how close to the ground should the unun be on an end fed half wave?

For an end-fed half-wave, the transformer box itself can be near the ground; what matters more is that the wire is elevated and the feedpoint is practical to keep dry and secure. A common rule of thumb is to mount the UNUN about 1 to 10 feet above ground, with lower being fine if you can manage moisture, strain relief, and RF safety.

Practical placement

• Ground level to a few feet up: Usually workable for portable setups or temporary installs, especially if the box is weatherproof and the coax/choke are arranged cleanly.

• About 3 to 6 feet up: A very common compromise for home installs because it keeps the transformer accessible while reducing splash, snow, and accidental contact.

• Higher than that: Also fine if it helps with routing, but the UNUN itself does not need to be high for the antenna to work.

What matters most

The bigger performance factor is the height of the wire , not the transformer box. EFHW references note that the antenna needs a proper matching device, and that a counterpoise or ground arrangement can help the end-fed current balance at the feedpoint.

Simple recommendation

If you want a clean default answer: mount the UNUN as low as practical but off the wet ground, roughly 1 to 4 feet high for most installations. If you’re portable, even lower is acceptable; if you’re permanent, a weatherproof mount a few feet up is usually easier to live with.

If you want, I can also give you a best-practice EFHW install diagram with where to put the choke, coax, and counterpoise.