In the feature article ldquoA look at the concept of channel capacity from a maxwellian viewpointrdquo by Sarkar et al. , the authors compare Shann... Channel capacity - Voltage - Impedance - Quantization - Uncertainty - Receiving antennas - Noise figure - Signal to noise ratio - Silicon compounds - antennas - channel capacity - mobile communication - polarisation - radiofrequency interference - channel capacity - Maxwellian viewpoint - matching impedance - interference pattern - vertically-polarized antenna - mobile communication - Entropy - channel capacity - information rates - Maxwellian physics - Poynting theorem - near field - far field - communication systems - information transmission - wireless communication systems - land mobile radio cellular systems - wireless LAN - Hartley's law - Shannon channel capacity
Comment on "A Look at the Concept of Channel Capacity from a Maxwellian Viewpoint" Keywords: Entropy; channel capacity; information rates; Maxwellian physics; Poynting theorem; near field; far field; communication systems; information transmission; wireless communication systems; land mobile radio cellular systems; wireless LAN; Hartley's law; Shannon channel capacity
n the feature article "A Look at the Concept of Channel Capacity from a Maxwellian Viewpoint" by Sarkar et al. , the authors compare Shannon with Hartley-Nyquist-Tuller, but say that they are unsure (when the two formulas give different numerical values) which is the correct one to use. They should use the one that matches the physical situation. If the system is noise-limited, then use Shannon. If the system is limited by quantization, then use HNT. Their uncertainty may partly arise from the similar numbers they happened to obtain for their matched receiving antenna (pp. 30,31): I believe this was probably just a numerical coincidence. Their statements about the relationship between power and voltage depending on matching are true, but trivial. If they used the voltage across the resistive part of their matching impedance (2.77 mV) instead of the total voltage (i.e., use external matching), then their matched HNT figure becomes 15.0 B instead of 15.3 B. A severe mismatch from a very high input impedance would give them the full open-circuit voltage from the antenna (5.53 mV) and an HNT figure of 17.0 B. Their calculations do not take account of the effect of matching on noise; sometimes, a deliberate small mismatch can improve SIN ratio. They then find that modeling an antenna over a perfectly conducting plane ground gives rise to a strong interference pattern, hence height-dependent system gain and so height-dependent channel capacity. They also find that a vertically-polarized antenna
IEEE Antennas and Propagation Magazine, Vol. 51, No.1, February 2009
can work well close to the ground. From this they conclude that there are advantages in operating an antenna close to the ground. It would be much safer to conclude that if an antenna has to operate near the ground then one should choose vertical polarization. This is why short-range mobile communication systems normally use vertical polarization. Their results would have been quite different if they had used horizontal polarization, or an imperfect ground. By oversimplifying their model they are led to false conclusions, such as a denial of the well-known phenomenon of height-gain.
Reference 1. Tapan K. Sarkar, Santana Burintramart, Nuri Yilmazer, Yu Zhang, Arijit De, Magdalena Salazar-Palma, Miguel A. Lagunas, Eric L. Mokole, and Michael C. Wicks, "A Look at the Concept of Channel Capacity from a Maxwellian Viewpoint," IEEE Antennas and Propagation Magazine, 50, 3, June 2008, pp. 21-50. D. P. Kimber 83 Longsands Road St. Neots, Cambridgeshire PE191TWUK E-mail: [email protected]