joe wrote:

1K is just a guess as it is just one component in the antenna input
circuit.

Also, nobody observed that the input impedance of the radio can vary
significantly with the setting of the "RF Gain" control.

Crude measurements on a DX-398 show the impedance near 85 ohms at 'max'
gain and near 280 ohms at 'min' gain. The measurements were crude and
the error could be 20%. Use these numbers with caution.

There are no guarantees that the input impedance does not change with
frequency, either.

The Sangean ATS-909 appears to operate no higher than 30 MHz. In the HF
range, antenna efficiency and transmission line mismatch have no
significant effect on the signal/noise ratio (unless the system is
exceptionally lossy and/or the receiver exceptionally noisy, neither
very likely), hence they don't affect your ability to hear stations.

Roy Lewallen, W7EL