摘要:

This paper is concerned with the evaluation of errors on a wideaperture type of direction-finder. The errors considered are those due to the wave interference between two rays incident on the aerial system, such as may be produced in ionospheric propagation. Expressions are derived for the mean-square error taken over all phase relations of the down-coming rays, and also over a distribution in bearing of the smaller of the two rays. In all cases a comparison is made with the corresponding figures for an Adcock direction-finder.It is suggested that, for the d.f. system discussed, an aperture of four wavelengths would be sufficiently large to give a useful reduction in the component of error variance due to wave interference, over that obtained with an Adcock direction-finder.

DOI:10.1049/pi-3.1953.0053

出版商:IEE    

年代:1953

数据来源: IET

摘要:

Simultaneous bearing observations were made using a cathode-ray direction-finder of the U-Adcock type and a multi-aerial wide-aperture instrument (PV2) employing cyclical differential measurement of phase. The transmitters observed operated in the frequency band 8–12 Mc/s and were located at distances ranging from 750–1500 km. In the analysis of the data, where particular attention was paid to the rapidly varying component of the bearing changes, it was found that for the majority of the transmitters studied the spread of bearings was significantly greater with the Adcock than with the other direction-finder. The reasons for this difference are discussed and it is inferred that a large part of the superior performance of the PV2 was due to its relatively large aerial spacing. Although the Adcock instrument gave the better performance in the presence of interfering signals, a higher proportion of observations were lost with it owing to deep fades.

DOI:10.1049/pi-3.1953.0056

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The first part of the paper deals with some of the general problems involved in the design of a television transmitter, and the reasoning which has led to the adoption of a low-level modulation system.The remainder describes some of the particular features of two high-power low-level-modulation vision transmitters installed at the B.B.C.'s Kirk O'Shotts and Wenvoe television transmitting stations.

DOI:10.1049/pi-3.1953.0058

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The simple application of information theory to data transmission shows that when the proper scale-unit of an input signalX(t)has been established by the reference of all noise to the input, the smallest time-interval, Δt, in which one metron (i.e. a change of one scale-unit) must be transmitted is determined by the maximum possible rate of change ofX(t); this determines the required bandwidth Δf⋍ 1/Δt. Channel capacity is taken as the number of metrons that can be transmitted per unit bandwidth in unit time, and it is shown that, if provision is made for transmission of changes inX(t)at the maximum rate of one unit Δxin each basic interval of time Δt, there will be considerable waste of channel capacity, since, in general, the mean rate of change ofX(t)will be much smaller than the maximum rate. It is shown that, provided a small delay for coding and decoding and some loss of fine structure in the received signal can be accepted, some of the time wasted, whenX(t)is changing slowly, can be employed by integrating the changes inX(t)over a number of basic intervals Δt. The integrated change over the period τ =nΔtcan then be coded into binary form and transmitted during the following period while the current changes inX(t)are again being accumulated. In this way either the bandwidth can be reduced or the time saved can be used for transmission of other quantities,Y(t),Z(t), etc. It is shown that in this way the number of metrons transmitted can be increased by a factorMdependent onR, the ratio of the maximum rate of change to the mean rate of change without regard to sign, andn. There is some increase in the proper scale-unit, indicating some loss of accuracy, butMmay have values of the order of 4 with an increase of proper scale-unit of less than 50%.It is then shown that in some cases provision is being made for transmission of more binary digits than are usually necessary, and by limiting the total change inX(t)that can be transmitted in one period τ, a further saving can be effected with only a small increase in proper scale-unit whenRis of the order of 10 or more.

DOI:10.1049/pi-3.1953.0061

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The current distribution required for maximum directivity of an array with a finite number of elements and any specified geometrical configuration is shown to be completely defined by the self- and mutual resistances of the elements and by a certain component of the voltage (the “resistance voltage”) across the terminals of each element. This voltage component is required to vary from element to element in the same way as the instantaneous local values of a sinusoidal disturbance travelling across the array, in the direction under consideration, with the velocity of an electromagnetic wave.As a consequence, the maximum gain of the array is expressible either as a double sum containing only the mutual conductances between the individual elements multiplied by trigonometrical factors depending on their spacing, or as an expression identical (except for a numerical factor) with that for the distant field of the array.These theorems hold, slightly modified, for arrays of non-identical elements.The theory has been applied to the numerical calculation of certain simple arrays. It appears that, for arrays of a given size, directivities greater than those obtained by conventional design methods can be achieved without excessive losses.This has been substantially confirmed by an experimental array of four elements operating at 75 Mc/s. The theoretical gain was 10.1 db, while 8.7 db was measured. Of the discrepancy, 0.6 db was calculated to be due to losses in the feeder system and a further 0.2 db to losses in the dipoles. The bandwidth was about ±½ Mc/s for a drop in gain off ½ db. The degree of super-directivity is indicated by the fact that a physically identical array fed with equal-amplitude currents phased for maximum field strength in the end-fire direction would have a gain of 4.6 db.

DOI:10.1049/pi-3.1953.0062

出版商:IEE    

年代:1953

数据来源: IET

DOI:10.1049/pi-3.1953.0064

出版商:IEE    

年代:1953

数据来源: IET

DOI:10.1049/pi-3.1953.0065

出版商:IEE    

年代:1953

数据来源: IET

DOI:10.1049/pi-3.1953.0066

出版商:IEE    

年代:1953

数据来源: IET