Horn antenna

From Wikipedia, the free encyclopedia

Pyramidal microwave horn antenna, with a bandwidth of 0.8 to 18 GHz. coa!ial cable
feedline attache" to the connector vi"ible at top. #hi" type i" called a rid$ed horn% the c&rvin$
fin" vi"ible in"ide the mo&th of the horn increa"e the antenna'" bandwidth.
horn antenna or microwave horn i" an antenna that con"i"t" of a flarin$ metal wave$&ide
"haped like a horn to direct the radio wave". Horn" are widely &"ed a" antenna" at (HF and
microwave fre)&encie", above *00 +Hz.
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#hey are &"ed a" feeder" .called feed horn"/ for
lar$er antenna "tr&ct&re" "&ch a" parabolic antenna", a" "tandard calibration antenna" to
mea"&re the $ain of other antenna", and a" directive antenna" for "&ch device" a" radar $&n",
a&tomatic door opener", and microwave radiometer".
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#heir advanta$e" are moderate
directivity .$ain/, low 1W2, broad bandwidth, and "imple con"tr&ction and ad3&"tment.
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4ne of the fir"t horn antenna" wa" con"tr&cted in 1856 by 7ndian radio re"earcher 8a$adi"h
9handra :o"e in hi" pioneerin$ e!periment" with microwave".
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7n the 15*0" the fir"t
e!perimental re"earch .1o&thworth and :arrow, 15*</ and theoretical analy"i" .:arrow and
9h&, 15*5/ of horn" a" antenna" wa" done.
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#he development of radar in World War 0
"tim&lated horn re"earch. #he corr&$ated horn propo"ed by >ay in 15<0 ha" become widely
&"ed a" a feed horn for microwave antenna" "&ch a" "atellite di"he" and radio tele"cope".
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n advanta$e of horn antenna" i" that "ince they don't have any re"onant element", they can
operate over a wide ran$e of fre)&encie", a wide bandwidth. #he &"eable bandwidth of horn
antenna" i" typically of the order of 10?1, and can be &p to 00?1 .for e!ample allowin$ it to
operate from 1 GHz to 00 GHz/.
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#he inp&t impedance i" "lowly@varyin$ over thi" wide
fre)&ency ran$e, allowin$ low A1W2 over the bandwidth.
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#he $ain of horn antenna"
ran$e" &p to 0= d:i, with 10 @ 00 d:i bein$ typical.
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Description
horn antenna i" &"ed to tran"mit radio wave" from a wave$&ide .a metal pipe &"ed to carry
radio wave"/ o&t into "pace, or collect radio wave" into a wave$&ide for reception. 7t typically
con"i"t" of a "hort len$th of rectan$&lar or cylindrical metal t&be .the wave$&ide/, clo"ed at
one end, flarin$ into an open@ended conical or pyramidal "haped horn on the other end.
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#he
radio wave" are &"&ally introd&ced into the wave$&ide by a coa!ial cable attached to the "ide,
with the central cond&ctor pro3ectin$ into the wave$&ide. #he wave" then radiate o&t the horn
end in a narrow beam. However in "ome e)&ipment the radio wave" are cond&cted from the
tran"mitter or to the receiver by a wave$&ide, and in thi" ca"e the horn i" 3&"t attached to the
end of the wave$&ide.
How it works
9orr&$ated conical horn antenna &"ed a" a feed horn on a H&$he" Birecway home "atellite
di"h. tran"parent pla"tic "heet cover" the horn mo&th to keep o&t rain.
horn antenna "erve" the "ame f&nction for electroma$netic wave" that an aco&"tical horn
doe" for "o&nd wave" in a m&"ical in"tr&ment "&ch a" a tr&mpet. 7t provide" a $rad&al
tran"ition "tr&ct&re to match the impedance of a t&be to the impedance of free "pace, enablin$
the wave" from the t&be to radiate efficiently into "pace.
