Reliable communication between antennas and modems has never been more important. Sub-par performance due to poor signal quality can result in everything from angry customers and lost revenue to major government and military missteps. The good news is there is (usually) an easy fix. Here are some handy troubleshooting tips for the next time you need to improve signal quality on either the transmit or receive path between an antenna and the modem.
Inspect your coaxial cable and connectors
Issues along your transmission line are often responsible for loss of signal level and should be checked first:
- Broken or kinked cable – Make sure you visually inspect the entire length of the coaxial cable between the antenna and your indoor modem. Look for kinks, broken sheath or loose connectors. If your cable is damaged, swap in a replacement.
- Broken, corroded or bent center pins – Visually inspect the center pins on each connector. Not just the cable but the modem, LNB or BUC connectors as well. Also, make sure that the shielding is properly terminated on the connecter and no stray wire is shorting against the center pin. If you have a damaged connector, replace it.
Check your signal source
If your satellite transmission cable and connectors check out, move on to your signal source:
- First, use a multimeter to check that DC power is present at the terminating device (LNB or BUC) and that it’s at the voltage level required. If your power is fed via the coax cable, be careful not to short the center pin to the shield. Tip: use an alligator clip here. No power? Check the output of your modem or bias tee / Mux-Tee. Replace if necessary.
- If your DC is okay, make sure the L-band signal is present at both the source and destination of your circuit. This is a little trickier to measure as a spectrum analyzer is usually required. 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. What’s expected? Here is a quick reference guide:
|Coax cable model||Impedance||Loss per 100ft|
|RG-6||75 ohms||7.4 dB @ 400 MHz|
|RG58||50 ohms||11.7 dB @ 400 MHz|
|RG59B||75 ohms||9.0 dB @ 400 MHz|
|LMR 400||50 ohms||3.9 dB @ 900 MHz|
|LMR 600||50 ohms||2.5 dB @ 900 MHz|
- If you can’t detect an L-band signal, go to the source: modem, LNB or Mux-Tee. Replace if necessary.
- If using an external reference LNB, check for the presence of a 10 MHz signal. (LNBs are usually labeled as “external reference” if an external 10 MHz signal is required). Again, ensure the signal does not attenuate more than is expected for the cable in use. A missing reference signal is usually the fault of the reference itself. Find the reference and replace if necessary.
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. If, however, everything above checks out, there might be a more nefarious issue – like an impedance mismatch.
Correct an impedance mismatch
For satellite terminals, the impedance of the modem, coaxial cable and LNB or BUC are normally all spec’d the same. 75 ohms for example. But what if there’s a mismatch – and the LNB you’re using is rated for 75 ohms when your modem is 50?
Coaxial cable impedance mismatches can be harmless, but sometimes they result in a lot of reflected power, return loss and insertion loss. 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.
Cables are usually marked with their impedance values. F type connectors are 75 ohms and N and SMA are 50 ohms. BNC connectors (which are available in both 75 and 50 ohm variants) are the tricky ones. This picture can help you differentiate between the two:
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. Matching devices are generally less expensive than transformers, but transformers do a much better job of limiting insertion loss.
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. 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). Orbital has a great option for this called a “Mux-Tee,” which offers unparalleled 50/75 ohm impedance matching.
To re-cap, if you are experiencing poor signal quality on either the transmit or receive path between a satellite modem and the antenna:
First: Inspect your coaxial cable and center pins on each connector (cable, modem and LNB/BUC) for damage
Second: Check your DC power and ensure the L-band and 10 MHz signals are within range for your cable type
Third: Check for and correct an impedance mismatch with a matching transformer