On our local Relay there are from time to time transmissions, where only static can be heard. Most transmissions via relay can be heard clearly. What could be the cause of this?
I hear this from time to time on more than one of my local repeaters -- especially on one that has a significant height advantage over most (1400 feet up a TV transmitting tower). Given the length of some of the static-only transmissions, and their timing (about when I regularly hear two or three local hams have their morning conversation), I have a hypothesis.
I hear this from time to time when I'm driving (I only listen to repeaters when driving; I've got too much other stuff to do at home, and don't own a fixed location radio that can access repeaters in any case). When I hear it is when I'm driving though a local terrain depression -- a small valley -- in a location far enough from the repeater to produce at least a partial signal blockage.
I encountered this again this morning on my way to work, and was able to verify that the S-meter bargraph on my radio's display was showing its minimum level -- yet the squelch was open with no audible signal. I took this as confirmation of my hypothesis below.
What I believe is happening when this occurs is that someone transmits and keys the repeater; the repeater opens and sends its CTCSS tone, which my radio detects and opens its squelch -- but the signal from the repeater is attenuated enough by terrain that instead of clear voice, I hear only the noise floor that would always be present without squelch. The CTCSS tone is "subaudible" -- below most radios' frequency response limit -- but it can also be detected in pretty heavy noise because the receiver is filtering for a single, narrow audio frequency.
This is not to say you can't hear static-only when someone "kerchunks" the repeater (illegally makes a short unidentified transmission), but in my experience, what I get then is a short period of "full quiet", followed by the repeater's courtesy tone and the very short static burst before my radio's squelch closes. That's very different from a long period of static with the squelch open.