Antennas Electrical length

1 antennas

1.1 changing electrical length loading
1.2 advantages
1.3 disadvantages
1.4 technical realization

1.4.1 application

antennas

while there wideband antenna designs, many antennas classified resonant , perform according design around particular frequency. applies broadcasting stations , communication systems confined 1 frequency or narrow frequency band. includes dipole , monopole antennas , of designs based on them (yagi, dipole or monopole arrays, folded dipole, etc.). in addition directive gain in beam antennas suffering away design frequency, antenna feedpoint impedance sensitive frequency offsets. transmitting, antenna intended operate @ resonant frequency. @ resonant frequency, definition, impedance pure resistance matches characteristic impedance of transmission line , output (or input) impedance of transmitter (or receiver). @ frequencies away resonant frequency, impedance includes reactance (capacitance or inductance). possible antenna tuner used cancel reactance (and change resistance match transmission line), avoided complication (and needs controlled @ antenna side of transmission line).

the condition resonance in monopole antenna element odd multiple of quarter-wavelength, λ/4. in dipole antenna both driven conductors must long, total dipole length of (2n+1)λ/2.

the electrical length of antenna element is, in general, different physical length example, increasing diameter of conductor, or presence of nearby metal objects, decrease velocity of waves in element, increasing electrical length.

an antenna shorter resonant length described electrically short , , exhibits capacitive reactance. similarly, antenna longer resonant length described electrically long , exhibits inductive reactance.

changing electrical length loading

loading coil in cellphone antenna mounted on roof of car. coil allows antenna shorter quarter wavelength , still resonant.

an antenna s effective electrical length can changed without changing physical length adding reactance, (inductance or capacitance) in series it. called lumped-impedance matching or loading.

for example, monopole antenna such metal rod fed @ 1 end, resonant when electrical length equal quarter wavelength, λ/4, of frequency used. if antenna shorter quarter wavelength, feedpoint impedance include capacitive reactance; causes reflections on feedline , mismatch @ transmitter or receiver, if resistive component of impedance correct. cancel capacitive reactance, inductance, called loading coil, inserted in between feedline , antenna terminal. selecting inductance same reactance (negative) capacitative reactance seen @ antenna terminal, cancels capacitance, , antenna system (antenna , coil) again resonant. feedline sees purely resistive impedance. since antenna had been short appears if resonant, addition of loading coil referred electrically lengthening antenna.

similarly, feedpoint impedance of monopole antenna longer λ/4 (or dipole arms longer λ/4) include inductive reactance. capacitor in series antenna can cancel reactance make resonant, can referred electrically shortening antenna.

inductive loading used reduce length of whip antennas on portable radios such walkie-talkies , short wave antennas on cars, meet physical requirements.

vertical antenna may of desired height : less one-half wavelength of frequency @ antenna operates. these antennas may operate either transmitting or receiving antennas


advantages

the electrical lengthening allows construction of shorter aerials. applied in particular aerials vlf, longwave , medium-wave transmitters. because radio waves several hundred meters many kilometers long, mast radiators of necessary height cannot realised economically. used whip antennas on portable devices such walkie-talkies allow antennas shorter standard quarter-wavelength used. used example rubber ducky antenna.

disadvantages

the electrical lengthening reduces bandwidth of antenna if other phase control measures not undertaken. electrically extended aerial less efficient non-extended antenna.

technical realization

there 2 possibilities realisation of electric lengthening.

often both measures combined. coils switched in series must placed in middle of aerial construction. cabin installed @ height of 150-metres on blosenbergturm in beromünster such construction, in lengthening coil installed supply of upper tower part (the blosenbergturm has in addition ring-shaped roof capacitor on top)

application

transmission aerials of transmitters working @ frequencies below longwave broadcasting band apply electric lengthening. broadcasting aerials of longwave broadcasting stations apply often. however, transmission aerials of ndbs electrical lengthening extensively applied, because these use antennas considerably less tall quarter of radiated wavelength.