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The reason the efficiency also suffers with a blip at about 40-60 MHz is that some of the incident RF from a 1mW reference source will end up in the 2.7R damper resistor via the resonant path to R1. This means the DC substitution (used by the HP 432A meter) can't correctly indicate the incident power in this frequency range unless the efficiency change is known across this tight frequency range. It's the equivalent of the sensor losing efficiency and in my case this was a loss of over 1% efficiency at the peak of the bump. This was always really annoying for me because it meant I had to have an efficiency chart showing how the efficiency sharply changed by over 1% across 40-60 MHz and this required lots of cal points.

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Adding the ferrite achieves two things. It removes the bump in the input VSWR and it also flattens the efficiency. At first, it might appear that adding the ferrite would suck even more energy from the thermistors as a ferrite bead looks resistive up at VHF.? This would make the efficiency even worse. However, as long as the ferrite introduces 'lots' of series resistance (ESR), there will be a big mismatch loss at 50 MHz so only a tiny amount of incident RF will get burned up in the high resistance of the ferrite. The efficiency won't be perfectly flat but there should be a big improvement in efficiency across 40-60 MHz and I was able to achieve this.

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Obviously, the choice of ferrite is fairly critical here as it has to provide a high ESR at 50 MHz and it also has to be fairly immune to temperature changes. This means that the initial permeability needs to be reasonably stable wrt temperature changes across about 10-50degC.

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The latest manual for the 478A sensor lists this ferrite as E1 but it doesn't give a part number for it. It would be really nice to know what the official ferrite material is.

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Regards

Jeremy