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7f a "imple open@ended wave$&ide were to be &"ed a" an antenna, witho&t the horn, the
"&dden end of the cond&ctive wall" ca&"e" an abr&pt impedance chan$e at the apert&re, from
the characteri"tic impedance of the wave$&ide to the impedance of free "pace, *66 ohm".
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When radio wave" travellin$ thro&$h the wave$&ide hit the openin$, it act" a" a bottleneck,
reflectin$ mo"t of the wave ener$y back down the $&ide toward the "o&rce, "o only part of the
power i" radiated.
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7t act" "imilarly to an open@circ&ited tran"mi""ion line, or to a bo&ndary
between optical medi&m" with a hi$h and low inde! of refraction, like a $la"" "&rface. #he
reflected wave" ca&"e "tandin$ wave" in the wave$&ide, increa"in$ the A1W2, wa"tin$
ener$y and po""ibly overheatin$ the tran"mitter. 7n addition, the "mall apert&re of the
wave$&ide .aro&nd one wavelen$th/ ca&"e" "evere diffraction of the wave" i""&in$ from it,
re"&ltin$ in a wide radiation pattern witho&t m&ch directivity.
#o improve the"e poor characteri"tic", the end" of the wave$&ide are flared o&t to form a
horn. #he taper of the horn chan$e" the impedance $rad&ally alon$ the horn'" len$th.
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#hi"
act" like an impedance matchin$ tran"former, allowin$ mo"t of the wave ener$y to radiate o&t
the end of the horn into "pace, with minimal reflection. #he taper f&nction" "imilarly to a
tapered tran"mi""ion line, or an optical medi&m with a "moothly@varyin$ refractive inde!. 7n
addition, the wide apert&re of the horn pro3ect" the wave" in a narrow beam
#he horn "hape that $ive" minim&m reflected power i" an e!ponential taper.
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C!ponential
horn" are &"ed in "pecial application" that re)&ire minim&m "i$nal lo"", "&ch a" "atellite
antenna" and radio tele"cope". However conical and pyramidal horn" are mo"t widely &"ed,
beca&"e they have "trai$ht "ide" and are ea"ier to fabricate.
Radiation pattern
#he wave" travel down a horn a" "pherical wavefront", with their ori$in at the ape! of the
horn. #he pattern of electric and ma$netic field" at the apert&re plane of the horn, which
determine" the radiation pattern, i" a "caled@&p reprod&ction of the field" in the wave$&ide.
However, beca&"e the wavefront" are "pherical, the pha"e increa"e" "moothly from the center
of the apert&re plane to the ed$e", beca&"e of the difference in len$th of the center point and
the ed$e point" from the ape! point. #he difference in pha"e between the center point and the
ed$e" i" called the phase error. #hi" pha"e error, which increa"e" with the flare an$le,
red&ce" the $ain and increa"e" the beamwidth, $ivin$ horn" wider beamwidth" than plane@
wave antenna" "&ch a" parabolic di"he".
t the flare an$le, the radiation of the beam lobe i" down abo&t @00 d: from it" ma!im&m
val&e.
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#he increa"in$ pha"e error limit" the apert&re "ize of practical horn" to abo&t 1= wavelen$th"%
lar$er apert&re" wo&ld re)&ire impractically lon$ horn".
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#hi" limit" the $ain of practical
horn" to abo&t 1000 .*0 d:/ and the corre"pondin$ minim&m beamwidth to abo&t = @ 10D.
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Optimum horn
Ear$e pyramidal horn &"ed in 15=1 to detect the 01 cm .1.;* GHz/ radiation from hydro$en
$a" in the +ilky Way $ala!y.
For a $iven fre)&ency and horn len$th, there i" "ome flare an$le that $ive" minim&m
reflection and ma!im&m $ain. #he reflection" in "trai$ht@"ided horn" come from the two
location" alon$ the wave path where the impedance chan$e" abr&ptly% the mo&th or apert&re
of the horn, and the throat where the "ide" be$in to flare o&t. #he amo&nt of reflection at
the"e two "ite" varie" with the flare angle of the horn .the an$le the "ide" make with the a!i"/.
