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In the case of L-band, look for insertion or thru loss of around 0.5 dB and return loss of -20 dB (equivalent to VSWR of 1.3:1 or better). When choosing a matching transformer, make sure it covers the frequency band in question and can handle the DC if you need to power your LNB. Matching devices are generally less expensive than transformers, but transformers do a much better job of limiting insertion loss. If you find a mismatch, you can use a matching device – or better yet, a matching transformer – to correct the issue and improve signal quality. This picture can help you differentiate between the two: BNC connectors (which are available in both 75 and 50 ohm variants) are the tricky ones. F type connectors are 75 ohms and N and SMA are 50 ohms. You can use a network analyzer to identify mismatches, but if you don’t have one on hand you can visually inspect your network elements.Ĭables are usually marked with their impedance values. But what if there’s a mismatch – and the LNB you’re using is rated for 75 ohms when your modem is 50?Ĭoaxial cable impedance mismatches can be harmless, but sometimes they result in a lot of reflected power, return loss and insertion loss. Correct an impedance mismatchįor satellite terminals, the impedance of the modem, coaxial cable and LNB or BUC are normally all spec’d the same. If, however, everything above checks out, there might be a more nefarious issue – like an impedance mismatch. Should you discover that your cable or connectors are in less-than-perfect physical shape, or that your L-band or 10 MHz signals are lower than expected across the length of your coax – eureka! You’ve discovered the culprit behind the poor signal quality and it’s an easy fix.
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Find the reference and replace if necessary. A missing reference signal is usually the fault of the reference itself. Again, ensure the signal does not attenuate more than is expected for the cable in use. (LNBs are usually labeled as “external reference” if an external 10 MHz signal is required). If using an external reference LNB, check for the presence of a 10 MHz signal.If you can’t detect an L-band signal, go to the source: modem, LNB or Mux-Tee.What’s expected? Here is a quick reference guide: Ensure that the L-band signal is within the specified levels for your equipment, and that signal loss over the cable is not more than expected for the type of cable being used. This is a little trickier to measure as a spectrum analyzer is usually required. If your DC is okay, make sure the L-band signal is present at both the source and destination of your circuit.No power? Check the output of your modem or bias tee / Mux-Tee. If your power is fed via the coax cable, be careful not to short the center pin to the shield.
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