摘要:

Short-term measurements are described of signal amplitude, fading characteristics, correlation distances and height gain at three ranges (130, 258 and 398km) from a transmitting system radiating horizontally polarized waves. Although the results were variable and at times difficult to interpret, it is considered that 150λ should be an adequate horizontal spacing, even at the shortest ranges, for directional receiving aerials operated in diversity, with a tendency towards smaller values for vertical spacing. Fading rates lying generally between 1 and 10 per second are found, with Rayleigh-type fading tending to occur at the longer ranges. No consistent relationship between signal amplitude, fading and correlation distance is found, although the highest fading rates are often accompanied by low correlation distances. Cross-correlation measurements show definite evidence of the steady drifting of scattering centres. Diffraction effects leading to height gain are found, and there is some indication of standing waves due to ground reflections. Aerial siting is discussed and the transportable equipment which was developed for the tests is briefly described.

DOI:10.1049/pi-b-2.1961.0061

出版商:IEE    

年代:1961

数据来源: IET

摘要:

In h.f. broadcasting, frequencies and aerials are chosen to give the optimum signal strength to listeners within the desired service area. The varying nature of the ionospheric transmission medium makes it desirable to measure the distribution of energy actually achieved over that area.In the experiments described, pulse echo measurements of the back-scattered energy from ground irregularities in the service area were made, the echoes being observed on a range/amplitude display. In tests with two aerial systems, the echo patterns clearly showed the differing energy distribution. The patterns agreed well with those calculated for the two aerial systems from parabolic ionospheric-layer theory and the radar equation.Further tests at night under varying ionospheric conditions showed that the propagation modes to 5 000km range were correctly evaluated by back-scatter sounding at times when ionospheric predictions, corrected by local vertical-incidence soundings, gave erroneous results.

DOI:10.1049/pi-b-2.1961.0063

出版商:IEE    

年代:1961

数据来源: IET

DOI:10.1049/pi-b-2.1961.0066

出版商:IEE    

年代:1961

数据来源: IET

摘要:

The paper describes the construction of a Birnbaum-type microwave refractometer and its application in studies of refractive-index variations at heights up to 20000ft. Practical problems encountered in airborne measurements are discussed in detail. Some typical results are given which illustrate the performance obtained in measurements of refractive-index profiles and discontinuities.

DOI:10.1049/pi-b-2.1961.0067

出版商:IEE    

年代:1961

数据来源: IET

摘要:

The basic methods of aerial radiometric survey are described and the factors determining their sensitivity, precision and utilization are discussed. Particular reference is made to the problem of discriminating between radiations from the earth's surface and those from other sources. It is demonstrated that the technique is of value in uranium exploration and for surveying deposited radioactive contamination.

DOI:10.1049/pi-b-2.1961.0068

出版商:IEE    

年代:1961

数据来源: IET

摘要:

The problem of measuring continuously the thermal-neutron flux intensity in high-power nuclear reactors at temperatures up to 500°C considered, and it is concluded that the mean-current ionization chamber provides the best solution.The development of a suitable chamber is described, which is made from stainless steel and has a gas filling of xenon. When the electrodes are coated with uranium oxide (U3O8) containing 0.48 mg of uranium 235, the current sensitivity is 7 × 10−17A/n/cm2/sec, and at a neutron flux intensity of 1013n/cm2/sec the equilibrium residual current measured in a graphite-moderated reactor is 0.6% of the total output. Life tests have been carried out successfully to a total neutron dose of 3 × 1020n/cm2.At a flux intensity of 1014n/cm2/sec a chamber with uncoated titanium electrodes has the same fraction of residual current as one containing uranium 235 and is preferred as it is not subject to depletion effects.

DOI:10.1049/pi-b-2.1961.0069

出版商:IEE    

年代:1961

数据来源: IET

摘要:

A machine is described consisting of a universal non-linear filter, which is a highly adaptable analogue computer, together with a training device. The analogue machine has 18 input quantities from which it can compute in about 2 millisec 94 terms of a polynomial, each term containing products and powers of the input quantities, with adjustable coefficients, and can form their sum. The input quantities may be, for instance, 18 past samples of the values of a stochastic variable which is fed into the machine, and the result of the computation is an output function which contains 94 free variables. The training device optimizes the output by successive adjustment of the variable coefficients, until it has approached a target function as closely as can be achieved with a polynomial of 94 terms, by the criterion of the least mean-square error. This is done by repeatedly feeding into the machine a record of the stochastic process, long enough to be representative, and adjusting the variable coefficients, one at a time after each run, by a strategy which ensures that the error will monotonically decrease from run to run.In order to make the machine an optimum filter it is trained on a record of a noisy process, together with a target record which contains the signal only. It is taught as a predictor by taking as the target function a value of the stochastic process advanced by a certain time interval beyond the last value which goes into the input. It is trained as a simulator, for instance of an unknown mechanism, by feeding it with the input of the mechanism to be simulated at one end and presenting it at the other with its output as target function. The machine will then make itself into a model of the device to be simulated and the non-linear transfer function of the device can be read off from the final setting of the coefficients, as nearly as it can be represented by a 94-term polynomial. The machine is not confined to single-input systems.The machine incorporates 80 analogue multipliers of a novel ‘piezomagnetic’ type which, in its present form, can perform over 1000 multiplications per second with an error of 0.5% or less.A few examples of the first test applications of the machine are described.

DOI:10.1049/pi-b-2.1961.0070

出版商:IEE    

年代:1961

数据来源: IET

DOI:10.1049/pi-b-2.1961.0072

出版商:IEE    

年代:1961

数据来源: IET

摘要:

A general technique of self-adaptation is presented which adjusts one parameter of a control system to obtain the best response as determined by some built-in optimizing criterion. The method has been applied to a position-control servo mechanism in which the damping factor is automatically adjusted to give minimum mean squared error. The response of the control system is considered for repetitive step functions and sinusoidal and random signal inputs.The addition of the self-adjusting feature requires little knowledge of the controlled process dynamics or applied signal characteristics. The optimization mechanism compensates for variations in system parameters, non-linear effects and changes in the applied signal.It is shown that the introduction of a self-optimizing element effects a substantial improvement in system performance. Experimental results using an analogue computer are in good agreement with theoretical predictions.

DOI:10.1049/pi-b-2.1961.0073

出版商:IEE    

年代:1961

数据来源: IET

摘要:

Absorption of moisture causes paper to change its electrical resistance and permittivity. The second effect is approximately linear and little affected by changes in furnish. It is most conveniently measured by means of a fringing capacitor. Instruments for use statically and on a moving web are described. Theories of the water-cellulose bond are discussed.

DOI:10.1049/pi-b-2.1961.0074

出版商:IEE    

年代:1961

数据来源: IET