spectrum analyzer dbm
Yeah, a high Z scope can give you a lot of errors. First of all the marconi might put out way too much power, because it is not loaded down with 50 ohms. But the Marconi internal power meter may not realize this.
Then at the end of the coax line running into a high z load at the scope, theoretically the voltage there will double (you have a forward wave, and a reverse wave or equal amplitude and the same phase on the line, so at the termination open circuit, they linearly add to double).
Put the T junction and a 50 ohm load on the unused port and you will get a true reading.