Think of the signal flow as liquid traveling through a hose. You want as smooth a “flow” as possible. The plot on the left is good to 26.5 GHz, 1.40:1 VSWR, the one on the right is very bad almost 3.5:1 @ 3 GHz
Phase Matching and Phase Stability A signal travelling through a medium will have a particular phase length in degrees (one wavelength adds 360° to the phase). Phase length is determined by the transmission medium (cable type) and the mechanical length. Cables that are made from the same material and matched in phase will also be mechanically matched. Matching can be undertaken to a particular value, or in relation to other cables— important in cases where signals need to arrive at a given point at the same time (i.e. antenna arrays). The phase length of an assembly can vary with flexure and temperature, which needs to be compensated for. Different materials and cable constructions can help reduce the problem, often the goal is to eliminate the “Teflon Knee”. The mechanical properties of PTFE affect electrical length (phase) dramatically in the 18° – 25° C range, causing the phase shift described above. A plot of this change resembles a knee. Glenair 962-011 cable greatly reduces this “knee” from 1300–1500 (ppm) to below 200 (ppm). Available in -402 and -200 sizes, with FEP or Tefzel jacketing.
Back reflections in a transmission line also impact electrical performance and are the result of discontinuity in the RF line caused by changes of impedance. This can be a result of changing conductor and dielectric diameters, interface dimensions, or gaps between parts. Reflections are generally measured in two ways: Return Loss —this is expressed in dB and indicates the ratio of transmitted to reflected power. Ideal return loss is a high number (infinity = no reflection). An RL of 3dB means 50% of the power is reflected, 6dB means 25% reflected. An RL of 0dB would be caused by a short or open circuit (100% reflected) VSWR —Voltage Standing Wave Ratio. Two waves will propagate along a mismatched line -- one travels forward, while the other is reflected. Both waves have the same frequency, and the reflected voltage is added to the transmitted voltage. This causes a standing wave. The size of the reflected wave to the transmitted wave (max to min) is the ratio. An ideal line gives a VSWR of 1 (no reflection). More reflections equal a higher ratio (e.g. 1.2:1). dBi —Decibel isotropic is the logarithm of the ratio of power emitted by the antenna, divided by the power of an isotropic radiator. Power Handling Larger cables can handle more power than smaller types (when using the same materials). Changing dielectric and jacket materials can help improve power handling, but this can have an impact on insertion loss. Trapped air pockets within cables or other components are of concern in power transmission on satellites. Power handling needs to be de-rated for frequency, ambient temperature, pressure, and reflections (VSWR/Return Loss). The Velocity of Propagation or Vp of the cable is represented as a percentage of the speed of light. Extruded PTFE (Teflon) cables have a Vp of 69.5%. Taped PTFE and ePTFE have more air and have much higher velocities (80% & higher)
1500
1000
The Dread “ Teflon Knee ”
500
0
-80°
-60°
-40°
-20°
-0°
20° temperature
40°
60°
80°
100°
120°
Standard PTFE
Glenair 962-011 Low Phase Change cable
The graph above plots the performance of standard PTFE coax cable (the red line) vs. Glenair 962-011 Low Phase Change Cable (the blue line). Note the “Teflon Knee” phase shift.
QwikConnect • January 2023
6
Powered by FlippingBook