Hi Charlie,

welcome to the world of HF and digital communications! your symptoms are typical and there are a few measures to be taken. unfortunately, much of the noise spectrum resides in the 80/75 meter amateur band! you'll typically find improvements above 10 Mhz.

your installation is problematic due to the proximity of your antenna tuner and autopilot. mine too, as I have most electronics installed in the closet in the aft cabin, on the same side as the antenna. there are several measures that can help. alternator noise (another culprit) and switching noise on the 12VDC bus can be reduced by using an inline filter(s) (Newmar or equivalent) at the electronics bus. the electronics should have a separate bus and protection which facilitates this filtering greatly. filtering the 12VDC and using flexible copper strap for grounding will provide a low-impedance source. take care with grounding and keep RF and DC grounds separate.

two sources of autopilot noise can be partially eliminated through shielding. unfortunately, most autopilots are not shielded and radiate into your antenna tuner. you might experiment with aluminum foil, to help isolate the noise source and better model the problem. one issue is the unshielded GTO-15 out of the tuner, so you have to reduce the offending noise source. you'll find, that if you use a balanced coaxial feed antenna, that many of your woes diminish. another source of noise is from the DC motor cabling from autopilot to linear drive. I recommend shielding these wires in a sheath or using a shielded cable. I also recommend using very large wire to the drive, and there are only a few cable manufacturers that offer a suitable shielded cable of that size. only ground one end, generally at the source, as noise currents can flow along the shield if both ends are grounded. keep this in mind for all your shielding, and it applies to Seatalk and NMEA outputs as well. I also use capacitors and L/C filtering at the alternator. the Balmar regulators can generate a wide spectrum of noise if not attended to.

having said this, the reality is that the proximity of the components and the digital spectrum produced from switching currents, makes for a noisy environment. the automated antenna tuner is a godsend on a sailboat, but is also the source of most of the problems. you'll find, experimenting with antennas, that coaxial shielding from source to above decks is quite effective. Icom makes a balanced-output tuner, but we are now faced with a broadband antenna design. I've found the isolated backstay works well from 2Mhz to 30Mhz. it will even load on 6 meters!

shielding inverters is a hopeless endeavour. even Class A inverters radiate from DC to daylight. eliminating noise from the 12VDC and some judicious shielding should greatly benefit your radio installation. you'll probably start favouring the higher frequencies where noise is not the issue it is below 4Mhz. most of my Winlink operations are on 10.1Mhz with no noise issues, and I operate AM/SSB on 75 meters without problems.

good luck and 73's

daniel