especially the output network. Also my TX is built around the 6BQ5 / EL84 rather than the 6AQ5 / EL90 as shown in this schematic below.

carrierTX-2.png

All-in-all it seems it works rather well, and the modulation sounds good. Sadly the recording is not a direct feed like I've done in the past, but the fidility is excellent in person.

From what I see the signal from the antenna goes directly to the neutral of the socket.

[Zozo]

From what I see the signal from the antenna goes directly to the neutral of the socket.

Yes the design you posted does indeed use a method of injecting the signal into your home domestic electrical wiring, however I can’t agree to taking this approach and especially with a carrier of 2 Watts; Usually with RF, and AM in particular it’s advisable to stop this from happening if your wanting a hum free clean signal.

It's worth noting that amplitude modulation “being such a simple method of modulation” has a habit of getting into things you may not wish it to, especially if connected to the same ring-circuit, (your direct next door neighbour may also be in for an unwanted surprise).

The design you did post, and solely the series cathode modulation approach did get me looking for other examples using the 6AQ5 / EL90’s. I see the wayback machine first indexed it back in March 2010, however there's a very similar design dating back to April 2000 by a fellow Ham, which I’ve posted below.

seriestx.gif

As you can see from the design it shares some similarities with the one you posted, but it also has aspects I don't like, But one I do like and recommend you should do if building a valve / tube TX is the Pi Network for the output. I could elaborate for some time as to why you should do this, but I'm sure the internet will be able to provide a bountiful take on the subject.

The other part of the design I don't like in the first one you posted is the “Pierce Oscillator”, and reading back over your threads regarding the troubles with PLL receivers, your likely be in the same situation with no variable load capacitance to bring the Xtal on frequency;
It’s also worth noting that “some” valve xtal oscillator circuits are not suitable for modern xtals which are not like the older types that could withstand higher drive current levels in such high voltage circuits like this.

[yellowbeard]

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