| WINDOM ROZPORKI |
sq5gvy pisze: SP6ML pisze: Ważne jest: Unikać długości ramion 1/2 Unikać długości lini zasilajacej 1/4 Linia zasilajaca MUSI być prostopadle do płaszczyzny anteny Dlaczego należy unikać długości 1/2 ? Pozdrawiam Andrzej SQ5GVY Bardzo trafne pytanie! Wszakże promiennik polfalowy to wysokosprawna antenna. Tak, to prawda. Jednak na zaciskach takiego promiennika czy antenny występuje wysokie napięcie. I tu jest problem... Wysokie napięcie wymusza stosowanie zupełnie innych materiałów konstrukcyjnych i izolacyjnych Jeśli jesteśmy w posiadaniu takowych no to nie ma wogole zagadnienia. Natomiast jeśli stosujemy CG3000 lub "zabawki" od MFJ-ta, (albo wlasne elementy) to warto doczytać w instrukcji obsługi i stosować sie do zawartych tam zaleceń o unikaniu 1/2 lambda długości promiennikow oraz 1/4 i wielokrotnosc lini zasilajacej Na uwiarygodnienie moich słów pozwalam sobie wkleić fragment txt z manuala: Antenna System Hints Location For the best performance, an end-fed longwire wire antenna should be at least one quarter-wavelength long at the operating frequency. Horizontal center-fed antennas should be at least a half-wavelength long and located as high and clear as possible. While good RF grounds help the signal in almost any transmitting installation, it is extremely important to have good RF grounds with long wire or other Marconi style antennas. Matching Problems Most matching problems occur when the antenna system presents an extremely high impedance to the tuner. When the antenna impedance is much lower than the feedline impedance, an odd quarter-wavelength feedline converts the low antenna impedance to a very high impedance at the tuner. A similar problem occurs if the antenna has an extremely high impedance and the transmission line is a multiple of a half-wavelength. The half- wavelength line repeats the very high antenna impedance at the tuner. Incorrect feedline and antenna lengths can make an otherwise perfect antenna system very difficult or impossible to tune. One example where this problem occurs is on 80 meters when an odd quarter-wave length (60 to 70 feet) of open wire line is used to feed a half-wave (100 to 140 foot) dipole. The odd quarter-wave line transforms the dipole's low impedance to over three thousand ohms at the tuner. This is because the mismatched feedline is an odd multiple of 1/4 wavelength long. The line inverts (or teeter-totters) the antenna impedance. A problem also occurs on 40 meters with this same antenna example. The feedline is now a multiple of a half- wave (60 to 70 foot) and connects to a full-wave high impedance antenna (100 to 140 feet). The half-wave line repeats the high antenna impedance at the tuner. The antenna system looks like several thousand ohms at the tuner on 40 meters. This places enormous strain on the balun and the insulation in the tuner, since voltages can reach several thousand volts. This can cause component arcing and heating. The following suggestions will reduce the difficulty in matching an antenna with a tuner: • Never center feed a half-wave multi-band antenna with a high impedance feedline that is close to an odd multiple of a quarter-wave long. • Never center feed a full-wave antenna with any feedline close to a multiple of a half-wave long. • If this tuner will not "tune" a multi-band antenna, add or subtract 1/8 wave of feedline (for the band that won't tune) and try again. • Never try to load a G5RV or center fed dipole on a band below the half-wave design frequency. If you want to operate an 80 meter antenna on 160 meters, feed either or both conductors as a longwire against the station ground. • To avoid problems matching or feeding any dipole antenna with high impedance open wire lines, keep the lines around these lengths. [The worst possible line lengths are shown in brackets]: 160 meters; dipole 80 meters; dipole: 40 meters; dipole: 35-60, 170-195 or 210-235 feet 34-40, 90-102 or 160-172 feet 42-52, 73-83, 112-123 or 145-155 feet [Avoid 130, 260 ft] [Avoid 66, 135, 190 ft] [Avoid 32, 64, 96, 128 ft] Some slight trimming or adding of feedline may be necessary to accommodate the higher bands. 6 MFJ-974/974H WARNING! To avoid problems, a dipole antenna should be a full half-wavelength on the lowest band. On 160 Meters, an 80- or 40-Meter antenna fed the normal way will be extremely reactive, with only a few ohms of feedpoint resistance. Trying to load an 80-Meter halfwave dipole (or shorter) antenna on 160 Meters can be a disaster for both your signal and the tuner. The best way to operate 160 Meters with an 80- or 40-Meter antenna is to load either or both feedline wires (in parallel) as a longwire. The antenna will act like a “T” antenna worked against station ground. Technical Assistance If you have any problem with this unit first check the appropriate section of this manual. If the manual does not reference your problem or your problem is not solved by reading the manual, you may call MFJ Technical Service at 662-323-0549 or the MFJ Factory at 662-323-5869. You will be best helped if you have your unit, manual and all information on your station handy so you can answer any questions the technicians may ask. Proszę jednak zwrócić uwagę ze jeśli będziemy sie trzymać tego zapisu: "To avoid problems, a dipole antenna should be a full half-wavelength on the lowest band." to wpedzimy sie sami w maliny... Natura podstawowych pasm amatorskich ma charakter harmoniczny - jest to umotywowane historycznie Tak więc jeśli wykonamy dipol polfalowy na 80m czyli składający sie z dwóch cwiercfalowych ramion to na paśmie 7MHz mamy kilka tysięcy Ohm impedancji a co za tym idzie nawet kilkanaście tysięcy Volt w instalacji antenowej. Tak wiec zupełnie z praktycznego powodu warto unikać takich sytuacji. Długości które spełniają założenie unikania wysokich napięć oraz transformacji lini zasilajacej to: Dla pasm 7MHz do 28MHz 6,8m Dla pasm 3,5MHz do 28MHz 13,5m lub jeśli jest miesce to 17m Dla pasma 1,8MHz 27m Jeśli są pytania, chętnie odpowiem. 73's ml |