Q: Do you have to be right under the eclipse to see effects? A: No. In fact, the greatest impact on propagation in most bands will probably be noticed by observers who are well off to the side of the eclipse, but who are listening to signals originating on the opposite side of the eclipse track.
Examples: A listener in Tucson might see a day-night transition of signal levels from WWVB in Boulder, then a return to daytime levels mimicking a sunrise transition, all in under an hour. Listeners from the Pacific Northwest to the northern Great Plains could briefly experience night-like reception of the 50 kW AM broadcast giants in Los Angeles or San Fransisco.
That's not to say those right under the eclipse will miss out on anything. In addition to about four minutes of the dramatic ring-of-fire annularity effect itself, they should encounter a whole series of interesting propagation effects from any HF ham stations that also lie along the track of the eclipse. It would be fantastic, for instance, to be a ham in Eugene, OR, or on South Padre Island, TX, communicating and/or beaconing with your counterpart at the other end of the path the whole time that the shadow is over land!