7n narrow horn" with "mall flare an$le" mo"t of the reflection occ&r" at the mo&th of the horn.
#he $ain of the antenna i" low beca&"e the "mall mo&th appro!imate" an open@ended
wave$&ide. " the an$le i" increa"ed, the reflection at the mo&th decrea"e" rapidly and the
antenna'" $ain increa"e". 7n contra"t, in wide horn" with flare an$le" approachin$ 50D mo"t of
the reflection i" at the throat. #he horn'" $ain i" a$ain low beca&"e the throat appro!imate" an
open@ended wave$&ide. " the an$le i" decrea"ed, the amo&nt of reflection at thi" "ite drop",
and the horn'" $ain a$ain increa"e".
#hi" di"c&""ion "how" that there i" "ome flare an$le between 0D and 50D which $ive"
ma!im&m $ain and minim&m reflection.
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#hi" i" called the optimum horn. +o"t practical
horn antenna" are de"i$ned a" optim&m horn". 7n a pyramidal horn, the dimen"ion" that $ive
an optim&m horn are?
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For a conical horn, the dimen"ion" that $ive an optim&m horn are?
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where
a
E
i" the width of the apert&re in the C@field direction
a
H
i" the width of the apert&re in the H@field direction
L
E
i" the "lant len$th of the "ide in the C@field direction
L
H
i" the "lant len$th of the "ide in the H@field direction.
d i" the diameter of the cylindrical horn apert&re
L i" the "lant len$th of the cone from the ape!.
i" the wavelen$th
n optim&m horn doe" not $ive ma!im&m $ain for a $iven aperture size% thi" i" achieved by
a very lon$ horn. 7t $ive" the ma!im&m $ain for a $iven horn length. #able" "howin$
dimen"ion" for optim&m horn" for vario&" fre)&encie" are $iven in microwave handbook".
Gain
Horn" have very little lo"", "o the directivity of a horn i" ro&$hly e)&al to it" $ain.
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#he $ain
G of a pyramidal horn antenna .the ratio of the radiated power inten"ity alon$ it" beam a!i" to
the inten"ity of an i"otropic antenna with the "ame inp&t power/ i"?
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For conical horn", the $ain i"?
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where
A i" the area of the apert&re,
d i" the apert&re diameter of a conical horn
i" the wavelen$th,
e
A
i" a dimen"ionle"" parameter between 0 and 1 called the aperture efficiency,
#he apert&re efficiency ran$e" from 0.; to 0.8 in practical horn antenna". For optim&m
pyramidal horn", e
A
F 0.=11.,
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while for optim&m conical horn" e
A
F 0.=00.
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1o an
appro!imate fi$&re of 0.= i" often &"ed. #he apert&re efficiency increa"e" with the len$th of
the horn, and for apert&re@limited horn" i" appro!imately &nity.
Types of horn antennas
pert&re@limited corr&$ated horn, &"ed a" a feed horn in a radio tele"cope for millimeter
wave".
#he"e are the common type" of horn antenna. Horn" can have different flare an$le" a" well a"
different e!pan"ion c&rve" .elliptic, hyperbolic, etc./ in the C@field and H@field direction",
makin$ po""ible a wide variety of different beam profile".
Pyramidal horn @ a horn antenna with the horn in the "hape of a fo&r@"ided pyramid,
with a rectan$&lar cro"" "ection. #hey are the mo"t widely &"ed type, &"ed with
rectan$&lar wave$&ide", and radiate linearly polarized radio wave".
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1ectoral horn @ pyramidal horn with only one pair of "ide" flared and the other pair
parallel. 7t prod&ce" a fan@"haped beam, which i" narrow in the plane of the flared
"ide", b&t wide in the plane of the narrow "ide".
o C@plane horn @ "ectoral horn flared in the direction of the electric or C@field
in the wave$&ide.
o H@plane horn @ "ectoral horn flared in the direction of the ma$netic or H@
field in the wave$&ide.
9onical horn @ horn in the "hape of a cone, with a circ&lar cro"" "ection. #hey are
&"ed with cylindrical wave$&ide".
