Dieter Kiel wrote:
The tuner doesn`t change the antenna specification, e.g. gain,
impedance, or pattern.
If you buy a beam antenna you will get this data, and the pattern of the
antenna is most time a free room pattern.

As the tuner is used to match the impedance of the radio to the line it
will effect the power getting to the antenna or the signal received from
the antenna if it is passed through the tuner.
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Agree with the above because the matching device (tuner) is NOT 100%
efficient , hence some (hopefully very little) RF power from the
transmitter will be converted in heat inside the matching device.

Frank GM0CSZ / KN6WH.