Need Help Tuning 6 Meter Yagi

I'm trying to dip my toes into VHF-land starting with six meters. I've built a 3-element Yagi, plumber's delight style with a gamma match and it's now sitting on the mast at about 6 feet off the ground.

6 meter Yagi at six feet elevation for testing
In testing position (please disregard that funky gamma clamp)

The half-inch aluminum elements were cut to a calculated length for 50.124 MHz and it's not far off in the test position. Trouble is, though I can tune it fine at the antenna, at the other end of about 100 feet of RG-8 PE core coax, the readings are way off, though there is a distinctive dip (actually two).

At the antenna, I have an SWR of 1.8 at 50.190 MHz. Close enough for government work I say. The SWR curve looks "natural" to me. All good.

However, when I hook up the feed line and apply the AA-54 to that end, it's all "patas arriba" as they say here in Costa Rica (feet in the air).

There's a nice dip about 800 KHz lower and a lesser one abut 1 MHz above the center frequency I had at the antenna.

Finally, at the first dip center frequency of 49.410 MHz, the specs look very good, almost no reactance, SWR of 1.09. In theory, I could move up the center frequency as measured at the antenna to get above 50 MHz at the rig end and I'm sure my rig would be happy with that. However, that sounds like a plan fraught with peril.

So, thoughts from the experts please! If it was just lossy cable, I could understand this, but that would more likely show a very broad trough in the SWR, not distinctive dips. Is it possible the line is not choked sufficiently and common mode current is the culprit?

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Thanks very much for your comment! 73, Casey


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