how to measure cable loss

how to measure the path loss of a cable using a spectrum analyzer and a signal generator

thanks in advance

Connect signal generator directly to spectrum analzyer. Measure the amplitude in dBm.
Insert cable between generator and analyzer. Measure the amplitude in dBm.
Subtract the two measurements to get cable loss in dB.

The attenuation of the coax strongly depend of the measuring frequency. At the smaler frequency use relative long piece of the coax. Search with google "coax+parameter"

Before doing any measurement and subtraction, be sure that the impedance of the cable is equal to the input/output impedance of the Spectrum Analyzer / Signal Generator (usually 50 ohms). Otherwise the calculation would be an error.

thanks for the replies.

so here's a follow up to my problem

i have to measure the pathloss between a certain microstrip.at one end of the mistrostrip is a sma connector connected to the module (by default) while at the other end , i soldered a cut coax to the solder mask, so now i was able to measure the total loss of the setup. the problem is, how am i suppose to measure the loss of a coax(abt 2 inches long), with its other end cut ( thus exposing it's ground and main wire ) ?

btw, the reaL purpose of the setup is to measure the path loss ( through subraction ) of the microstip from the solder mask to the sma connector connected to the module by default ....

Using only a signal generator and a spectrum analyzer would be hard for your case to do an accurate measurement.
You need a Network Analyzer to be able to de-embed each segment.
Also don?t forget that is possible to have a Missmatch Loss between microstrip line and coax cable.

How to connect both the devices without using cable..and find the amplitude? i set the SG to 500 MHz and 0 dB and connected it to SA it gived some harmonics what this means to tell plz any body make me clear i am new for the system. i also want to check as the noise that obviously produces from some where in or out of the system so how to detect this?
i will be greatfull plz.
thanking in advance..

Depending on what frequency you are measuring, perhaps a significant concern is the impedance mismatch of your coax 'pigtail' connection. It is very difficult to make a pigtail connection without adding a few nanohenry of series inductance in the line.

One way to get good accuracy is make a test microstrip twice as long with SMA connectors on either end. Measure then divide by 2.

Also, if you are looking to measure a very low loss accurately you need to ensure your test equipment source and load have a very good match.

One way to achieve this is to fit good quality 10dB attenuators at the end of your test equipment cables before calibrating. Then insert the test item between the attenuators to get pretty good accuracy of measurement.
The match of a signal generator and a spectrum analyser might not be as good as you think. Also your test cables can make things worse so the 10dB attenuators can help a lot here.