Powdered iron cores typically have a 55 degree C ambient temperature limit. Ferrites have differing temp limits. Some powder iron mixes are ususable up to 70 degrees C with the knowledge that the core's performance will degrade and life cycle will shorten. Further the larger the core volume the faster the core will age. (See http://www.lodestonepacific.com/distrib/pdfs/Micrometals/desguide.pdf
You will want to minimize your core losses. That is in part achieved by the following:
1 Selection of optimum iron powder mix for frequency of operation. You might use -2 material for 80 meters and -6 for 40 and 20.
2 The diameter of the wire used.
3 The number of turns used.
4 The spacing between the turns.
5 Using a single layer of turns.
If Micrometals still have the curves available look at the differing Q curves for differing core materials, number of turns, wire size and core sizes. A nice inductor with a Q of 250 at 4 Mhz can quickly sag at 5 or 7 Mhz to a Q less than 100. You can wind a toroidial inductor to provide less of a Q change over frequency but invariably you sacrifice the peak Q performance.
When looking at the core information on www.lodestonepacific.com
you can see that different core materials are used at different frequencies. Generally -2 material is good for 80 meters. I personally prefer -6 material for use in the 7 Mhz to 30 Mhz range. -2 material can be used at 160 Meters but -1 is my choice in most applications.
Check the following url for Micrometals data on their cores.http://www.lodestonepacific.com/distrib/pdfs/Micrometals/RFA_Cat.pdf