Tuesday, September 30, 2008

80m QRP AM Transmitter - first contact!

Encouraged by the success of the beacon transmitter, I've built another version that is amplitude modulated using a 600 ohm telephone transformer.

The transmitter is just 2 transistors, (not counting the mic pre-amp and audio power amplifier), here's how it looks:

AM Transmitter.jpg

Whistling into the microphone looks like this on the cro:

AM Waveform.jpg

I'm certainly not getting 100% modulation.

I've just had a contact with Sam, VK2BVS, who reported that with his pre-amp in I'm 10dB over S9, with excellent quality audio "like the BBC".

Next step is to box all this up and make it a bit easier to switch from receive to transmit. (Currently I have to clip a clip lead on to power it and switch a co-ax switch).

The circuit is closely based on the beacon by Alan VK2ZAY. My construction is huge compared to his.

80m AM transmitter.png

For the audio amplifier to drive this I used a Jaycar "Short Circuits 3" #13 High Power 12V amp which seems to do the trick quite nicely.


Alan Yates said...

One "inefficient" by extremely simple way to get full modulation depth is to replace the transformer with an NPN emitter follower. Bias its base such that its emitter sits about mid-rail (ie carrier output 1/4 of PEP), then AC couple your AF signal to the base so the emitter tracks it about 600 mV less. It can then swing right up to the rail minus the saturation voltage and right down to cut-off, giving you pretty linear modulation up to essentially 100%. Trapezoid tests will let you set the bias pot for optimal modulation but "mid rail" will be quite close to optimal.

Of course the transistor drops 1/2 the rail voltage across it at the quiescent collector current, so it simply burns up that power as heat. The device must be suitably rated for the power dissipation and collector current, but a BD139 with a small heatsink is probably suitable for an initial try.

Such a class-A modulator is frowned upon as inefficient, but it is extremely simple, easy to build and easy to drive. It skips all the high-power AF amplifier stages and the heavy modulation transformer.

The base input impedance of the modulation transistor will be a little low - about 5K I'd estimate from the load and beta. You'll need to provide a full swing into that load as the emitter follower has no voltage gain, but that is easy to achieve with an op-amp (or even just an common emitter amp if you don't mind not quite making it to 100%).

Peter B Marks said...

I'm certainly no where near 100% modulation at present, so it would be worth a try.

Watching my waveform on the cro, I'm thinking that the other useful stage would be some sort of speech processor, audio peak limiter to keep levels up and not too high.

Sam 2BVS was being generous I suspect.

Today I'm off to visit VK3ASE who is a very experienced AMer.

Kitchen Benchtops said...
This comment has been removed by a blog administrator.
Peter Marks said...

Dear Mr Benchtops,

or can I call you Kitchen?

I hope you're not not trying to promote your page with comment spam....


Jack Jones said...

Hi, I’ve updated my QRP website! I'm sure it's of interest to fellow shortwave radio enthusiasts.

Here’s the link: www.stationqrp.com


Norbert Mázik said...

I am a new ham, just waiting for my callsign to be assigned.
Instead of a telephone transformer, I use small unidentified transformer, probably audio, whose ohmic resisetance of the primary is about 60 ohms and 0.6 ohms for secondary. I put a 6 ohm resistor in series with the seondarty windig so that my lm386 won't burn.
Would that be ok: I also have a power transformer, whose primary is about 260 ohms and secondary is about 5 ohms. This somwhat limits the output power of the tx. Which in your opinion is better?


Norbert, OM1A??

Norbert Mázik said...

Would that be ok?

Additional data for the transformer:

Primary: 147 mH, 60 ohm
Secondary: 270 uH, 0.6 ohm

Norbert Mázik said...

I tried the transformer and it works. Is it necessary to turn off the power for this tx
when on receipt. I mean, it transmits the carrier even if you do not modulate it using mike
and apmlifier. I hear it on my regenerative receiver.
Please give me some advice, as I am going on the air in a few weeks and I do not want to sound bad, let alone breach the conditions for amateur radio transmission.

I am thinking about a pushbutton with which I will connect the power source when I talk to the
microphone. Is that all right?
Also, I am going to use an RC low pass behind the mike preamp, to supress frequencies above 3KHz.
Anything else I should take care of?

Thanks for any and every input.


Peter Marks said...

Well done Norbert. I think at such low power you won't cause any problems. Turning all of the power off will certainly stop the carrier but might "chirp" a bit when you start transmitting.

Yes limiting the audio bandwidth is a good idea - you waste power transmitting high and low frequencies that people listening won't hear.

You might be over thinking all of this - just give it a go!

Best wishes,