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IIRC 122.75 is the only legal air-to-air fixed wing frequency outside of controlled airspace.
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Sort of. The problem is that the FAA never assigned a universal working (not emergency) frequency between all types of aircraft and aircraft operations. This has lead to the unofficial frequency of 123.45, which some call "fingers". And 122.75 is so noisy that 123.45 is simply more usable. Interestingly, it is a legit frequency for oceanic operations and all transoceanic aircraft monitor it.
Helicopters will often monitor 123.025 as a "point of pride". It's like waving to people who have the same car as you. Plus helicopters do tend to flock together, with news, EMS and military often over the same emergency.
The best legit answer is to get your private airport registered with your state and the FAA, on the sectional chart, and assigned a frequency. Then in the AFD notes be sure it says "Ultralight operations frequently in the vicinity", etc. This is not as hard as it sounds. I have a private heliport registered on my property. However that is still no guarantee that anyone will monitor or self announce as they pass by.