https://wb3gck.com/2018/12/09/revisitin ... -vertical/
a simple and easy to build vertical antenna offering good performances over the 10...80 meters bands.
Also, a good field of ground radials and/or stakes will definitely improve the performances
as for radials, four of them will suffice for a simple portable setup; the lenght will be 5.20m (17ft) and 2.15m (7ft) the reason for those sizes is simple; 5.20 meters is near to 1/16 wave on 80, near to 1/8 on 40 and 1/4 on 20; so on all the bands between 20 and 80 meters it will offer a low impedance path to RF "return" currents
do your math with 2.15m for the "upper" HF bands notice that those lenghts are ok with "random" endfed too 
just in case
((300/3.5)/16)=5.35 we repeat the calc for 40 and 20 meters and "round" those 5.35m to a lower value to fit the other bands too
by the way we don't want to shorten the wire too much or it will be uneffective for our purpose (which is also to avoid common mode currents on the feedline)