I think horizontal line from cable loss.
Very good free simulator program:
Simulate lossy coax (for example RG-58), now subract with ideal lossless coax. Negative coax lenght supported. Now near horizontal line, not dot the zero and infinity impedance on smith chart.
I already requested not only electrical lenght compensation, loss compensation important too. With firmware mod or/and external program.
Best coax is a precision air line, very expensive.
