Quote Originally Posted by KU0DM
We're putting up an 80m dipole at the K0KU club station, but need to cover most of (if not all) of 75/80 meters.

My current thoughts on achieving this is a parallel dipole, with the upper wire cut for 3.550 and the lower for 3.900 (which are our target areas). If the (-) is the wire, it'd look something like this:

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What are your thoughts?
Any other recommendations?

Only requirement is that it's coax fed.
If you have the room (presumably on the building roof?) I'd build two dipoles -- one cut for 3550, one for 3900 -- that share a common feed point. Mount one in one direction (say, NS), the other 90 degrees to the first (say, EW).

Energy flows along the path of least resistance, or in the case of antennas reactance, so while there will be some interaction between the two, it won't be much. The effect will be that one antenna will be doing the work when the other (due to high reactance on that particular frequency) will be pretty much inert.

I've done this before, and it works well (both in the 80 CW/75 Phone configuration, and 80 CW/40 CW configuration). You may have to do some minor tweaking of the antennas to get them to play on the frequencies you want, so cut them a touch long and trim accordingly.

I don't know what would happen if you hung the two antennas in parallel with each other, such as this:
80 CW ------------------||------------------------ ____________(similar to a catenary wire used on electric trolleys and rail lines)
75 PH __ --------------- ||--------------------
(ignore the __ inserted to get the spacing right)

The interactions would be different, but I think in general it could work as well.

Either way, you would need to plan on a transmatch to work the DX phone part of the band.

Also, either way, try to raise the center feed as high as practical, to make the antennas more of an "inverted V" than a true "in plane" dipole. Brings the feedpoint impedance closer to 50 ohms than 75, and the better the feedpoint impedance, the easier it is to match the antenna with 50 ohm coax.