Navtex-Fan wrote:
Z = 241-j1812 Gain = -28.6 dB SWR = 278
I'm assuming that the mismatch is "solved" by the use of the 4:1 UnUn..

Nope, not necessarily. Most 4:1 designs assume a resistive
load and do not handle reactance very well. A 4:1 device
might make the problem worse.

If I model a base loading coil of +/- 550 microH , the following results:
Z = 165-j14 Gain = -26.9 dB SWR = 3.3

As long as your receiver AGC has enough gain to overcome
the loss in the antenna, why worry? Internal receiver noise
is usually negligible compared to atmospheric noise at
518 KHz.
--
73, Cecil http://www.w5dxp.com