It's tricky to measure RF loss. This requires the use of a signal generator and a spectrum analyser. A signal of known strength at 1575 MHz (50 ohms input impedance) is injected into one end of the RF cable assembly, and the RF signal strength is measured at the other end of the RF cable using the spectrum analyser. The difference between the injected signal strength and the resultant signal level measured by the spectrum analyser is the net loss of the RF cable assembly. The OEM4 GPS receivers like to see a minimum input gain of +18 dB at their RF inputs. It's ok to err on the high side (greater than +18 dB), but less than +18 dB causes tracking problems. Example: With a GPS-512 sourcing +26 dB of gain, the RF cable assembly, including all splitters, attenuators, amplifiers, etc (if any), must not exceed a net loss of -8 dB. For more information on various RF cable scenarios, the following application note may prove useful: http://www.novatel.ca/Documents/Bulletins/apn003.pdf One other minor factoid: The RF cable run should be as straight (direct) as possible -- if there is excess RF cable, and this cable is neatly coiled, it will act as a stray RF receiver/antenna by accident, which could swamp the GPS signal if there are other RF generating sources nearby.