Could you describe your source ? Is it CW or pulse ? What is your frequency ?

Why do you use a spectrum analyser? What do you expect to see with this analyser ?

what types of coaxial cable? If the cable has a frequency limit up to 500MHz and you pump the signal of 1000MHz, off course an attenuation will occur

Information is missing as it was already mentioned, range of frequency, type of coaxial cable, termination load. At first impression, it looks like there was a matching problem due to one of the three elements involved in the experiment. 

It seems your channel (coaxial cable) is not homogeneous media, in real sense, at frequency of operation. The coaxiallity of centre conductor is doubtful or dielectric material thickness or density has a variation in spatial direction. This creates lot of current leakage from between centre and outer conductor over a spatial direction which can reduce the current

If you check the spec of the cable, if the cable is given the spec like this:-

Attenuation  100MHz     3dB / 10m  that meant 0.3dB loss at 1 m

If attenuation 1000MHz  30dB /10m that means 3 dB loss at the end of 1 m.

Dear responders

Please take note of the following with respect to all answers

 Cut of  frequency of the line is above 3GHz

Characteristic impedance of line is 46 to 50ohms

Attenuation of the line should not exceed 0.1dB/m in measurement frequency range

Ideally no current is expected on the outer surface of outer conductor of coaxial line

Current probe and spectrum analyzer is used in absence of differential voltage probe and RF volt meter.

It is obvious that there is mismatch for the current on outer surface of the line and standing wave is formed. Answer is expected for this issue.

You are driving a common-mode current onto the outer-surface of the coax. There are several ways this could be happening. It is likely that your source does not have a metal enclosure (allowing fields to couple to the cable) OR that your source-end connector does not have a 360 degree shield connection (allowing return currents flowing on the pigtail to develop a voltage between the cable shield and the enclosure).

For measuring current amplitude in the cable we should use Vector Voltmeter. Channel A as reference is placed near the input and channel B move along the cable. If the current attenuate that means your frequency of signal that you pump above the limit. If the current fluctuate that means the cable is not fully matched. The standing wave occurred.

You are absolutely correct Prof.Hassan. But I am trying to replace vector voltmeter by current probe and spectrum analyzer. Issue is measurement of current on outer surface of outer conductor of coaxial transmission line of 50 ohms Zo. Line is matched at input and output terminals. I am absolutely in dark in respect of source and load impedance for the targeted current.

may be the cable acting like antenna. It radiates as it goes along. Some current remains

Most likely. I am also thinking in that line

I have another theory. Transmission line is 50 ohm and your terminating load is also 50 ohm, so it likes a power divider. So what you measured is actually the voltage which is half of the input voltage. Is it right?

apart from that the attenuation of the cable, slightly matched and radiation make it lower

           Your suggestions are well noted. Will look into that.

First, there should be no measurable current on the outside of the outer conductor attributable to your source if the connectors have been installed properly; both on the cable itself and on the measurement instrument.

Assuming the cable assembly is in proper working order, any measured current may be from the external environment. As others have mentioned, without further information it's impossible to speculate further on the source, frequency range, or strength of the external signal(s) that could be inducing a current on the outside of the shield and measured by the current probe.

The apparent variation in current readings is typical on most "real" transmission lines since a perfect match is not achievable. And since there is a possibility your measurement is actually external signals of unknown frequency, that coax shield is not resonant. There will be some standing wave on the outside of the of the shield; I'm guessing that is what your probe is measuring. The source, frequency range and amplitude all unknown - a product of the external environment you are in.

For a very practical explanation of transmission lines, look at the ARRL Antenna Book, any edition will due. www.arrl.org. There is probably a copy in a nearby library.

-Mike

I would like to emphasize again that the theory I used is the maximum power transfer. when the impedance of termination is equal to the impedance of the line. So the maximum power transfer to the load is a quarter of the input or equivalent to half of the input voltage.  The voltage at the output should be half. However because the attenuation in the cable, slightly mismatched between load and the line and some radiation , the voltage at output is further reduced.