The 18 gauge wire is the same size as the center conductor of RG6 coax. Actually, the piece of wire in the photos came from a short length of RG6 coax.
The meter switch was set to the 0-50 VDC range.
This measurement is called measuring the output voltage without a load, because the preamp is not connected.
When the preamp is connected, the voltage will drop to its normal operating level, called voltage under load.