9orr&$ated horn @ horn with parallel "lot" or $roove", "mall compared with a
wavelen$th, coverin$ the in"ide "&rface of the horn, tran"ver"e to the a!i". 9orr&$ated
horn" have wider bandwidth and "maller "idelobe" and cro""@polarization, and are
widely &"ed a" feed horn" for "atellite di"he" and radio tele"cope".
2id$ed horn @ pyramidal horn with rid$e" or fin" attached to the in"ide of the horn,
e!tendin$ down the center of the "ide". #he fin" lower the c&toff fre)&ency,
increa"in$ the antenna'" bandwidth.
1ept&m horn @ horn which i" divided into "everal "&bhorn" by metal partition"
."ept&m"/ in"ide, attached to oppo"ite wall".
pert&re@limited horn @ a lon$ narrow horn, lon$ eno&$h "o the pha"e error i" a
fraction of a wavelen$th,
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"o it e""entially radiate" a plane wave. 7t ha" an apert&re
efficiency of 1.0 "o it $ive" the ma!im&m $ain and minim&m beamwidth for a $iven
apert&re "ize. #he $ain i" not affected by the len$th b&t only limited by diffraction at
the apert&re.
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("ed a" feed horn" in radio tele"cope" and other hi$h@re"ol&tion
antenna".
=0 ft. Holmdel horn antenna at :ell lab" in Holmdel, Gew 8er"ey, (1, with which rno
Penzia" and 2obert Wil"on di"covered co"mic microwave back$ro&nd radiation in 15<;.
Ear$e 166 ft. Ho$$ horn antenna at #H# "atellite comm&nication" facility in ndover,
+aine, (1, &"ed in 15<0" to comm&nicate with the fir"t direct relay comm&nication"
"atellite, #el"tar.
Ho$$ microwave relay antenna" on roof of #H# telephone "witchin$ center, 1eattle,
Wa"hin$ton, (1
Ho$$ antenna"
Hogg horn antenna
type of antenna that combine" a horn with a parabolic reflector i" the Ho$$ antenna,
invented by B. 9. Ho$$
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at :ell lab" aro&nd 15<0. 7t con"i"ted of a horn antenna with a
reflector mo&nted in the mo&th of the horn at a ;= de$ree an$le "o the radiated beam i" at
ri$ht an$le" to the horn a!i". #he reflector i" a "e$ment of a parabolic reflector, "o the device
i" e)&ivalent to a parabolic antenna fed off@a!i".
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#he advanta$e of thi" de"i$n over a
"tandard parabolic antenna i" that the horn "hield" the antenna from radiation comin$ from
an$le" o&t"ide the main beam a!i", "o it" radiation pattern ha" very "mall "idelobe".
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l"o,
the apert&re i"n't partially ob"tr&cted by the feed and it" "&pport", a" with ordinary front@fed
parabolic di"he".
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#he di"advanta$e i" that it i" far lar$er and heavier for a $iven apert&re
area than a parabolic di"h, and m&"t be mo&nted on a c&mber"ome t&rntable to be f&lly
"teerable. #hi" de"i$n wa" &"ed for a few radio tele"cope" and comm&nication "atellite
$ro&nd antenna" d&rin$ the 15<0". 7t" lar$e"t &"e, however, wa" a" fi!ed antenna" for
microwave relay link" in the #H# Eon$ Eine" microwave network.
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1ince the 1560" thi"
de"i$n ha" been "&per"eded by "hro&ded parabolic di"h antenna", which can achieve e)&ally
$ood "idelobe performance with a li$hter more compact con"tr&ction. Probably the mo"t
photo$raphed and well@known e!ample i" the 1= meter .=0 foot/ lon$ Holmdel Horn ntenna
at :ell Eab" in Holmdel, Gew 8er"ey, with which rno Penzia" and 2obert Wil"on
di"covered co"mic microwave back$ro&nd radiation in 15<=, for which they won the 1568
Gobel Prize in Phy"ic".