Preprint/Draft: Submitted to 2011 IEEE Radio & Wireless Symposium, Phoenix, AZ 16-20 Jan., 2011 The Q-Track Corporation, © 2010 On the Origins of RF-Based Location Hans Gregory Schantz Q-Track Corporation, Huntsville, Alabama 35805, USA Abstract— This paper will provide a brief survey of the origins of RF-based location technology through the beginning of the Second World War. Direction finding (DF) was invented by John Stone Stone in 1902 and improved upon by Lee de Forest, Ettore Bellini and Alessandro Tosi. Both radar and amplitude ranging date to 1904, although these concepts were in advance of the ability of RF technology to implement. DF played a critical role in the First World War, most notably in the naval Battle of Jutland. The requirement for accurate night-time direction led classicist and cryptographer Frank Adcock to invent an improved DF system. In the 1920’s, DF and related concepts came of age for civilian applications like navigation. Inventors of the period introduced a variety of other techniques were introduced including time-of-flight or transponder ranging. By the time of the Second World War, DF was a mature field and additional novel RF-based technologies were ready to be developed. Keywords - Navigation, Position measurement, Radio position measurement. I. INTRODUCTION Communications may have been the first commercial application of wireless technology, but RF-based location was close behind. This paper will provide a brief survey of the origins of RF-based location technology. Fundamental techniques like Direction Finding (DF) and amplitude ranging date back over a hundred years to the early days of radio. DF in particular played a critical role in both World Wars, influencing the course of history. II. FALSE STARTS AND MISUNDERSTANDINGS In the first few years of radio, a variety of aggressive inventors recognized the problem of RF-based location and leapt to offer solutions. Some of their ideas illustrated the inventors’ misunderstanding of the behavior of radio waves. Inventors assumed (erroneously) that long wavelength RF signals would cast sharp shadows in an optical fashion. Isidor Kitsee and Charles E. Wilson, for instance, proposed a spherically end-loaded antenna with a shield to block signals from a particular direction (see Figure 1a). [ 1 ] Hermon W. Ladd similarly proposed a whip antenna with a rotatable shield. [2] In Ladd’s proposed system (shown in Figure 1b), a narrow slit in a rotating shield is supposed to allow the antenna to be illuminated only when the slit is aligned with the direction of incidence of the signal. Both these DF antennas fail to work, because the low frequency signals (typically <300kHz) they aimed to detect have wavelengths too long to be shadowed by such a small shield or to illuminate such a small slit. Figure 1a (left): Kitsee and Wilson’s US 651,014 (1900) direction finding antenna relied on shielding of a spherical capacitive end load to “shadow” signals. Figure 1b (right): Ladd’s US 733,910 (1903) direction finding antenna employed a rotating slit intended to allow the antenna to be illuminated only if the slit were aligned with a distant transmitter. The well known radio inventor, Lee de Forest (1873- 1961), fell prey to a similar confusion. De Forest assumed that the larger the antenna’s physical cross-sectional area, the more signal would be collected. This is not necessarily true even for an antenna comparable in dimension to a wavelength. For an electrically small antenna, actual and effective aperture are often two different things. Nevertheless, de Forest argued: “…a large screen will collect a larger amount of energy than a small one…. When such a screen is broadside onto the waves - that is, normal to their direction of travel - it will manifestly collect the largest possible amount of energy, while it is edge on, or in a plane coinciding with the direction of travel of the waves, it will collect the smallest amount of energy.”[3] Figure 2a shows de Forest’s 6ft by 15ft capacitive ―collecting screen.‖ [4] This antenna actually exhibits an omni-directional reception pattern for frequencies below 25MHz. Only at 30MHz where the antenna begins to be an appreciable fraction of the 10m (30ft) wavelength does the antenna pattern begin to deviate. Even there, the deviation enhances reception in the plane of the antenna, not broadside to it. Figure 2b shows a NEC simulation of the azimuthal pattern of de Forest’s antenna at 10MHz and 30MHz. De Forest’s claim to be able to detect a seven-mile-distant station to an accuracy of within ten degrees using this system seems unsupportable given the LF (<300kHz), long-wavelength nature of the transmit signal.