Fibers vs. Coax for RF Delay line Applications Er'el Granot (1,2) , Raviv Weber (1) , Shai Tzadok (1) , Guy Gertel (1) and Niv Narkiss (1) (1) Finisar-Israel, 3 Golda Meir St. Nez-Ziona, 74140, Israel (2) Department of Electrical and Electronics Engineering, Ariel University Center of Samaria, Ariel, Israel Abstract — A comparison between the RF-over-fiber and the RF-over-coax technologies is presented. It is shown that while in relatively short delay lines the coax technology will do, for delay lines longer than a certain distance the fiber technology has a clear advantages: Beyond several microseconds it is better in terms of volume, Noise Figure and cost. Index Terms — RF over fiber, Optical fibers, Radio over fibers, delay lines. I. INTRODUCTION Recently there has been a growing development of analog communication systems in the RF domain[1-3]. As the channel capacity and bit-rate increase the complexity and cost of the transfer line increase as well. Not only that these devices become extremely expensive and cumbersome, but for certain applications even simple parameters such as weight and volume become crucial factors, which may hinder any project feasibility. An example of great importance is Analog RF Radar, which consists up-to several hundreds of RF antennas in array. To transmit its data via coaxial cable would result in extremely heavy cables (80-600kg per km) and high attenuation (at least 450dB/km @10GHz)[2-3]. These two main deficiencies (weight and loss) can be overcome by replacing the electrical cable with optical ones. Optical fibers have extremely high transparency and the losses (~0.2dB/km) are negligible with comparison to coaxial cables, and obviously are extremely lighter (only 1.7kg/km)[2,3]. Therefore, usage of optical fibers instead of electrical ones allows transmitting the data to longer distances, and to integrate the devices in smaller and lighter packages. II. RF OVER FIBER AND COAXIAL CABLES In photonics microwave links the data is transmitted via optical carriers, which are modulated by the RF signal. At the transmitter side the RF signal is transcribed over the optical carrier, and therefore can be transmitted over long distances via optical fibers. At the detection end the optical data is re- converted to RF signal by optical detectors (see Fig.1). Clearly, the conversion from RF signal to optical one and eventually back to RF signal cost a considerable loss of power. Even in a back-to-back configuration, such a setup has a Noise Figure (NF) of ~30dB. It is therefore not a surprise that it took the industry few decades (from the invention of optical fibers) to adopt the RF over fiber technology. During the last decade several parameters have changed. On the one hand, the optical communication technology improved considerably (in terms of performances and low cost) and on the one hand the RF applications demands increased (in terms of spectral bandwidth, distances and number of elements). Fig. 1. Basic components of a fiber optic link: modulation device, optical fiber and photodetection device Fig. 2. A comparison between the volume of different coax cables and smf28 (fiber) for the same delay lines. The low absorption of optical fibers makes them advantageous solutions to many long distance data transport applications. One of the RF applications is a delay line. Such an element is required in many Antenna, Radar and RF systems. While it is indeed impractical of using fibers for a delay line of several RF modulator Laser Detector Optical fiber RF in RF out 10 -2 10 -1 10 0 10 1 10 2 10 3 10 -2 10 0 10 2 10 4 10 6 10 8 delay time (μ sec) Volume (cm 3 ) smf28 fiber RG174 RG142 RG214