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I need to have a long (100') stretch of cable from my antenna to a GSM box (remote control for heating via GPRS or SMS), and I need to minimize signal loss. In another (totally different) question someone told me to use a lower-loss cable, such as a RG6, instead of the standard RG58 that is normally used for telecom applications.

Some reading up told me that 75 ohm cables are usually used for longer stretches of cable when you need low signal loss, but that connecting any 75 ohm equipment (cable/connector) will lose you about 5% signal loss (in every such link) due to the "standing wave effect".

5% is of course a loss, but nothing compared to dropping attenuation considerably (like going from 13db to 6db/100'), so I was wondering about doing just that. Cost is not really an issue, so I just want to get the optimal cable.

Are there any other effects, but signal loss, that I need to consider? Can I break my equipment? Should I do something else, like an alternative low-loss 50ohm cable (RG213?)? Can I use some kind of "impedance converter" (tips!) to down-convert from 75 to 50?

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  • \$\begingroup\$ You have a break in impedance which will cause all kinds of funny electromagnetic wave effects, most commonly signal reflection. It may or may not make the whole thing unusable, not knowing anything about the hardware its hard to tell. Why not make everything the same impedance? or at least add some impedance converting circuitry? \$\endgroup\$ – PlasmaHH Dec 20 '17 at 12:50
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    \$\begingroup\$ Can you move the GSM box to the antenna and use, say, Ethernet to connect to the heating control? \$\endgroup\$ – Andrew Morton Dec 20 '17 at 13:17
  • \$\begingroup\$ @Plasma, Sure, but I am not intimately familiar with that, hence my question on what the best choice would be if the cable could cause other issues (such as those mentioned). Whatever would work and give minimal signal loss. \$\endgroup\$ – oligofren Dec 20 '17 at 13:17
  • \$\begingroup\$ @Andrew, thought about moving it, but then I might have other issues. The GSM bit is integrated into the remote control, btw. Anyway, I'd like to know my options before choosing, including optimal cable choices. \$\endgroup\$ – oligofren Dec 20 '17 at 13:20
  • \$\begingroup\$ Presumably the signal is very weak at the desired location for the controller - is it so weak that a high-gain antenna wouldn't work either? If you could use one of those, you wouldn't have the hassle of installing 100' of cable. \$\endgroup\$ – Andrew Morton Dec 20 '17 at 13:33
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Some notes on using mismatched RF transmission lines, as opposed to using a digital link for moving your data closer to the antenna.

As mentioned previously, using a transmission line with a characteristic impedance other than your radio's impedance and antenna impedance can cause signal reflections in your transmission line. This will cause standing waves, as you've read, and lead to power losses in your system as the reflected signals bounce back and forth before finally radiating from the antenna. Those losses are greater at higher frequencies.

The issue gets more complicated depending on your exact situation and requirements. There are many things you can do to match impedances between sources (for example, your GSM box), loads (your antenna), and transmission lines (the cable). With impedance matching networks you can use a transmission line with a different characteristic (75ohm) impedance than your source (50ohm).

For example, I recently installed an antenna system with 300ohm transmission line matched to a 50ohm radio for an amateur radio setup. However, that matching only works at a specific frequency and was manually tuned. Cellular signals might require more bandwidth than a matching network of a particular design can provide and performance would suffer outside of the frequencies that are matched well. (I don't have any practical experience with cellular systems, so I can't say much about them.)

Without measurements of the system as it is installed, it isn't possible to say exactly what you'll need to do, if anything, to make your 75ohm system work well. You might get lucky and it'll work on the first install, but likely not. To answer the headline question, it isn't likely to break your GSM box, but that really depends on the power levels it uses and the impedance mismatch of your installed system.

The device used to "easily" measure impedances at RF is a Vector Network Analyzer (or VNA), and they aren't cheap, with some exceptions, like the "mini VNA Tiny". They take some time to learn how to use. The use of a Smith chart greatly aids in designing matching networks. Some math and circuit theory is involved.

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Your best choice for long runs of coaxial between 50Ω systems is really to use impedance matched cable with low loss. No-one is forcing you to use RG58 just because you need 50Ω! There's other cable types. What you want depends on frequency, (power is probably not too much of an issue here), weather resistancy, and cost – all three things that you'd probably have to answer by consulting a electronics distributor. GSM/GPRS sadly doesn't say whether it's 900 or 1800 MHz, and there's enough cable types that work well for the first, but not as well for the second frequency range.

Of course, low-loss microwave 50Ω cable might simply be prohibitively expensive. In that case: better lose a few dB to impedance mismatch at the antenna (not sure how you'll connect your GPRS device) than lose more dB in the cable. Notice that if you know the frequency, you can get/build an impedance matcher that will not be lossless, but pretty low-loss. Additional cost, complexity, and source of errors ensue.

But Andrew is absolutely right: Go analog as shortly as possible, digital for the rest. So, place your RF equipment close to the antenna. I'd rather switch a small current at the antenna which is used to actuate a relay that switches the heating 30m away (why, btw, are you using feet? I'd assume Norway has the good, non-freedom units of measurement?).

By the way, you're going through great lengths to get GSM into a remote location, just to switch a heating.

Maybe that doesn't pay. I have positively no idea what a Iridium satellite pager IoT device costs. That might be a bit overkill, but would work as long as you're not bound to hide your antenna in a cave.

Another option might be to have your GSM/GPRS device close to the cell phone tower, and use another technology for the extremely low-rate communication with your home. That of course requires you can put up a GPRS device somewhere close to the tower, and add a transmitter for a different technology to it. I'm thinking about something like 433 MHz modules, which are heavily restricted in transmit power (much more severely than GSM phones!), but can be made to send long sequences at very low rates, which you could build a correlation receiver for at your house.

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  • \$\begingroup\$ True, but as an amateur it's the practical issues that put a stop to DIY solutions, like getting SMA connectors that fits the cable type. For RG58 you'll find lots of hits, whereas more "exotic" variants don't exist. Maybe it's a non-issue: you can make it work, but I wouldn't know from practical experience :-) \$\endgroup\$ – oligofren Dec 20 '17 at 14:10
  • \$\begingroup\$ Regarding finding other 50-ohm coax and connectors, I happen to work at a distributor that has LMR-400 and a compatible SMA connector in stock. It exists in the marketplace. \$\endgroup\$ – davidmneedham Dec 20 '17 at 14:17
  • \$\begingroup\$ There are plenty of adapters between different RF connectors as well. \$\endgroup\$ – remicles2 Dec 20 '17 at 14:21
  • \$\begingroup\$ There are USB->Cat5 connectors that are rated for 50 metres, and available for under €10. You could put a USB GSM adaptor on the end of it, and then run the Cat5 to control box. \$\endgroup\$ – CSM Dec 20 '17 at 15:59
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    \$\begingroup\$ uh, "patching" is what I'd expect not to work. The whole point of impedance matching is that the geometry and physical properties of the transmission line stay constant. So, proper coax connectors with the right impedance is your only option. \$\endgroup\$ – Marcus Müller Dec 20 '17 at 18:57

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