Looking for a semi-strong SW transmitter on 6-7MHz~

[crashuncle]

I plan to build/get a shortwave transmitter soon because SW is harder to backtrace than AM/FM. There exist hundreds of schematics on the internet but its difficult to find out which ones are good and which ones are bad.

So what would you recommend me? What do you use personally?

The transmitter should have at least 5-10W, but I also don't mind something stronger like 20-100W

Suggestions for a fitting antenna are also welcome.

[Kage]

If you understand the science behind class E amplification I would go that route.
It's relatively easy to get 10 watts or more with a single FET in switching mode class E and using a square wave oscillator as the signal generator.
The troubles come in with filtering its output to get a nice sine and no harmonics.
Using the method in push-pull you can get some serious power up around 100 watts but requires much more careful planning and power supply current.
Of course the output network of a class E amplifier is the limiting factor in frequency transition and will need a retuning each time you switch frequencies.

It is also fairly easy to design a linear amplifier using MOSFETs and amplify a simple signal generator either by means of xtal control or VFO which is what I chose to do with my setup.
This can work very well with preventing harmonics as it's linear from the first stage of oscillation and modulating a linear or previous stage before it can be done easily, even with a class A "series modulated" stage giving you clean flat audio across the whole audio spectrum.

There are so many ways to design a transmitter lol.
What exactly are you familiar with designing and what would work best for you?
The thing is there are lots of transmitter schematics online like you say. The best one to work with is what happens to be easiest for you to construct and using designs you are familiar with.

Also you should keep in mind that AM is a bit tricky in that you need to keep that output waveform rock solid from -100% modulation to +100% modulation with no carrier swing which means the thing should be built to give a solid unforgiving RF output even when fully modulated. The audio sine wave should appear exactly the same on the RF output, no flat topping or bottom rounding of the wave.

Buying an SW transmitter is not easily done. I can't think of a single company that sells one for hobbyist. Your best bet at getting one pre-made would be had at a cost to someone that can either design you one or willing to pass on a grenade or corsair type transmitter, and even at that those transmitters are peanut whistles and can't usually hold their own in modulation without popping a MOSFET output unless designed perfectly from what I have heard.

[Kage]

Oh and as far as the aerial... I would go with the basic dipole with a balun, or even an inverted V as it has some vertical projection along with its horizontal path.
Make sure to point the broadside towards your aimed audience as that's where most the signal is coming from.

[RFBurns]

Class E is a great way to go for best efficiency. Class E is however a bit tricky as it requires your MOSFET to work in both zero voltage and zero current ratio (cross point) during the switching. It's also quite tricky to maintain that zero zone when loaded, so your output network will have to be designed well, providing enough transfer efficiency while keeping a level of load isolation on the MOSFET. Load swings will throw the Class E out of whack, thus a very well matched antenna is needed to tie in to the very well designed output network. Use toroid transformers on the output network and antenna balun. Plan to spend some time tweaking everything for a good match while maintaining the maximum RF transfer efficiency to the antenna as well as the maximum output device efficiency.

Good luck!