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The standard output impedance for a RF generator would be 50 ohms, so that would be the best coax type. However low cost generators like the SG-8 don't bother with exact impedance. An attenuator that is marked in db will not be correct if the connecting impedance is wrong, but otherwise it doesn't matter unless the coax is very long. 75 ohms is used for a few things like TV RF and video signals.
The same modulation is used on all bands in the SG-8. The modulation puts a audio sinewave on the output tube grid along with the RF. This is not the best way to modulate the RF, but it works OK. It is possible that the audio is changing the tube bias somehow and effecting the operation.
The frequency counter will only function correctly without modulation. You need to use a working radio or an oscilloscope to check fo modulation.
I don't know exactly what you did and I don't know how you did it, so I can't know what the problem might be.
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