Hi all,
It's great to see this subject come up again!
The easy stuff first: The bonding bars are 340mm long and 8.7 mm in diameter with a 9 mm X 10 mm knurled slotted head at one end and a 1/4" BSW thread at the other. And I don't think there's anything special about the coaxial connecting cable other than its length. I saw a suggestion somewhere that the length was kept short to avoid transmission line effects at the higher frequencies but a wavelength in coax at 50 MHz is much longer than the cable so I doubt that it's a real issue.
Now the harder stuff: I don't want to modify the TF1245A because I have the VHF exciter and live in hope of finding a TF1246. The only modification I might consider is to replace the metering diodes with modern hot carrier diodes if necessary. My synthesized signal generator tops out at about 10 dBm so I need a bit more than that to compensate for inefficiency and losses in the coupling circuit
I have had mixed success making the exciter. I duplicated the amplifier from the HP 4342A Q meter and fed it through a 10:1 transformer made from a FT50-43 toroid but the results were not satisfactory at the extremes of the frequency range.? I'm not sure what ratio is needed for such a transformer. The output of the HP circuit is an emitter follower so it will have a low source impedance and it needs to feed the TF1245 input at 0.5 ohms. If I can get it to supply enough power over the entire frequency range I will work towards building in an ALC using a photoresistor optocoupler. I have recently picked up a power amplifier module on ebay that is rated at 2 watts (33 dBm) and will do some more experiments with it although it might need some modification to get down below 1 MHz.
I'm impressed with Peter's transformer design - it's a much better arrangement than what I have been using so I will try and duplicate it and maybe that will improve things.
More news as it breaks!
Morris
