Good afternoon, all.

I would like to put up a dipole antenna for 80 and 40 meters, and a
frequency near 4.5 MHz (for non-amateur use), coax fed, without the
use of traps or a tuner.

I expect to use three dipoles (one for each band), all coming together
in the middle and fed with a single feedline through a 1:1 W2AU-type
balun.

For illustration purposes, let's say that the 80-meter dipole will run
east-west, the 4.5-MHz dipole will run northeast-southwest, and the
40-meter dipole will run north-south.

Questions...

Is the standard 468/F formula likely to be anywhere near accurate, or
will the three antennas interact with one another to throw the
calculations off?

In particular, is the 4.5-MHz frequency too near the 80-meter band
such that interaction will be greater than otherwise?

If the interactions are sufficient to throw the formula off, will my
antennas end up being longer or shorter than the formula length?

Thanks.... CJ