SMA to BNC adapter

Does anyone know if these adapters effect the receive of handheld scanners? I am using one now and it seems to degrade the receive. Or the antenna may be defective, I can't tell because I do not have a scanner with SMA connector.

Not all adapters are created equal. I've had RS adapters that degraded SWR when in line. The same type of adapter made by Amphenol had not effect. So the answer is that it is possible. However, I'd try a different antenna, too.

Check if the adapter is making a good connection. This can be an issue with SMA, depending on the make and model of the adapter.

What I do is program something always on into the radio, like the weather, and start screwing the adapter into the radio with the coax or antenna already on it. If the receive don't pick up before I can still get a couple more turns on the adapter into the radio, I look for another adapter.