Let's take another look at AM

[Albert H]

We all got into radio because you enjoy the magic of playing a CD or talking into a microphone and having it heard over a wide area. We've all (mostly) come to the conclusion that DAB is a waste of time and money, and Band II is so full of BBC Regional repeaters that most of it is unusable.

Why not try AM? If you look up and down the medium wave band, you'll see that all the French stations have gone and there are innumerable clear frequencies. It's even possible to find clear frequencies with two channel widths free on either side!

The audio quality on AM is inferior, and stereo isn't a possibility (in this country, anyway), but for casual listening, or listening in a car, the quality can be quite acceptable. Part of the trick to make it sound good is to boost the mid and upper frequencies, and apply plenty of compression.

Down at medium wave frequencies, it's easy to get plenty of power out of very cheap components. Many of the semiconductors are aimed at the audio or industrial control rather than the RF market, and generating medium wave frequencies from standard crystals is really easy with modern PLL methods. The most expensive parts of the rigs I built recently were the mains transformer, the heatsink and the box. The rest of the parts cost almost nothing.

There are only two bits that make transmitting medium wave a bit tricky - the aerial and the earth!

You need as high a vertical aerial as you can achieve, and the transmitter needs to be connected to a good earth. Back in the early 80s, we invented the "Tower Block Sloper", which was a simple wire aerial that gave spectacular results. I'll be happy to explain how that's built and calibrated! Using a TBS and a just small 10 Watt carrier / 45 Watt peak rig, we covered most of South-East England - in the daytime.

Obviously, night time coverage can be reduced due to continental stations getting here by means of ionospheric "skip", but by careful choice of frequency (avoiding the big continentals), you can get your signal into Europe!

If there's any interest in medium wave (and remember - most car radios and domestic radios can receive it), I'll put some designs up here. These are really easy to build, don't require tricky VHF construction techniques, and are simple to get going.

[NOYB]

By coincidence I was playing with Stereo Tool audio processor the other day to simulate that "AM sound". What surprised me was how much difference stereo made, even with audio band-limited to 4.5 or 5kHz. Shame there aren't more receivers around that can do AM stereo. If you pushed the audio bandwidth a bit, I'm sure many people could not tell the difference, or might even prefer the sound of AM. (I recall in the 80's many of my friends continued listening on AM ages after FM came along because they said the preferred the sound of AM).

I must admit when driving down the motorway a few weeks back listening to a football match I preferred the sound of 5 Live on AM than on DAB - probably because it is speech-based and the nice compression cut through the road noise better.

Yes it would be interesting to see some solid state AM TX designs.

Worth also mentioning ground conductivity for the aerial ground. In contrast to FM where height is king, for AM the ideal site would be a tower block built in a salt marsh! (or a radio ship in the North Sea!). Also I'm told care is needed to avoid the antenna being too high Q - or it could limit the bandwidth and cut the high frequencies.

It would be great to see some new AM stations on air - I'll get my loop aerial out of the loft!

[thewisepranker]

I've given this a bit of thought before but it never went any further than that as I just can't help getting distracted by something else.

It could be possible to make a very efficient AM amplifier using some (very) modern FETs with class G or H topology. Not sure whether this has been done already. It wouldn't be easy and would need to be designed properly, not just chucked together, but could make for an absolutely ideal battery powered setup if that's your thing.

[sinus trouble]

Hmmm? Come to think of it, Although i understand all the theory behind AM I have never actually built anything AM based!
Id like to give it go! Even just as a low power project!

[Gum]

I've always wondered about cutting a lightning conductor near the base of a building and using the top section as the aerial and the bottom as an earth...

An old BT junction box or something could be fixed to the wall over the break to contain equipment and with a bit of care, I'm sure would go completely unnoticed by anyone.

[Gigahertz]

Ive been trying one of these AM 1000 Rangemaster. Used alot in the USA for part 15

Two issues first its only 100mW and second I don't have a crystal for UK section. Not having much luck finding a supplier of crystals but I did hear that there was a company in Ireland many moons ago.

Anyway Ive done tests and had range so far of 1 mile mounted 10ft off the ground. It has a built in ATU, I'm using a firestick mobile CB whip as I could get hold of the recommend 102" straight whip. I could make something just haven't had the chance and just wanted to get it on air.

Ive attached a pic if youve never seen one before

[Albert H]

The "Rangemaster" gets good reports. I'll look through my Contacts list and see if I can find someone who can cut you a crystal at reasonable price.

If a crystal can't be easily obtained, it's quite easy to build a PLL and VCO (to give you sinewave drive if that's what's needed). The crystal we all use for Europe is 4.608MHz (which divides down easily to 9 or 18kHz). This crystal is a standard part from Farnell - it's used in Baud-rate generators.

