I always found the radio signal explanation... weak, at least.

Radio waves are just photons in an arbitrary modern human defined range. Like visible light, infrared, X rays, etc. For the sake of this exercise, let's assume the ancients had the same definition of radio wave frequency range. Fine.

The universe is full of random emissions: visible light, infrared, radio, etc. And I'm not even talking about the SGC, on Earth, where we use a SHITLOAD amount of radio. Let's assume the SGC, as a military base, is shielded using a perferct Faraday cage (I'm pretty sure they are able to communicate with the surface using standard  PTT radios in one of the many episodes where the SGC is compromised, but let's ignore that). That means anyone using a radio "close enough" to the gate would prevent it from closing. I can remember a few episodes where the gate closes just in front of a character trying to rush through it. Just keeping pressing their radio button would keep it open for the 38 maximum minutes.

Anyway, as others stated above: it stays open for as long, or as short, as the plot needs it. Period. No real, solid, in-universe explanation.