Very interesting results, Jeroen. Thanks for posting them.
Is the MF resonance due to the inductive and capacitive coupling cancelling each other? (They're 180 degrees out of phase, of course.)
The frequency is way too low to be a transmission line effect in a 1-m length.
Cheers
Phil Hobbs
To summarize:
Apparently Jeroen uses CK50 sheathed in copper tubing while his colleague's cable is CKB50. The transfer impedance of all tested coax cables converge at about 54 MHz - a cable channel's common lower frequency range.
Don Y's primary takeaway from this thread may be to solder both ends of his conformal copper to the coax screen underneath.
Danke,
The original data came from an HP3577 and I recorded only the magnitude. Since this looks like a resonance, that's also what I'd expect.
I can't easily go back and look again. I did this in 2009, and I'm now retired. At the time, I was trying to make a choice for cables connecting beam trajectory pick-ups in the CERN PSB to their pre-amplifiers.
I suppose -but did not verify- that the dip is a resonance of the outer inductance with a parasitic capacitance of my setup, with the screen resistance as the damping element. I can't quite make it fit that model though. The screen resistance doesn't differ enough between, for example, UT141 and RG58 to explain a deep resonance for the former, and its total absence for the latter.
Jeroen Belleman
RG402 and RG405 semirigid coaxial cable has been around for ages.
Plus you had some pretty frou-frou RG58 there, with foil and two braids.
The normal stuff is one tinned-copper braid with about 80% coverage. You can probably make a directional coupler with a pair of patch cords and some heat shrink. (I should try that.)
Cheers
Phil Hobbs
A practical question is what might the coupling be between two close, parallel coaxes.
Since I now have a 3-GHz VNA, I might have a try measuring that. The coupled amplitude goes like a cosine, so it’s easy to calculate how much interaction length you need for 100% coupling from just one measurement.
Cheers
Phil Hobbs
This was Draka's version of RG58, I think it was made to CERN's specs, fire retardant and rad-hard. From an RF standpoint, it wasn't so fancy and it didn't fit very well in standard RG58- compatible connectors.
Speaking of directional couplers, I made one with bits of UT85. It was inspired by a 1979 publication by Udo Barabas in IEEE Transactions on Microwave Theory and Techniques. It worked surprisingly well, with a measured transmission bandwidth of
40kHz to 9GHz, although the directivity deteriorated beyond a GHz or so. It was a rough demo of the concept. I'm sure it's possible to do much better. It was my suggestion for a GHz bandwidth beam position signal processing front-end in the LHC.I wrote it up in
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