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This antenna:

enter image description here

comes with an balun (or unun) that has three connectors. One for the coaxial cable to the radio, one for the antenna and one for the earth. My understanding is that the earth is an RF earth, correct? If that's the case, why isn't it just using the mesh of the coaxial cable, isn't that connected to the RF ground in the radio?

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    $\begingroup$ According to their website, it has a 9:1 balun at the feedpoint. With a 20m long wire, it almost certainly requires a tuner. The comment there indicates he could not get it to load on 80m. I'm afraid that this one "requires" a lossy ground to "prevent" very high VSWR. ;-) $\endgroup$
    – Mike Waters
    Dec 28, 2018 at 16:21

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The ground/earth connection is for a length of wire that acts as a counterpoise. The instructions for the antenna (if there are any) may specify how long this wire needs to be (typically 1/4 wavelength). If you don't have one, the coax shield will end up being the counterpoise.

Does a 1/2 wavelength end-fed antenna need a good RF ground?

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As imabug says, without a RF ground, the coax may become part of the antenna.

You don't want the shield of the coax acting as a counterpoise for two reasons. You may not have good control of its length which may detune the antenna, and using it that way puts common mode current on the coax and may bring RF into your shack, raising the risk of RFI and radiation burns.

Antennas that intentionally use the coax shield as RF ground typically interrupt the coax with an un-un at a measured distance from the antenna feed point both to control the length and to limit common mode current on the rest of the coax.

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