AM Micro Transmitter
 

Hello, calling all RF experts,

I've recently been messing about trying to put together a micro transmitter to distribute MW AM around my house.

After much exprimentation I've come up with the circuit attached. This gives good sound quality (due to the overall NFB) and upto 50% modulation without any problems. The signal at the aerial is about 20V pp unmodulated when its running from a 30V bench supply.

What I've no idea how to do is to distribute it around the house nicely without radiating too far abroad. At the moment a roll of wire taken up the step from my basement lair has to be placed right on top of a set to get a decent signal.

Would it be possible to send the signal along some co-ax and then have take offs on each floor and plug in mini aerials? (my house is a 4 floor terrace). If so how should I match impedences, when it comes to aerials I'm completely lost!

Any help or suggestions would be good. I'd like to be able to get some decent service out of all my sets and here deep in a valley bottom in W.Yorks the only thing worth listening on AM is R4; its lovely to hear clear undistorted music coming from a big 40's set!

Dom

Re: AM Micro Transmitter
 

You might want to try a leaky feeder antenna. Just hook up some rubbish quality co-ax, the sort with almost no braid cover and trail it around the house. The signal doesn't get out very far. Don't forget to terminate the far end with 75 ohms, assuming it's 75 ohm co-ax.

Re: AM Micro Transmitter
 

If I do this how do I match the fairly high impedance of the output valve to the low 75R impedance of the co-ax? Would it be best to wind a low number of turns around the final tank circuit?

Also will this work for such long wavelengths? 400m is where I've got it tuned at moment.

Cheers

Dom

Re: AM Micro Transmitter
 

Dom,

Yes, you can wind a few turns around the final tank coil. Make sure you re-tune the tank circuit back to resonance. If you put a milli-ampmeter in series with the tank circuit, you'll see it dip at resonance. Then, introducing the co-ax link turns will "load" the tank circuit, which means that, at resonance, the dip won't be so low as it is without the link turns. Then, in effect, you will be drawing power from the tank circuit down the co-ax to the termination resistor.

You may need to increase the modulation a little to compensate for the increased d.c. input to the final valve.

Have fun!

Aub

Re: AM Micro Transmitter
 

Hi Aub, the ammeter - is this measuring the AC or DC currents to find resonance?

At the moment I've just been tuning for maximum voltage across the tank, though I've noticed that it "sounds nicer", more top-end notes, if tuned slightly to one side of the peak (slightly higher C) though.

Cheers

Dom

PS: Very ashamed at not remembering any of this from my University Physics Degree, I've been trying to play with complex numbers and differential equations to understand all this tonight but it just made me fall asleep!

Re: AM Micro Transmitter
 

hi Dom,

It's DC milliamps. A 0 to 50mA meter should be suitable. My theory's a bit thin as well. I remember that parallel tuned circuits have a very high impedance at resonance. I've never measured voltage across the tank coil before, but I guess the voltage would increase at resonance.

Regards

Aub

Re: AM Micro Transmitter
 

Just to add,

Off load, the dip should be quite sharp at resonance, assuming you're not tuned to a harmonic by mistake. It will also depend on the Q of the tank coil.

Aub

Re: AM Micro Transmitter
 

Max dip ie minimum current, doesn't always correspond with maximum RF out, but it's as near as makes no difference.

400 metres is 750 Khz, so I'd be inclined to listen for harmonics around 1500 Khz or 200 metres to make sure the transmitter is clean.

Re: AM Micro Transmitter
 

About 40 years ago I ran a radio club in a boarding school for the educationally sub-normal (as they then were known). One of the members made the simple TX from Clive Sinclair's "Transistor Circuits Manual No.1". It worked well so he then hooked it up to the Central Heating system,using the radiator pipes as an aerial and was able to broadcast "Radio *******" (name of school) to all the dormitories. Very popular it was as well! Some of those lads really took to electronic circuitry. A couple with reading ages of 5-6 could build a two or three transistor circuit on breadboard or veroboard with minimal help from me straight from the circuit diagram. One of those two got a job with Henry's Radio after leaving us.
And they called them "sub-normal"!!!!
Jim

Re: AM Micro Transmitter
 

I don't know what you're trying to say about my electronics skills! ;)

Yes, more young kids (and people with learning difficulties etc) should try these things out - even if you don't fully get the theory it's great fun making something that works and inspires to get that understanding. Something that being given a ready made, sealed plastic box just doesn't do.

My first "toy" transmitter made from various electronics kits I'd been handed down and got from jumble sales managed to transmit over the area of about a mile, due to my hooking it up to some disused telephone lines....until my father found out and gave me a good hiding! I was about 6 at the time!

I'll definitely try and keep this one's range down somewhere reasonable. And back on topic if I make a frame aerial that I reckon should be roughly ~75ohms at 400m and I deliver a few volts pp into it what sort of range would I get? Please could somebody explain how do I convert this to mv/m or whatever and what sort of figure should I be trying to acheive to cover a single floor of a medium sized terraced house?

Cheers


Dom

Re: AM Micro Transmitter
 

Dominic - looking at your (very interesting!) circuit, you've got a DC path to earth through your oscillator coil primary from your second PCL82 triode anode.


