Analysis and Prediction of Sky-Wave Field Intensities in the High Frequency Band

The focusing gain parameters associated with radio waves reflected obliquely from the ionosphere as derived theoretically are compared with experimental observations. The description of the field in the neighborhood of the caustic depends on the shape of the ionosphere with respect to the earth. For an ionosphere concentric with the earth and a surface location transmitter it is convenient to describe the focusing gain in terms of the evaluation angle of arrival. For a tilted ionosphere, or for an elevated transmitter in which the caustic is elevated above the earth's surface, it is convenient to describe the field as a function of the distance behind the cusp, and above the caustic surface. Experimental observations indicate both situations may exist in nature. The concentric ionosphere is termed the normal situation, and a gain inversely proportional to the approximate cube of the elevation angle was indicated by the experimental data as opposed to a straight inverse relationship by geometric optics. The effect of a distorted ionosphere was evidenced by strong focusing gain at positive elevation angles. It is hypothesized that an increase in curvature of the ionosphere can result in the observed gains which would be consistent with the theory. The required curvature has been observed on other experimental programs.

Section: Resultsmentioning

confidence: 99%

Horizon Focusing Effects Observed on Ionospherically Reflected Radio Waves

Humphrey¹

1968

“…The VOACAP predictions are semiempirical in that the excess system loss is taken from the predicted signal power based on commonly applied ray path calculations. The excess system loss table which provides the long-term fade values evolved in the early 1960s based on measurements of 43 years of mass plot data [Laitinen and Haydon, 1962]. The circuit measuremeres were reduced to monthly median values for a given hour and the upper and lower observed decile values.…”

Section: Paper Number 97rs00843mentioning

confidence: 99%

Required signal‐to‐interference ratios for shortwave broadcasting

Lane¹

1997

Abstract. The required signal-to-Imerference (RSI) ratio for a specified grade of HF radio service is the hourly median warned signal power at the input of the receiver needed relative to the sum of the hourly median unwanted signal power and the hourly median radio noise power in the RF bandwidth of the receiver, adjusted so that the hourly median ratio will not fall below the RSI ratio more than a certain percentage of the time due to minute-to-minute fading within the hour. Shortwave listeners are well aware of the deleterious effects of cochannel and adjacem channel imerference. This type of interference is especially prevalent in the overcrowded international broadcast bands where it is manifested by cross talk and a beat note produced in the receiver by the carrier of the unwanted signal. Yet little agreemere exists as to the magnitude of the amplitudemodulated, double sideband (AM-DSB) interfering signal that can be tolerated by the listener. Numerous protection ratios have been proposed in the literature, as well as by elemems of the International Telecommunication Union. These values tend to range from 17 dB [International Frequency Registration Board, 1989] to as high as 50 dB for "good commercial quality, " offset in carrier frequency of 500 Hz and 10 dB short-term fade protection [CCIR, 1970]. In this paper, several significam experiments are reviewed for the purpose of normalizing their findings to a common set of parameters. The parameters relate to articulation scoring, type of noise (if used), fading of warned and unwanted signals, type of interference, listener skill, bandwidth of the receiver, carrier frequency offset, etc. From this compilation of normalized data, RSI values are recommended as they relate to the desired broadcast quality and the signal-to-noise ratio of the warned signal. The RSI ratios are compatible for use in HF sky wave prediction programs that comain appropriate RF noise and imerference combining subroutines. The recommended minimum acceptable for cochannel imerference is 36 dB, in order to maimain a fair grade of service where voice reception is understandable but with difficulty for at least 90 % of the hour. In order to provide good service ( i.e., the interference is perceived not to be annoying) to 90% of the listeners, the RSI should be 48 dB. The RSI needed for "no perceptible" interference is 53 dB.

“…(b) No allowance was made for horizontal gradients, i.e., for changes in the electron-density distribution along the great-circle path from transmitter to receiver (although it could easily have been included). (c) D-layer absorption was accounted for by use of the skywave absorption nomogram [Laitinen and Haydon, 1956]. (d) Isotropic antennas were assumed for both transmitting and receiving.…”

Section: Fundamental Assumptionsmentioning

confidence: 99%

Computer simulation of HF frequency‐selective fading and performance of the mode‐averaging diversity combiner

Kawachika¹,

Villard²

1973

A ray-tracing technique for simulating the performance of the mode-averaging diversity combinet (MADC) [Villard et al., 1972] is described, which extends the methods developed by Croft [1969] to the analysis of frequency-selective fading. It is shown that the computer procedure satisfactorily predicts observations made during a test representative of conditions encountered in practice. Using the parameters of this test, the method is applied to the original MADC configuration to investigate the dependence of performance on antenna spacing and ionospheric profile. Two MADC schemes which incorporate alternative methods of forming the running average are described. Their performance is also evaluated for the above parameters and shown to be roughly equal to that of the original. It is concluded that for the given parameters at least, a choice between the three alternative configurations can be made on the basis of convenience. INTRODUCTIONOne purpose of this paper is to describe a method for computer simulation of frequency-selective fading in HF signal propagation, based on realistic models of the ionosphere (electron density versus altitude). A further purpose is to apply the method in simulating and studying the action of a mode-averaging diversity combiner capable of reducing minima and nulls in the transmission passband caused by transmission-mode interference [Viilard et al., 1972]. These nulls are of practical importance because they affect the quality of complex signals, such as speech, having frequency components of which the duration is long compared with the differential time delay of multipath modes. EFFECT OF THE IONOSPHERE ON FADINGThe electron-density profile of the ionosphere determines the path or paths whereby a transmitted HF signal reaches its destination and thus determines the occurrence and severity of frequency-selective fading. For example, if the transmitted signal components received via the two paths in Figure 1 happen to be comparable in amplitude, then at fre-Copyright ¸ 1973 by the American Geophysical Union.quencies for which the difference in path lengths is equal to an odd number of half wavelengths, the components will almost completely cancel. In most cases the transmitted signals will reach their destination via two or more modes or hops. Possible angles of arrival of signals transmitted over a 1000-km path are indicated in Figure 2. In deriving this figure, a typical operating frequency and ionospheric profile have been assumed. Path lengths in general will differ, and frequency selective interference will result. An advantage of a computer simulation of fading is that a representative assortment of ionospheric conditions can be explored. In the study described here, the six possible models of the ionosphere shown in Figure 3 were used. Each model consists of a profile of electron density versus altitude up to 300 km or more. The profiles were those of Croit [1965, 1966] and are derived from rocket measurements. BASIS FOR COMPUTER SIMULATIONSThe ground range from transmitter...