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?
Answer
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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