Hi Andrea,
Welcome to the forum and welcome to the world of the X108G!
Let me know if you have any questions as I use my X108G (outdoor OLED version) as my main rig for SOTA now, taking over from an FT817ND from Yaesu. Do you intend to use the rig from home and portable or just "in the field"?
ATU - there is a cheap £9? manual QRP ATU kit out of China (
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) that may do what you want but of course a better route is to create a resonant antenna if that's possible, as an ATU doesn't tune an antenna. It matches an antenna to the 50 ohms that the rig needs - so they're better described as Antenna Coupling or Matching Units. The exception to this rule are things like a screwdriver mobile antenna or the SteppIR wher the actual element lengths on the antenna change to match the frequency in use.
Do you already have an portable antenna? If not and if you want to buy one, take a look at sotabeams.co.uk in the UK, Richards products are well built. If you prefer to build yourself, an effective, simple, cheap antenna is a linked dipole and Rob has a calculator on his SOTAMaps website here:
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click on the linked antenna designer tab. In fact you can combine both appraoches as Richard does a kit of parts as well, so you can build your own linked dipole - you measure and cut your own wire and coax and Richard's £17,20 kit provides all the other little bits you need.
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If you don't have the coax or the antenna wire, he can supply those as well. See what you have and see what you need or make it all up yourself with whatever you can find laying around - I find that's always more fun.
73 Ed DD5LP (G8GLM).