yes, Cairnpapple Hill SS-254 was going to be my first suggestion and a very easy walk. There are also a few hills immediately south of Edinburgh in The Pentlands. Quite steep approaches for a non-hillwalker but short walks and well established paths GM/SS-171 Allermuir Hill might be the easiest with good views north over the Capital. Best to start from Castlelaw Hill Fort an Iron Age settlement.
For Arthurs Seat GM/SS-272 you need to ask permission from the Holyrood Park Rangers before using a radio in the Park, but I know of no one who has been denied permission to broadcast, just a formality. The top is a short but quite a steep walk from Dunsapie Loch car park, so the summit is typically busy at any time of day with tourists and runners! There is a large flat area of grass just to the south of the summit which would allow use of a large HF antenna and is in the activation zone.