I am putting up an inverted V beam tomorrow for 40 meters. At first it will be 2 ele and if all is well I’ll add the 3rd. My question is: Can I add 20 meter elements to the same boom without significant interaction? Eventually I will have a rotatable yagi for 20 so the 20 meter part would be temporary. Thanks!

This is tricky since there is strong interaction, especially when the bands are harmonically related like 20m and 40m are. But, it can be modeled to determine spacing and element lengths. Lacking the ability to do very good modeling, you might just copy what others have done with 20/40m yagis using flat linear elements; shouldn't really change if the elements are drooping like inverted vees.

If you can put up two elements, why not make them the same length and phase them? In this image, you can see two dipoles that are phased. But you should be able to do the same thing with the inverted vee's. Make sure you use all the same type of coax (obviously, so that the electrical lengths work out with proper phase): https://www.google.com/imgres?imgur...oTmAhUKca0KHcnsBc8QMwhNKA0wDQ&iact=mrc&uact=8 After some encouragement by VA2GU (check out his QRZ.com page), I actually did this on 80 meters last season using a phased dipole design from ON4UN's Low-Band DXing book (5th edition), pages 12-2 and 12-3. I used phasing coax lengths of 1/4 wave and 3/4 wave at the desired frequency (3.510 kHz) so that I would get the same current magnitude to both antennas, 180 degrees out of phase (this is referred to as "current-forcing" using properties of 1/4 wave coax multiples, really neat stuff! Described in ON4UN's book on page 11-11, and in the ARRL Antenna Book by Roy Lewallen, W7EL). For my setup, the two wires were separated by about 30 feet. They were at a height of 25 feet. Recommended height is 1/3 wavelength and separation 1/8 wavelength, which comes out to be about 55 feet high and 32 feet apart, respectively. I was not expecting much, with the wires so close to the ground...I figured ground absorption would be too high. I was wrong! I fired the thing up on 80 meters with my FT-817 QRP, and easily worked into Europe. From wires at 30 feet above the ground? Yep. Unfortunately, the trees came down in an ice storm shortly after installation...and I have been so busy with work that I never got back to reinstalling the setup. The link I sent above for Bob Heil's antenna is NOT using current-forcing, it's using Christman phasing method if I remember correctly. His lengths are cut based on the electrical length of RG-213 to get his desired phase difference. Just remember that if you're doing the current-forcing method, 1/4 wave coax is not true 1/4 wave of the frequency...it's actually a quarter wave at the frequency you want MULTIPLIED by the velocity factor of the coax! Nice part about the phasing is that you can flip directions with a switch in the shack! Let us know how it goes.