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Cable loss higher than expected

Jun 17th 2018, 10:54

ON4DD

Joined: Aug 2nd 2014, 09:58
Total Topics: 0
Total Posts: 0
Dear,

I am testing a long cable run (about 90m). This cable has been used outside in rather harsh environments at times.

I've set up my VNA with the correct values (velocity factor,...) and the distance to fault is correct when I just connect one side to the RF out connector (S11).
However, the datasheet specifies 25.8dB loss per 100m at 1Ghz, but I am measuring 43dB loss (S21).
DC resistance of the center conductor and shield is according to the manufacturer specifications for that length. There is no (measurable) DC loss between shield and center. Capacitance is defined at 56pF/m, I measured 5.82nF which seems to correct.

I am wondering if this could be caused by:
- possible moisture in the foam PE dielectric
- other reasons?

Anything I might be missing?
Thank you,

Dieter ON4DD
May 26th, 21:58

WA8NVW

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Moisture under the jacket will cause corrosion of the copper shield surface, increasing losses exponentially at higher frequencies. One cure is to utilize a cable with flooding compound around the braided shield. Another is to select a cable with a solid outer conductor, either 'hardline' or layers of foil along with the braid, such as RG6/QS used by cable TV companies at frequencies at or above 1 GHz .

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