Log-periodic antenna

A log-periodic antenna known as a log-periodic array or log-periodic aerial, is a multi-element, directional antenna designed to operate over a wide band of frequencies. It was invented by John Dunlavy in 1952; the most common form of log-periodic antenna is the log-periodic dipole array or LPDA, The LPDA consists of a number of half-wave dipole driven elements of increasing length, each consisting of a pair of metal rods. The dipoles are mounted close together in a line, connected in parallel to the feedline with alternating phase. Electrically, it simulates a series of two or three-element Yagi antennas connected together, each set tuned to a different frequency. LPDA antennas look somewhat similar to Yagi antennas, in that they both consist of dipole rod elements mounted in a line along a support boom, but they work in different ways. Adding elements to a Yagi increases its directionality, or gain, while adding elements to a LPDA increases its frequency response, or bandwidth. One large application for LPDAs is in rooftop terrestrial television antennas, since they must have large bandwidth to cover the wide television bands of 54–88 and 174–216 MHz in the VHF and 470–890 MHz in the UHF while having high gain for adequate fringe reception.

One used design for television reception combined a Yagi for UHF reception in front of a larger LPDA for VHF. The LPDA consists of a series of half wave dipole "elements" each consisting of a pair of metal rods, positioned along a support boom lying along the antenna axis; the elements are spaced at intervals following a logarithmic function of the frequency, known as d or sigma. The successive elements decrease in length along the boom; the relationship between the lengths is a function known as tau. Sigma and tau are the key design elements of the LPDA design; the radiation pattern of the antenna is unidirectional, with the main lobe along the axis of the boom, off the end with the shortest elements. Each dipole element is resonant at a wavelength equal to twice its length; the bandwidth of the antenna, the frequency range over which it has maximum gain, is between the resonant frequencies of the longest and shortest element. Every element in the LPDA antenna is a driven element, that is, connected electrically to the feedline.

A parallel wire transmission line runs along the central boom, each successive element is connected in opposite phase to it. The feedline can be seen zig-zagging across the support boom holding the elements. Another common construction method is to use two parallel central support booms that acts as the transmission line, mounting the dipoles on the alternate booms. Other forms of the log-periodic design replace the dipoles with the transmission line itself, forming the log-periodic zig-zag antenna. Many other forms using the transmission wire as the active element exist; the Yagi and the LPDA designs look similar at first glance, as they both consist of a number of dipole elements mounted along a support boom. The Yagi, has only a single driven element connected to the transmission line the second one from the back of the array, the remaining elements are parasitic; the Yagi antenna differs from the LPDA in having a narrow bandwidth. In general terms, at any given frequency the log-periodic design operates somewhat similar to a three-element Yagi antenna.

However, the system is somewhat more complex than that, all the elements contribute to some degree, so the gain for any given frequency is higher than a Yagi of the same dimensions as any one section of the log-periodic. However, a Yagi with the same number of elements as a log-periodic would have far higher gain, as all of those elements are improving the gain of a single driven element. In its use as a television antenna, it was common to combine a log-periodic design for VHF with a Yagi for UHF, with both halves being equal in size; this resulted in much higher gain for UHF on the order of 10 to 14 dB on the Yagi side and 6.5 dB for the log-periodic. But this extra gain was needed anyway, it should be noted that the log-periodic shape, according to the IEEE definition, does not align with broadband property for antennas. The broadband property of log-periodic antennas comes from its self-similarity. A planar log-periodic antenna can be made self-complementary, such as logarithmic spiral antennas or the log-periodic toothed design.

Y. Mushiake found, for what he termed "the simplest self-complementary planar antenna," a driving point impedance of η0/2=188.4 Ω at frequencies well within its bandwidth limits. The log periodic antenna was invented by Dwight E. Isbell, Raymond DuHamel and variants by Paul Mayes; the University of Illinois at Urbana–Champaign had patented the Isbell and Mayes-Carrel antennas and licensed the design as a package to JFD Electronics in New York. Channel Master and Blonder Tongue Labs ignored the patents and produced a wide range of antennas based on this design. Lawsuits regarding the antenna patent which the UI Foundation lost, evolved into the 1971 Blonder-Tongue Doctrine; this precedent governs patent litigation. The log periodic is used as a transmitting antenna in high power shortwave broadcasting stations because its broad bandwidth allows a single antenna to transmi

London Labour and the London Poor

London Labour and the London Poor is a work of Victorian journalism by Henry Mayhew. In the 1840s he observed and described the state of working people in London for a series of articles in a newspaper, the Morning Chronicle, that were compiled into book form. Mayhew went into deep pedantic detail concerning the trades, habits and domestic arrangements of the thousands of people working the streets of the city. Much of the material comprises detailed interviews in which people candidly describe their lives and work. For instance, Jack Black talks about his job as "rat and mole destroyer to Her Majesty", remaining in good humour despite his experience of a succession of near-fatal infections from bites. Beyond this anecdotal material, Mayhew's articles are notable for attempting to justify numerical estimates with other information, such as census data and police statistics, thus if the assertion is made that 8,000 of a particular type of trader operate in the streets, Mayhew compares this to the total number of miles of street in the city, with an estimate of how many traders operate per mile.

