In the middle are four diodes arranged in a bridge configuration. If you think it looks a bit like a bridge rectifier you would almost be right. I never look for conversion gain, that's what a quality I.F. Amplifier is all about. No F1 or F2 signals should appear at the output. Similarly because of this balance the F1 and F2 signals do not appear at each others port.
Some mixers require even higher levels.
A pure 50 ohms would be ideal. High quality, high speed diodes which will take the necessary saturating current and large reverse voltages across the non conducting diodes are an absolute must where performance counts. The actual construction will determine the bandwidth. Diodes make ideal harmonic generators. Noise figure is rarely an issue in H.F. sets.