What isn't clear to me is that if your unmarked field is within the CTAF of a marked airfield what frequency do you use?
Okay the F in CTAF stands for frequency. A CTAF is not a volume of airspace. It is a frequency to use in the vicinity of an aerodrome (landing area etc).
So it follows than you cannot be "within" a CTAF. If the unmarked field is withing the "normal" circuit size of the aerodrome which has a CTAF then as you say, common sense says use the CTAF.
https://www.casa.gov.au/file/182536/...token=ev1DY9ng
After exploring options for change and considering all feedback, we believe the safest and simplest system is the one currently in place. In other words, the recommended radio frequency to use in non-controlled airspace are:
- ‘in the vicinity’—within 10nm, and at a height where your operations could be in the way of other traffic—of any non-controlled aerodrome published on aeronautical charts, pilots should use the CTAF (126.7 MHz or discrete frequency) as published
- anywhere within a Broadcast Area, pilots should use the dedicated Broadcast Area CTAF
- in all other non-controlled airspace, pilots should be on Area VHF .