Gevonden!
Het heeft even geduurd, maar ik heb het artikel gevonden waarvan ik wist dat ik het ergens gezien had.
Ik beweerde niet voor niets dat de US Marine zowel de antenne als zelfs de naam 'Nord' had overgenomen van het zendschip. Dat mag dan grappig klinken, maar het is wel waar.
Dit komt uit een langer verhaal over antennes door Cpt. Paul H. Lee, voor amateurs gepubliceerd in een reeks artikelen in 'CQ' in 1968 en 1969. Dit stuk stond in augustus 1968.
Let op, Lee beschrijft dus de marine- en amateurtoepassing; het schip wordt maar eventjes genoemd.
De gegeven uitleg hebben we hierboven natuurlijk ook al gezien (afgestemde kring, folded unipole, et cetera), maar het is leuk de originele tekst weer terug te zien. Ik moest wel verder terug zoeken dan ik dacht.
Een scan is beroerd (tekst is flets grijs op donkergeel verkleurd papier), dus ik tik het even over.
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NORD
There is yet another type of short vertical antenna which has recently come into use. This is the NORD®.
The original NORD was designed for shipboard use, for radio broadcasting in the m.f. band in European waters. It has since been built for shore use at high power, in the l.f. band. To date, NORDs are in use at Naval Communication Stations in Japan, Guam, Philippines, Hawaii and Annapolis, Md., providing l.f. multichannel transmitting capability.
A schematic of the NORD is shown in fig. 24. A center tower is used, with its base grounded. The tower is fed by the folded unipole method, with a feed wire going to the top of the tower. The upper set of guys (three of them) is connected to the tower, and they extend out to three short perimeter towers, where they are secured to strain insulators. From these the cables drop down to tuning units which consist of capacitors.
For sake of clarity in fig. 24, the perimeter towers are not shown.
The NORD is basically nothing but three over-coupled tuned circuits with a common element, the center tower.
The purpose of the NORD for l.f. is to provide sufficient bandwidth for multichannel teletype transmissions, while still retaining reasonable radiation efficiency. The electrically short series fed tower cannot do this at high power. Its Q is very high and its bandwidth is limited. Also, with top-loading, the top-loading guys are corona-limited due to the very high voltages which are developed on them.
The NORD has no such problems. By adjustment of the capacitors Ca, Cb, and Cc, optimum bandwidth and/or efficiency can be obtained, with reasonable input impedance being provided by the folded unipole feed.
For Naval l.f. purposes we require bandwidth. For amateur c.w. and voice at h.f., efficiency is paramount. The NORD can fulfill both of these, by suitable adjustment, in a tradeoff of efficiency for bandwidth or vice versa. By going to the NORD at l.f. we have rid ourselves of the high voltage helix with its inherent losses, and its bandwidth limitations.
The input coupling unit with the NORD can be nothing but low loss capacitors. In fact, in the high power NORDs vacuum capacitors are used throughout.
A full-size NORD, designed for 50 to 150 kc, now in use, has dimensions as follows:
Height of center tower - 450'
Height of perimeter towers - 150'
Spacing of center to perimeter towers - 500'
For use on 2 and 4 mc by amateurs, the NORD would not have to be so short electrically. It could have the following dimensions:
Height of center element - 30'
Height of perimeter supports - 10'
Spacing of center to perimeter - 33'
This antenna would have excellent efficiency on both bands. Guy tuning capacitor values would have to be determined by experiment, but should be of at least 1500 mmf total capacity, adjustable in steps, with a portion variable. The input impedance at the feed point will be R + jXL of a reasonable value, and this can be matched to coaxial line by a simple capacitor network.
The antenna is tuned by connecting an impedance bridge to the input, then starting with the guy tuning capacitors at minimum they are brought up in value simultaneously until a good R + jXL is obtained. It may be possible to obtain 50 + jXL, in which case only a series capacitor will be required for matching to the line.
Heavy ground busses must be used to link perimeter tuning units with the center tower.
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