Simple MW Transmitter help

Everything technical about radio can be discussed here, whether it's transmitting or receiving. Guides, charts, diagrams, etc. are all welcome.
MiXiN
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Re: Simple MW Transmitter help

Post by MiXiN » Mon May 07, 2018 4:26 pm

I recall around 500 > 600mW up at that frequency, so your result tallies with mine.

Do you find that the audio quality varies quite a bit across the band as well? I was getting distortion like clipping on the lower output power parts of the band, but on the higher power parts of the band (1200 Khz, etc) the audio was pretty good. This was with the same audio level.

Hope I'm making sense.

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yellowbeard
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Re: Simple MW Transmitter help

Post by yellowbeard » Mon May 07, 2018 8:18 pm

On an AM transmitter the amount of audio you need for 100% modulation is directly proportional to the amount of RF, if you are getting less carrier out then you need to lower it down alright!

MiXiN
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Re: Simple MW Transmitter help

Post by MiXiN » Tue May 08, 2018 12:24 am

You learn something new every day!

Nice one, Yellowbeard. 👍

Albert H
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Re: Simple MW Transmitter help

Post by Albert H » Mon May 14, 2018 2:29 am

A crude rule of thumb from the valve days - the modulator needs to be rated at about half the carrier power of the rig for anode modulation.

Caroline and a lot of the other offshore stations used transmitters built on the Ampliphase / Cheirix modulation system. The carrier was generated by a crystal, then split into two paths. One path was phase delayed by 135° (with no modulation) and the two paths went through identical amplifiers to a combining network at the output. Modulation was achieved by phase modulating the two carriers in opposing directions. At the output network, you got phase addition or cancellation which equated to AM going to the aerial! The phase modulators were a complete pain to align (I know - I spent days getting them right), but the big trick - introduced on the BTA50 which Caroline favoured - was envelope feedback. A tiny proportion of the output power was rectified to recover the audio, and fed back into the modulators in the correct phase to (effectively) provide negative feedback to clean up the audio. It also enhanced the audio bandwidth too.

I came up with a simple (relatively) Cheirix transmitter using an "Outphasing" technique (similar to Ampliphase) with transistor / FET power amplifiers (Class D for efficiency) and just using fast comparators to pulse-width modulate the carriers. Envelope feedback really cleaned it up, and the prototype is in daily use in Eastern Europe. It develops 225 Watts carrier and about 1kW peak, using 16 output transistors that cost just £1.50 each! The aerial was the most expensive part of the whole set-up.
"Why is my rig humming?"
"Because it doesn't know the words!"
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