Why is the SWR only 2.0, but cannot be tuned lower?

Q:

I have an AT1000M Pro.
It works fine on all other bands and can be tuned to an SWR of 1.0-1.3.
However, at 21 MHz, the SWR is only 2.0, and it can't tune SWR any lower. It displays "FAIL."
Why is this?

A:

There are two possible reasons why the actual SWR is ~2.0 but cannot be adjusted.

1. Antenna type: This problem can occur with some unbalanced antennas, typically GP/LW antennas. In this case, you can try adjusting the array length (extending or shortening the antenna) and adjusting the ground plane. This method can resolve the problem in most cases.
   If tuning still fails after adjusting the array or ground plane, try tuning with a different antenna to determine if there is a problem with the ATU's tuning capability at freq

2. Minimum tuning range: Even though the SWR is 2.0 or even lower, the reason it cannot be adjusted may be that it falls below the ATU's minimum step size of 10pF or 0.05uH. This means that the actual required capacitance and inductance values ​​of your antenna are lower than 10pF or 0.05uH, which can prevent the ATU from adjusting to a good SWR.

Therefore, please adjust the array or ground plane first.