Chinese Lulu transmitter?

[Nige from Brum]

I found this earlier on Ali Express.

https://www.aliexpress.com/item/1005005 ... ry_from%3A

It looks very much like the Lulu shortwave transmitter, which got me thinking, If you use the DDS input on the chip, & add a cheap class D audio amp with a 12v transformer and 2 capacitors, you have an almost off the shelf shortwave rig. Obviously you could replace the crystal if you had a suitable one too.

Or am I barking up the wrong tree here?

[EFR]

I have build one of these for fun.

FET is chinese fake, makes about 3Watts of RF.

LPF toroids were fake, needed about 1.5 times more turns to get it to band.

I used genuine IRF530, bolted it to huge heatsink, added resistor from gate to ground, I cannot remember why? And used cheap chinese 100W audioamp to power FET with bigger shoke on line.

Made about 15W carrier, around 50Wpep.

One good thing is that You can wire your dds directly to the SMA connector.

If you already have DDS, just build some simole buffer amp, lots of them in internet, trow one transistor stage with 2SC1969 and modulator.
Set carrier to around 2-3W.

Add BPF behind it, and get one those cheap chinese 180W HF amplifiers, hook it up and set modulation so it peaks around 150-170Wpep.
Add nice LPF after it.

Full HF transmitter, costs under 80€ here....

[Albert H]

Unfortunately, the linearity of the "170W" Chinese SW amplifiers is rather poor. If you bias them carefully, you can get the distortion down to around 8%, but no better. I used one of those boards driven to about 150W on modulation peaks, then added envelope feedback to the modulator. I got the measured audio distortion from about 10% down to just under 0.1% - better than acceptable! Unfortunately, the efficiency was horrible, so I decided to modulate the Final. fed to the Chinese PAThe modulator I used was a Class D mono amplifier module that was rated at "300W" - which seems to be Chinese for about 75W RMS. I used a somewhat mangled and re-wound ILP mains toroid as the modulation choke, and the whole thing ran reasonably cool when supplied from a 28V supply.

Two car batteries in series ran the rig at about 145W peak, 40W carrier for a few hours. The rig is built into an Eddystone diecast box, with an aerial tuner built into a plastic icecream carton, which makes a nice waterproof housing. The audio comes from a cheap Chinese MP3 player.

The rig uses a simple single IC synthesiser (using an MC145106), and tunes from 5.8 - 6.5 MHz, with the frequency selected by a row of DIP Switches. The PLL voltage-tuned oscillator is followed by a three-stage transistor amplifier that takes it up to around 2.5 Watts which gets fed to the Chinese amplifier.

The whole rig cost more in experimentation time than it did in parts! The whole rig cost around €70 (but that negects a few of the components that were cannibalised from old gear). Running it into a ½-wave sloper gives pretty good coverage in the daytime. The RTI guys who're using it have received mail from most of Europe, and I could hear it at both of my homes.

They've had problems lately with another station in their vicinity, who seem to be running much more power, who landed on top of their frequency a few weeks ago. They've been having difficulty in finding another equally good frequency, so I've suggested extending the aerial and moving down from 6.3 MHz to 5.8 MHz, since there seems to be more room there at the moment.

When they finally "lose" this rig, I'll build something quite different, using outphased PDM for the modulation. It'll still use FETs for the output, but will develop more power, and be much more efficient.

[EFR]

Hah, and I just did a short testrun with that old 3-500Z rig on 6300Khz, nice 100W carrier, about 400Wpep.

But if we think around FETs and tubes, they are quite close...

[Albert H]

It's amazing how FETs have developed over the last few years.

[shuffy]

It's amazing how FETs have developed over the last few years.

Tell me about it. I've been working on an HF design for a few years now and keep redesigning the "PA" because whenever I get enough free time to do some construction, there's a bunch of new FETs out - most recently a bunch of new GaNs. There's currently some chatter on AMFone for anyone interested.

[Albert H]

I know exactly what you mean. However, the very latest FETs tend to be expensive. If you don't mind (slightly) reduced efficiency, you can use FETs from the previous generation (about 4 - 5 years ago) and save a lot of money!