[MiXiN]

For Xtals cut to any specification, I've always used Quartslab - http://www.quartslab.com

A nice personal service & no order is too small for him.

[Albert H]

There's another firm I've used http://www.golledge.com/ - they're a bit more expensive than QuartSLab, but their delivery is very quick and their products are top-notch.

[Gigahertz]

Thanks for the info will check them out.

Albert as you mentioned in your previous post about a few French stations have now gone. Could you put forward some recommendations on good frequencies I could look at "yes I will check them out before using one"
I doubt that I will get anymore than 2 miles with this rangemaster tbh as that seems there limit from research on the Web.

Not really into construction like I was plus don't have the test gear now but would like something more powerful than QQRP. Rather buy something than build it.

[Albert H]

Try 846kHz - one of my old favourites. Also look at 1125kHz (right between the two Talksports) or 1395kHz. Drop me a PM when it's on and give me an approximate location. I'd be interested in seeing just how far a little rig like the Rangemaster will go.

[Albert H]

Incidentally, I'll put up a simple-to-build small MW transmitter design is anyone's interested. The only instruments you'll need to set it up will be a multimeter, a 'scope (though that's not essential), and (ideally) a frequency counter. A basic field strength meter also helps.

The crystal I use is a standard, off-the-shelf one (so is cheap), there's a bit of cheap CMOS logic, there's a power FET (costs 90p!) for the output device, three ferrite rings, and an audio power amplifier chip. You'll also need some heatsink, but you should be OK just bolting stuff to the box you house it in. The power supply is a transformer, bridge rectifier,smoothing capacitor and a couple of voltage regulator ICs for various parts of the circuit. You should get change out of £25 if you don't shop at Maplin!

[MiXiN]

Amateurradioshop.nl was selling a couple of MW AM Transmitters, but recently he told me that the Netherlands Government had intervened (or something like that) so had to take all Transmitters down off his website.

Pity really as he had a nice range of kits for sale.

@RF-Head - I don't want to derail this topic, but as you're from NL, please can you tell us more on this?

[Albert H]

The little AM rig that the Dutch guys were selling was really just for domestic use - very low power (just a milliwatt or two).

I'll give you details of something that will give you useful coverage over a wide area with a good aerial and earth.

[radium98]

Albert i have a good pll of a 25 watts AM transmitter digital i will post soon who may want to buitl and i want to try doing one i have no idea on AM MW and it is more hard on coil level and if you help about the aerial it is good i am newbie in aerial .
I'll give you details of something that will give you useful coverage over a wide area with a good aerial and earth
this is the one i said i have hope you have another one and i want to builot help us in others infos i think it is pretty good http://pira.cz/amtx.htm

[radium98]

T

i64-3.jpg

i64-2.jpg

i64-1.jpg

he AM tx 25watts ready built NOT by me .some pictures to get an idea about coils .

[radionortheast]

my first transmitter was a 1 transistor am transmitter, kit from the science thingy place, you where meant to attach the aerial to the reciever to make it legal, but who would do that kind of boring and I doubt it would stop the signal traveling into other rooms, me and my friend where fasinated by it. I put loads of batteries togeather, i’m sure it did reach my friends across town! just with a few feet of wire and connecting the earth to a radiator.

I’ve built am transmitter kits tried to replicate it using the part 15 0.1w us transmitters, mostly only got a signal weakly about 250 meters away, cordless phones used to use am transmitters just above 1600khz i've heard you could hear them 1 - 2 kms, must of been the phone line acting as an aerial, as i’ve not got further than 250 as I say with a simliar wire aerial hooked off a curtain pole. I don't think small aerials work on am, maybe best using a higher frequency I don't know, the whole aerial side to it is something of a black art..it just seems abit odd to have just a wire stuck on the output section without any co-ax going to aerial away from the transmitter..

[ronald001]

AM is becoming more and more popular in NL, because the dutch OFCOM has allowed low power stations to broadcast on particular frequencies. ( you had to apply for a low power license )

There are also quite some dutch pirate stations that are transmitting on 1620 or higher with 100-1000 watts. ( mostly in the evening or Saturday/Sunday morning )
Also shortwave is popular ( around 6250 and higher )
DDS transmitters and tube transmitters are mostly used.

You can find nice schematics on http://members.home.nl/radiomorningstar/index1.htm and PA's on http://eb104.ru/ for the ones who are interested

@Albert, yes i am interested in antenna diagrams!

[MiXiN]

So, does anyone know of any sources for any QRP AM Transmitter kits? By this I mean a couple of Watts or less..

[ronald001]

You Have to look for them in greece, i think AM is unlicenced over there.
I found one: http://users.otenet.gr/~nsavvas/pll_en.html
20 Watts PEP 160 euro