It's difficult to define source impedance as it will vary with frequency (even the slightest change) and power (even the slightest change). "Most" low power transmitters (e.g., below broadcast-station levels which are generally in kilowatts) are design optimized for a 50 Ohm resistive load, but that doesn't mean that with varying conditions their source impedance is actually 50 Ohms. Real antennas that aren't purely resistive loads vary in Z with frequency (even the slightest change), so will typically only exhibit VSWR = 1.0:1 at exactly one frequency. That doesn't mean they don't work fine over a range of frequencies. A good way to measure how they're actually working is using field strength readings. I've worked on many MF-HF commercial broadcast station sites and can't remember any of them that had resonant antennas. There may be some, somewhere but I've never seen one. That's why they all have antenna tuners at the feedpoints.