Electronics > RF, Microwave, Ham Radio

15m long RG-213 with a notch at 450 MHz

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biastee:

--- Quote from: gf on January 23, 2022, 02:29:42 pm ---EDIT: I see you added images. Much better now. It seems, though, there is an impedance mismatch already at the source, which would imply that the cable impedance were not 50 Ohm. If you switch the trace type to IMPEDANCE RESISTANCE then you can see the impedance profile over time (distance).

BTW: The start is outside the left border of the screen. Try to shift the traces a small amount to the right by applying say -20 ns Electrical Delay. I guess the horizontal part of the trace covering the cable length will go even a little bit more negative then.

How does it look with the "good" cable, for comparison?

--- End quote ---

When the format is switched over to "Resistance", the cable portion was ~ 40 ohm. Attached photos of the bad RG-213 cable and a good LMR-400 clone - both with "OPEN" at their opposite ends. A -20 nS delay has been added. The LMR-400 appeared shorter than actual because the propagation velocity was at default 67%; i.e. not changed to 85%.


--- Quote from: Bud on January 23, 2022, 03:22:32 pm ---I am thinking the RG213 may be leaky and what you see is a result of coupling the leaked fields to the surrounding environment. LMR400 is a double-shielded cable so leakage is much smaller. To test this theory put a few snap-on ferrite chokes on the RG213, at the ends and along the length, and see if that affected the resonance amplitude or frequency.

--- End quote ---

I don't have snap-on ferrite chokes that are large enough to fit the RG-213's 10mm OD. Can I use TDK T19-31-8 (ui = 290) toroid rings which is large enough to slip over the BNC connector instead?

Thanks to all for the responses!

Bud:
Use whatever magnetics you have , the point is to test the leakage idea, whether using magnetics affects the resonance.

coppercone2:
you know, how about testing that wire with a megger? If its contaminated dielectric maybe the leakage current will be higher.

Measure shield to center pin resistance, preferably first sweep it at a lower voltage (100V?) with a electrometer if it does not reveal anything, plug it into a megger.

biastee:

--- Quote from: Bud on January 23, 2022, 03:22:32 pm ---To test this theory put a few snap-on ferrite chokes on the RG213, at the ends and along the length, and see if that affected the resonance amplitude or frequency.

--- End quote ---

No joy with the ferrite rings (see attached photo).


--- Quote from: coppercone2 on January 24, 2022, 08:26:32 am ---you know, how about testing that wire with a megger? If its contaminated dielectric maybe the leakage current will be higher.

Measure shield to center pin resistance, preferably first sweep it at a lower voltage (100V?) with a electrometer if it does not reveal anything, plug it into a megger.

--- End quote ---

:-) I really didn't expect to hear "megger" on an RF forum! Ok, I am throwing in the towel at this point! I don't own one and I don't intend to splurge on one for the following reasons: 1. this test is probably not specific because a crushed cable would also exhibit a lower breakdown voltage, and 2. a positive diagnosis will not enable the problem to be rectified.   

Thanks to all for the responses!

coppercone2:
I mean, you can just put any voltage across it and measure current. does not need to be accurate just compare to another cable.

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