Satellite Signal Analysis: What if you have no loop-out port?

A number of people had expressed interest in the past in examining satellite signals from broadcast satellites using SDRs. While my normal advice is to use a regular satellite card to power the LNB and do the DiSEqC selection duties and take the signal from the loop-out port. This has been quite a handy trick, as the loop-out port is normally DC-blocked and buffered so as not to cause impedance problems or consume any RF signal. Unfortunately, it seems that most cards nowadays do not have a loop-out port.

So if you want to analyze satellite signals, what do you do?

The Hard Way

The hard way would be to build your own LNB power injector or inserter or purchase one. These units usually have either fixed or switchable voltage output for the LNB to change polarity and some even have 22khz tone injection switches to change the band of “universal” Ku LNBs. They generally don’t have provisions for DiSEqC injection, so you’re likely going to need an inserter per-LNB. The output is normally DC-blocked, with only the RF which you can use with an SDR or spectrum analyzer. This would give you the whole output dedicated to your equipment.

To design your own with good isolation and a pass-band of 950Mhz through to 2150Mhz without significant impedance mismatches is not a trivial task, so I’d advise against it unless you are adventurous or have the equipment and expertise to do it. While you can purchase a commercially-available solution, they’re not cheap depending on where you are. The lack of DiSEqC may make cabling a little more messy too.

The Easier Way

The easier way would be to split the signal from the LNB before it gets to the single port on the satellite card. To do this properly requires a bit of care – you can’t just wire things in parallel otherwise you’re going to have horrendous impedance mismatch!

Splitters can be found on eBay fairly cheaply, but not just any splitter will do! Many of the splitters are intended for terrestrial use and do not cover the satellite band of 950Mhz to 2150Mhz, resulting in high attenuation. Another issue is that many of the splitters are all ports power pass, on the premise that “more is better”. The downside is that many of these splitters with this feature expose the LNB power to the split ports which may fry your RF equipment if it doesn’t expect to see DC bias of up to 18V + a relatively strong 0.65V 22khz tone signal.

As a result, you’re best to look for a two port splitter with DC blocking on one leg. These too, have become somewhat rare. Otherwise, you could use an all-ports-power-pass splitter with a DC block for safety, but these aren’t that widely available here.

Luckily for me, I found the Matchmaster 07MM-GM02. This is exactly the type of splitter I describe – suitable for terrestrial and satellite with just one-port power pass.

They’re a fairly respected brand here, although the part is Made in China, possibly from the same place as many others.

The unit comes bagged, with ports covered, mounting screws and a grounding screw.

While the unit seems to be soldered shut, it’s not made for outdoor installation. The batch number suggests that it was made in May 2017.

F-connector ports, as usual, and a QC label round out everything about the splitter. The downside of using this technique is that the splitter’s insertion loss is up to 5dB on each leg, so the output signal will be reduced. However, this is probably not enough to prevent locking or decoding of the signal as many LNBs can push out -40 to -50dBm of signal and cards are happy even down to -75dBm at times.

One thing to watch out for is that the order of the ports matter – the power pass port must be connected to the satellite card otherwise the LNB would be unpowered (and the splitter could be damaged). Another thing to watch out for is that if you don’t use the other port, and don’t want to disconnect the splitter, you must terminate the port with a 75 ohm terminator or risk having impedance mismatches cause standing wave effects.

There is also another caveat – TV reception equipment is generally made for 75 ohm characteristic impedance, whereas amateur radio, spectrum analyzers and SDRs have a 50 ohm characteristic impedance. The mismatch is not particularly problematic in the case of signal reception and shouldn’t cause any damage, but it makes getting absolute figures a little difficult.

Unfortunately, while there are some in-line amplifiers, they seem to make things worse with a non-adjustable (and often excessive) level of gain which can cause overdriving and intermodulation which degrades the signal to noise ratio rather than improving it.

Conclusion

No loop out port? No problems, as long as you can find either an LNB power inserter with the features you need or an F-connector splitter with all-ports-power-pass and a DC-block or an F-connector splitter that only has power pass on one leg. Each of these options has their own advantages and disadvantages, but it means that not having a loop-out-port on a satellite card isn’t such a big pain after all.

About lui_gough

I'm a bit of a nut for electronics, computing, photography, radio, satellite and other technical hobbies. Click for more about me!
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