Here is my schema. I would like to connect a device using SMA connector with RG 174 to an attenuator of 30 dB with SMA connector. On the other side the attenuator is connected to a SMA-UHF converter to connect another device. This last device use normally PL 259 connector with RG-58 cable.

I think everything is good in this description, but I want to be sure. I forgot to specify that all the cables have a characteristic impedance of 50 Ohms.


RG-174 cable does not offer very good shielding. This can be a significant problem when you are attempting relatively high levels of attenuation as the cable leakage may provide more signal than the attenuator is allowing. Here is an illustration of coaxial cable leakage from Wikipedia:

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There will be losses in the "transport mode" that are due to the RF resistance of the conductors and losses in the dielectric material between the two conductors. This is generally expressed as dB/100 feet or dB/100 meters at a given frequency by the cable manufacturer. The radiating loss is much smaller by comparison and is generally not specified by the manufacturer. This needs to be determined experimentally. Cable TV companies, for example, learned long ago that at least two shields were needed in order to prevent signal leakage that could either be used to pirate cable TV service using a simple antenna pointed at the cable or to not run afoul of regulatory requirements.

Now consider your situation. You wish to attenuate a signal from a higher power source so as to reduce it to a level suitable for a sensitive receiver input. If the coaxial cable leaks only 1/1000 of the power that is traveling through the cable, for example, that is a -30 dB signal source that is radiating from the cable. If you now place a 30 dB attenuator at the end of the cable, the leaked signal and the transported signal are at the nearly same power level. Depending on many other factors, this may or may not affect your measurements.

A general rule for any well equipped RF lab is to use at least double shielded coaxial cables. As @ChrisK8NVH suggested, Times Microwave LMR-100 or LMR-195 would be good choices as they are both double shielded. The nice thing about Times Microwave is that they typically rate their shielding effectiveness - both cables are > 90 dB. These cables are also suitable for SMA connections.

When I am doing sensitive RF work, I will also include several ferrite toroid cores that the cable wraps around in order to reduce common mode current that can also upset sensitive experiments.

Can you get away with RG-174 cable for your experiments? Perhaps. But the results you observe may not be in line with your underlying assumptions.

  • $\begingroup$ I forgot to precise but i think the installation will measure about 20cm or 50cm max for a frequency of 160 MHz. Is it still a problem ? $\endgroup$ – fullcarton Feb 13 '19 at 8:06
  • $\begingroup$ I did calculation of the efficiency on this website timesmicrowave.com/calculator/… It is looking good. However I did not really understand the significant case with the attenuation $\endgroup$ – fullcarton Feb 13 '19 at 8:57
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    $\begingroup$ @fullcarton The calculator you referenced is for cable losses due to resistive losses. Here we are talking about cable losses due to radiation (leakage). $\endgroup$ – Glenn W9IQ Feb 14 '19 at 21:20
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    $\begingroup$ @fullcarton Can you be more specific regarding your application? $\endgroup$ – Glenn W9IQ Mar 5 '19 at 15:59
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    $\begingroup$ @fullcarton If this is an acceptable answer, then please mark the answer as "accepted' by clicking on the checkmark. $\endgroup$ – Glenn W9IQ Mar 7 '19 at 15:15

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