Here is an easy way to adjust your ATU without the need to transmit.
This small unit is powered from 12v. You insert it between your transceiver and ATU. Uf you are using an amplifier then insert it between transceiver and amplifier. All adjustment are made whilst your are receiving. The unit can be left in circuit whilst transmitting as it is bypassed automatically.
When you switch the init one you will hear a pulsating noise in the receiver. Then simply adjust the ATU until the pulse noise almost disappears. That is the point of optimum adjustment and you can go ahead and transmit. No interference caused to other users. Connectors are SO-239.