Antenna Tuning Unit (ATU)
ANTENNA TUNING UNITS (ATUs):
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http://en.wikipedia.org/wiki/Antenna_tuner:
"An antenna tuner, transmatch, antenna tuning unit, or ATU matches a transceiver with a fixed impedance (typically 50 ohms for modern transceivers) to a load (feed line and antenna) impedance which is unknown, complex or otherwise does not match. This mismatch is usually caused when using a non-resonant antenna (one that is not the correct electrical length as compared to the wavelength of the signal). An ATU allows the use of one antenna for a broad range of frequencies. A tuned antenna is never as good as a naturally resonant antenna due to additional induced losses on the feed line due to the SWR (multiple reflections), and losses in the ATU itself."
http://www.arar93.dsl.pipex.com/mds9.../aerials2.html
600 Ω balance XLR connector:
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I have never seen an XLR connector with specified impedance. The reason is that the physical length of the XLR connector is just a fraction of the wavelength of the highest audio frequency being used with the XLR connector. So impedance matching of the audio connector itself is not important. More important is low stray capacitance.