By Marcus C. Walden

Abstract: This paper presents a comparison of Chilton ionosonde critical frequency measurements against vertical-incidence HF propagation predictions using ASAPS (Advanced Stand Alone Prediction System) and VOACAP (Voice of America Coverage Analysis Program). This analysis covers the time period from 1996 to 2010 (thereby covering solar cycle 23) and was carried out in the context of UK-centric near-vertical incidence skywave (NVIS) frequency predictions. Measured and predicted monthly median frequencies are compared, as are the upper and lower decile frequencies (10% and 90% respectively). The ASAPS basic MUF predictions generally agree with fxI (in lieu of fxF2) measurements, whereas those for VOACAP appear to be conservative for the Chilton ionosonde, particularly around solar maximum. Below ~4 MHz during winter nights around solar minimum, both ASAPS and VOACAP MUF predictions tend towards foF2, which is contrary to their underlying theory and requires further investigation. While VOACAP has greater errors at solar maximum, those for ASAPS increase at low or negative T-index values. Finally, VOACAP errors might be large when T-SSN exceeds ~15.

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By Marcus C. Walden

Abstract: Signal power measurements for a UK-based network of three beacon transmitters and five receiving stations operating on 5.290 MHz were taken over a 23 month period between May 2009 and March 2011, when solar flux levels were low. The median signal levels have been compared with monthly median signal level predictions generated using VOACAP (Voice of America Coverage Analysis Program) and ASAPS (Advanced Stand Alone Prediction System) HF prediction software with the emphasis on the near-vertical incidence sky wave (NVIS) links. Low RMS differences between measurements and predictions for September, October, November, and also March were observed.

However, during the spring and summer months (April to August), greater RMS differences were observed that were not well predicted by VOACAP and ASAPS and are attributed to sporadic E and, possibly, deviative absorption influences. Similarly, the measurements showed greater attenuation than was predicted for December, January, and February, consistent with the anomalously high absorption associated with the “winter anomaly.”

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By Marcus C. Walden

Abstract: The design of a 16-element waveguide array employing radiating T-junctions that operates in the Ku band is described.

Amplitude weighting results in low elevation sidelobe levels, while impedance matching provides a satisfactory VSWR, that are both achieved over a wide bandwidth (15.7-17.2 GHz). Simulation and measurement results, that agree very well, are presented. The design forms part of a 16 x 40 element waveguide array that achieves high gain and narrow beamwidths for use in an electronic-scanning radar system.

I. INTRODUCTION: Design equations for optimum horn antennas have long been established [1]. This concept has been employed in an electronic-scanning ground surveillance radar system for nominal elevation beamwidths of 10° and 20°. Unfortunately, for narrower beamwidths, the optimum horn becomes impractically long for a man-portable system (e.g. for a desired 5° beamwidth, the length is ~1.5 m).

Because the antenna forms part of a 40-element array that defines the azimuth beamwidth, a 16-element waveguide array with a corporatefeed structure was selected to achieve the desired 5° elevation beamwidth with a short physical length.

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By Marcus C. Walden

Abstract: A lightweight, wideband tapered-slot antenna that uses an antipodal Vivaldi design and provides useable gain from ~5 GHz to in excess of 50 GHz is described. Simulations and measurements are presented that show excellent agreement. This antenna design is currently deployed in handheld test equipment.

I. INTRODUCTION: Numerous designs exist for wideband (multi-octave) antennas that also have good directivity. However, the selection pool reduces if the antenna is to be employed within handheld test and/or monitoring equipment. For example, the relative bulk and weight of standard gain or double-ridged waveguide horns is undesirable, as is their cost.

Microstrip antennas are attractive because they are, by comparison, lightweight and cheap. While a patch array is simple, its feed structure is more complicated and incurs losses, particularly at higher microwave frequencies. For desired operation from below ~20 GHz to above ~40 GHz, a tapered-slot or Vivaldi antenna was considered suitable [1]. Furthermore, an antipodal Vivaldi design was selected because it offers a simple microstrip-coax interface and provides good gain over a wide bandwidth [2].

Inevitably, some engineering design trade-offs are required.

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By Marcus C. Walden

Abstract: This paper presents the results of a significant number of propagation measurements performed at 3.5 GHz in urban environments. Furthermore, the data collected is used to derive a statistical path loss model over the range 100 m to 2 km. This work offers valuable propagation measurements in a frequency range that is globally being allocated for broadband wireless systems.

Introduction- Over the period from December 2003 to June 2004, a large number of propagation surveys were conducted by Plextek and LCC UK on behalf of a client operating a radio network at 3.5 GHz. Measurements were performed in urban environments within major cities of the United Kingdom (London, Birmingham, Liverpool and Manchester amongst others). Although numerous path loss models are available (including the Hata [1] and COST 231 Walfisch-Ikegami [2] models) that describe propagation in urban, suburban or rural environments, they tend to be limited to the lower frequency bands (up to 2 GHz) and to large ranges (1–20 km) in the case of the Hata model. This provided the motivation to use the measured path loss data at 3.5 GHz to derive a log-normal shadow fading model appropriate for the actual measurement environment. This was found to agree very well with published work at other frequencies [3].

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