The articles were collected and published in three volumes in 1851. A fourth "Extra Volume", published in 1861, was co-written with Bracebridge Hemyng, John Binny, Andrew Halliday, covered the lives of streetwalkers and beggars, though it departed from the interview format to take a more general and statistical approach to its subject. London in the 1840s was more like a 21st-century Third World megalopolis than a typical 19th-century city. A significant portion of the population had no fixed place of work, indeed, many had no fixed abode. In classic fashion, the city teemed with outsiders and migrants from other parts of Britain, with the British Empire's continued growth, people from all over the world began arriving in the city as well to seek their fortune. Items of commerce, such as food, drink and household goods, were distributed by an army of carts and wagons. While goods were sold from storefronts, there were thousands upon thousands of street-traders lumped together as costermongers.

Alongside these familiar forms of trade in consumer goods and services, Mayhew's work describes lesser-known trades driven by now-obsolete markets and by sheer poverty, such as gathering of snails for food, the extreme forms of recycling practised by pure finders, the mudlarks and'toshers'. Mayhew's perception as an observer is unsurpassed in early descriptions of London's street scenes, his richly detailed descriptions are able to give an impression of what the street markets of his day were like. Here is a typical description by Mayhew:'The pavement and the road are crowded with purchasers and street-sellers; the housewife in her thick shawl, with the market-basket on her arm, walks on, stopping now to look at the stall of caps, now to cheapen a bunch of greens. Little boys, holding three or four onions in their hand, creep between the people, wriggling their way through every interstice, asking for custom in whining tones, as if seeking charity; the tumult of the thousand different cries of the eager dealers, all shouting at the top of their voices, at one and the same time, is bewildering.

“So-old again,” roars one. “Chestnuts all‘ot, a penny a score,” bawls another. “An ‘aypenny a skin, blacking,” squeaks a boy. “Buy, buy, buy-- bu-u-uy!” Cries the butcher. “Half-quire of paper for a penny,” bellows the street stationer. “An ‘aypenny a lot ing-uns.” “Twopence a pound grapes.” “Three a penny Yarmouth bloaters.” “Who‘ll buy a bonnet for fourpence?” “Pick ‘em out cheap here! three pair for a halfpenny, bootlaces.” “Now‘s your time! Beautiful whelks, a penny a lot.” “Here‘s ha‘p‘orths,” shouts the perambulating confectioner. “Come and look at ‘em! here‘s toasters!” Bellows one with a Yarmouth bloater stuck on a toasting-fork. “Penny a lot, fine russets,” calls the apple woman: and so the Babel goes on.' The articles comprising London Labour and the London Poor were collected into three volumes in 1851. The 1861 edition included a fourth volume, co-written with Bracebridge Hemyng, John Binny and Andrew Halliday, on the lives of prostitutes and beggars, he wrote in volume one:'I shall consider the whole of the metropolitan poor under three separate phases, according as they will work, they can't work, they won't work'.

Mayhew interviewed everyone — beggars, street-entertainers, market traders, labourers, sweatshop workers down to the "mudlarks" who searched the stinking mud on the banks of the River Thames for wood, metal and coal from passing ships, the "pure-finders" who gathered dog faeces to sell to tanners. He described their clothes and where they lived, their entertainments and customs, made detailed estimates of the numbers and incomes of those practicing each trade; the books make fascinating reading, showing how marginal and precarious many people's lives were, in what, at that time, must have been the richest city in the world. Poet Philip Larkin used an extract from London Labour and the London Poor as the epigraph for his poem "Deceptions"; the extract details a rape: "Of course I was drugged, so I did not regain consciousness until the next morning. I was horrified to discover that I had been ruined, for some days I was inconsolable, cried like a child to be killed or sent back to my aunt." Writer Ben Gwalchmai was inspired to write

Myrceugenia leptospermoides

Myrceugenia leptospermoides is a species of small evergreen tree or large shrub in the genus Myrceugenia of the family Myrtaceae. It is known as mocollo, murtilla del malo or chequen, it is endemic to central Chile where it is found in riverine habitats in the coastal mountain range at altitudes below 300 metres. Myrceugenia leptospermoides grows to a height of about 3 m; the bark is pale greyish-brown and the small leaves are in opposite pairs. The twigs are densely pubescent when they soon lose their hairs; the leaves are up to 15 mm long and 3 mm broad, oblong or linear with bluntly-pointed tips, with entire margins. They are greyish-green above and yellowish-green below, with a prominent midrib on the underside; the flowers, which grow in the axils of the leaves, have short stems. The calyx lobes are sometimes hairy and the six petals are white. In the centre of the flower there is a boss of a single style; the fruit is a globular berry, ripening to red and purple, about 5 mm in diameter. The flowering period is the fruits ripen in July and August.

Myrceugenia leptospermoides is endemic to the coastal area of Chile. It is found from Ñuble Province southwards to Cautín Province, its typical habitat is in wet or misty locations and it is found growing with other undergrowth shrubs, near rivers and lakes or on damp forested slopes. It occurs at altitudes up to about 300 metres, it is an uncommon species and its conservation status is considered to be "endangered"