Missing DC blocking capacitor?

Cheers,
Neil :)

Re: AM Micro Transmitter
 

No criticism intended!!!
Talking of non-numerical/literacy skills - I taught one of the boys there to place chess one weekend - he beat me in every game for ever afterwards!!!
Jim

Re: AM Micro Transmitter
 

Thanks Neil,

Whoops - indeed I have, there should be a 390pf blocker there (value not selected scientifically), still getting used to using this Visio thing for making diagrams...practicing for work related stuff...honest!

Thanks for the interest, not sure if it's a sensible circuit, is a sort of mish mash of ideas I've picked up coupled with the fact that I've got a lot of "used good" PCL82's to hand!

No worries Jim, some of the cleverest people, in terms of engineering / problem solving, I know can barely read or write. Some of the most dim / lacking in any common sense have very expensive educations and PhD's. Not sure where I fall, probably a bit of the worst of both worlds! I nearly always start out with some sound scientific theory, about half an hour later you'll find me randomly substituting resistors, bunging a few extra turns on a coil and wetting my finers and holding the transistor in one hand and the power switch in the other!

Cheers

Dom

Re: AM Micro Transmitter
 

It would be interesting to see what would happen if you increased the HT voltage to, say, 150V, as everything's working in starvation conditions, especially as the RF amp is in series with the modulator valve.

I supspect that there would be more available RF energy to go that bit further round the house. Still, it's an interesting circuit which obviously works!

All the best

Aub

Re: AM Micro Transmitter
 

Dom - Must say I've never seen PCL82's being used like that before. Excellent use of resources!

Cheers,
Neil :)

Re: AM Micro Transmitter
 

Dom,

Looks to me like you are applying modulation to the B+ (HT - we are English!) of the oscillator valve.

This will also introduce FM to your transmitter, as the HT for the oscillator valve will be wobbling!

Im fairly certain this is not what you intended!

The Oscillator would be best fed from a stabilised source, with modulation applied to the "output stage"

I would think that the transmitter would prefer HT somewhere nearer 150-200v - you will find the signal will get everywhere without the need for complicated feeder systems.

Have Fun
Sean

Re: AM Micro Transmitter
 

Hi Sean, the Triode of the second valve (the oscillator) is powered straight from the HT (top end of the 22K anode load goes to HT) which at the moment is a regulated bench supply. The wave length stays rock solid and there is no discernable FM...I may have to put some extra decoupling in for this stage when I come to building a real power supply for the thing from scraps.

Sorry if the diagram is a bit unclear with the cross overs. The second pentode is a cathode follower to match the high impedance of the audio amplifier triode to the low impedance required to drive the modulator. Hence its anode and screen are connected to B+ and it's running as a beefy triode and only its cathode and grid should be modulated by the audio.

Thanks for the interest

Dom

Re: AM Micro Transmitter
 

Dom - What's the function of the feedback loop from the aerial - it doesn't seem to do anything?

Cheers,
Neil :)

Re: AM Micro Transmitter
 

Ah, yes, my mistake!

For some reason I did not take any notice of the choke in the cathode lead - Heising modulation in the SG of the PA valve.

Needs more volts though!, and yes, some decoupling might be useful, as I do think you will have FM lurking

Re: AM Micro Transmitter
 

Hi Sean, the Triode of the second valve (the oscillator) is powered straight from the HT (top end of the 22K anode load goes to HT) which at the moment is a regulated bench supply. The wave length stays rock solid and there is no discernable FM...I may have to put some extra decoupling in for this stage when I come to building a real power supply for the thing from scraps.

Sorry if the diagram is a bit unclear with the cross overs. The second pentode is a cathode follower to match the high impedance of the audio amplifier triode to the low impedance required to drive the modulator. Hence its anode and screen are connected to B+ and it's running as a beefy triode and only its cathode and grid should be modulated by the audio.


I must say they're one of my favourite valves. My HiFi Amplifier is the first valve project I designed from scratch (see http://brahms.demon.co.uk/software/e...2_1/index.html). And the sound is wonderfull almost up to audiophool standards, all for a few quid's worth of telly valves!

I probably will up the voltage a little but am very wary of upsetting the neighbours! These PCL valves work very well at low voltages though. My first attempt was with ECH42 as a modulator valve but this was rubbish at lower voltages (I guess due to the variable mu nature of the thing?). I plotted all the curves up to 30V and the pentode sections give about 10ma of usefull poke even at these low voltages.

Also I don't want to push my luck too much with the heater-cathode voltage, as I'm running the first pentode as a cathode follower and they're not really meant for this

It demodulates the final signal as close to the aerial as possible and feeds it back in anti-phase to the input audio signal to provide overall negative feedback. The diagram is wrong in this respect, there should be a 1000pf in parallel with the demodulating resistor - new diagram in a moment. This all, reduces the modulation by about 3dB (countered by turning up the CD player source) and makes a positive difference to the overall sound quality. (Without this feedback a 1kHz square wave source looked pretty ropey at the end, with it present its almost perfect!)

Thanks for the interest

Dom