My tuner doesn't exhibit the behavior you describe. You are the second person to mention the sudden changes in volume. I'd be curious to see what Hugo has to say about this?

Your antenna sound similar to the kind of antenna I had on my VW GTI. It was a Fuba antenna (OEM'd to VW) with a small power amplifier in its base. There was a power wire that paralleled the antenna wire-- the factory radio supplied some constant voltage to that wire. Without this, the power amplifier did not boost the signal and you got poor reception.

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Tony Fabris