Transmission lines at microwave frequency in VSAT systems
Introduction – this discussion document serves to bring awareness to persons involved in transmission line infrastructure and to the effect on quality of performance. The 1-5GHz microwave frequencies used in many of today’s marine communication systems demand the need to properly measure performance and identify areas of potential fault. Advanced Marine Systems engineers are certified by a leading authority and are experienced in transmission line sweep procedures at microwave frequencies. A question that continually arises among installers is 50 or 75 ohms, this is addressed in a simplified way.
Cable insertion loss may be broken into 2 categories, “real” loss referred to as attenuation and reflection loss referred to as mismatch loss. Attenuation is directly attributed to the physical characteristics of the cable construction and the signal frequency. Reflection loss occurs as a result of mismatch within the transmission line system.
Attenuation is made up of 4 components, metal loss, dielectric loss tangent, dielectric conductivity loss, stray radiation. Due to the very low conductivity of dielectric and the relative low power levels in the intermediate frequencies of yacht systems we will ignore radiation loss and dielectric conductivity loss, both losses being negligible in a properly performing circuit. The significant losses are therefore metal loss (of the signal transmission along centre conductor) which is proportional to the square root of the signal frequency, and the dielectric loss tangent. Dielectric loss tangent is directly proportional to frequency and as such, in microwave transmission systems, becomes the predominant loss (as the metal loss is only proportional to the square root of the frequency. From here on we will consider attenuation as a single loss which is proportional to frequency.
Reflection loss occurs where a high frequency signal is reflected back to the transmitter source. Consider a transmission line system with ZERO attenuation; the reflection loss would be the difference in power input to the transmission line to the output power. Reflection loss occurs when the characteristic impedance of the system is not consistent, this is mismatch. Mismatch will result from incorrect component to component connection or damaged cable. Reflection and mismatch are discussed separately later.
Voltage Standing Wave Ratio (VSWR) is set up in a transmission line due to reflections in the transmission line caused by impedance mismatch. The reflection(s) give rise to a voltage standing wave which opposes and interferes with the forward carrier impacting the phase and peak signal level. Mismatch and thus signal reflections occur when the characteristic impedance of the transmission line is altered due to damaged coax cable, incorrect connectors and poorly fitted connectors. In detail, damaged cable may be as a result of exceeding the bend radius, exceeding the tensile stress limit, physical deforming the cable through crushing or pinching from through deck glands or cable ties, as well as the more obvious direct damage of piercing insulation with a screw or other foreign object. Incorrect connectors include terminating a cable with connectors not manufactured for that cable type or worse case scenario connectors of different impedance, for example terminating a 75ohm cable with 50ohm connectors. Poorly fitted connectors include terminating cable using unsuitable tools or procedures, ie wrong dies sizes for a crimp ring or multiple crimping which invariably alters the ratio of the outer conductor to the inner conductor of the cable and thus alters the impedance.
To better understand VSWR, consider a 100meter cable run of LMR600, a popular industry 50ohm microwave cable with velocity of propagation 87%. A 1MHz carrier frequency will have a wavelength of 261metres whereas a 1GHz carrier will have a wavelength of 26cm. in the case of the 1MHz carrier, less than half a wavelength appears in the 100m cable run with a slight variance of signal voltage and current which may be considered as direct current during this instance, whereas 383 wavelengths occur at the higher frequency of 1GHz.. at this higher frequency the forward signal changes very rapidly and field voltage and current are continually variable and reverse, similar to alternating current. It is at these microwave frequencies that pronounced interference to the forward carrier occurs from voltage standing waves set up as a result of reflections within the transmission line. The effect of the voltage standing wave on the incident wave is to alter its peak and phase. The peak voltage of the incident wave to the peak voltage of the reflected wave is referred to as the Voltage Standing Wave Ratio or VSWR. VSWR may alternatively be recalculated and referred to as return loss and is measured in decibels, dBRL.
50 ohms versus 75 ohms impedance
Peak power handling versus impedance occurs at 30ohms whereas lowest loss versus impedance occurs at 77ohms. The arithmetical mean is 53.5 ohms, the mathematical mean is 48ohms. A round off at 50ohms can be assumed. A more detailed explanation is available.
The inexpensive CATV cables produced for the industry are at a 75ohm standard. This standard is adopted due to the very low power levels that the CATV cables need carry combined with the lowest losses achievable. In the case of the previously discussed LMR600, with dielectric constant of 1.32, the lowest loss would be at an impedance of around 70ohm. In consideration that low loss cable have a small centre conductor when compared to that of similar 50ohm cable, a significant consideration is cost saving of material.
Transmission line testing, analysis & measurement
Time Domain Reflectrometry (TDR) vs Frequency Domain Reflectrometry
The high frequency transmission lines onboard a modern large yacht plays a significant part in efficient operation of today’s modern VSAT systems. Whereas it’s easy to swap out a satellite modem, BUC or LNB, changing faulty or failed cable can be a time consuming and very expensive operation. Properly equipped field engineers specializing in RF utilize sophisticated test and analysis equipment to troubleshoot and characterize the systems.
Formerly TDR equipment was used to analyze transmission line systems however this technology has been replaced by FDR . TDR uses a half wave DC pulse source with a rapid rise on the leading edge of the pulse. Insufficient energy is delivered into the device under test (DUT) to adequately characterize and analyze the system. FDR injects a swept frequency range at the nominal operating frequency of the DUT.
Return loss and VSWR measurements
Return loss and VSWR measurement are key to every line sweep specialist making field test of transmission lines. These tests will clearly show the match of the system and alert the field engineer as to whether it meets the design specification of the system. As previously discussed, a bad match will result in excessive reflections that will end up back in the transmitter and distort the signal in phase and amplitude. There is a direct correlation between amounts of reflected power and mean time between failures (MTBF) of transmitters. In general the whole system performance is degraded.
A better than 20dBRL measurement on a transmission line system is very efficient as only 1% of the power is considered returned, whilst 99% is delivered. This is easily obtainable on the transmission lines including jumpers and connectors on any quality installation. if the return loss is 10dBRL then 10% of the power is returned and the transmission line system would be considered as poor. As previously discussed, return loss may be converted directly to VSWR. Return loss measurements are preferred by line sweep specialist as it is easier to compare small and large numbers on a logarithmic scale. A return loss of 0dB is considered a full open or short circuit, whereas 60dB is considered a perfect match.