Toroidal helix antenna

Toroidal helix antenna
AP21-3
TOROIDAL HELIX ANTENNA
James F. Corum
Department
of
Electrical
and Computer Engineering
West V i r g i n iU
aniversity
Morgantown, WV 26505-6101
Kenneth L. Corum
CPG Communications, Inc.
158
Salem
St.
Wilmington, Mass.
01887
Introduction
The design and operation of Low P r o f i l e and E l e c t r i c a l l y
S m a l lA n t e n n a sh a st r a d i t i o n a l l yc e n t e r e da r o u n dt h e
p r o p e r t i e s of e f f i c i e n c y and bandwidth f o r . t h e s e s t r u c t u r e s .
Over t h ey e a r s a v a r i e t y of s i g n i f i c a n t t h e o r e t i c a l s t u d i e s
and experimentaleffortshavebeenperformed
by many w e l l
40
known i n v e s t i g a t o r s W
. heeler's
monumental
work
of
over
y e a r s , summarized i n Ref. 1, i s worthyofspecialnote.
He
hasprovided
one ofthe
most s p l e n d i dp r a c t i c a lt e c h n i q u e s
f o rc h a r a c t e r i z i n ga n dp r e d i c t i n gt h eb e h a v i o ro ft h e s e
i n t h i cs o n t e x t ,
means t h atth e
small
antennas.
(Small,
e n t i r ea n t e n n a ,
and i t s images,fLtwithintheradiansphere
a sphereofradius
A / 2 n . Small a t VLF o r ELF, ofcourse,
may e n t a isl u b s t a n t i arfe ael s t a t e .C
) l e a r l yt,h e
bottom
l i n e on a l l t h i s i s t h a t remarkableperformance i s c e r t a i n l y
practical,providedonlythat
one doesnot become too greedy
inthesize-efficiency-bandwidthtradeoff.
E l e c t r i c a l l y SmallAntennashavealwaysrepresentedone
RF engineeringprofession.
ofthemajorchallenges
t ot h e
( I n d e e d t, h et o p i c
seems t or e a p p e a ri nr e g u l a rc y c l e s . )
Traditionally,acceptableperformancehasbeencoaxed
and
tweaked o u to ft h e s ed e v i c e s
by e i t h e r (1) reducingsystem
l o s s e s( u s et h i c kw i r e s
on theantennas
and low loss e l e ments i nt h em a t c h i n gn e t w o r k s )o r
by ( 2 ) i n c r e a s i n gt h e
r a d i a t i o nr e s i s t a n c e( s a y ,
by t o pl o a d i n gs h o r t o w e r s ) .
Newmann hasdevised
a t h l r dl i n eo fa t t a c k
which i s v i a b l e
when a smallantenna i s t o be located on a larger conducting
support s t r u c t u r e . The technique,apparently,
i s t o ( 3 ) use
t h e small a n t e n n a a s a c o u p l e rt oe x c i t ec h a r a c t e r i s t i c
modes
on
t h es u p p o r t .
The method i s e s p e c i a l l yu s e f u li f
Unthe
small
antenna,
by i t s e l f , i s n ovt e r ye f f i c i e n t .
f o r t u n a t e l y ,o n l yt h ef i r s t
two techniquesareofvalue
to
AM b r o a d c a s t e r s or f o r VLF/ELF communications.
-
SmallAntennas
a t Low Frequencies
A t theselowerfrequencies,anothermajorelectromagi s ocf o n s i d e r a b l sei g n i f i c a n c eT. r a d i t i o n a l
netic
factor
small antenna analysis i s usually formulated in
a free space
environment.
A t t h e s e lowerfrequencies,
however, one must
alsocontendwiththe
Sommerfeld a t t e n u a t i o nf u n c t i o n .( I n
CH2435-6/87/[email protected] 1987 IEEE
832
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t h i sr e g a r d c
, o n s u l t h er e m a r k a b l e
work of
Norton,
who
reduced the calculation of ground
wave l o s s e s t o a p r a c t i c a l
form a p p r o p r i a t e f o r u s e
by AM b r o a d c a s t e r s i n t h i s c o u n t r y .
lowerfrequenciestheproblem
Ref. 3 ) Simply p u t ,a tt h e s e
i s notonly
t o reducesysw i t he l e c t r i c a l l ys m a l la n t e n n a s
tem losses, but simultaneously
t o produce v e r t i c a l p o l a r i z a A t AM b r o a d c a s tf r e q u e n c i e s ,h o r i z o n t a lp o l a r i z a t i o n
tion.
practical significance.
a t t e n u a t e s so r a p i d l y as t o beofno
Any r a d i a t e d HP, i n f a c t ,r e p r e s e n t s
a s e r i o u sd e g r a d a t i o n
A simple c o i lm
, ultiturn
loop
or
of
system
performance.
h e l i x , no m a t t e r how e f f i c i e n t , w i l l produce e l l i p t i c a l
polarization.
No m a t t e r how w e l c
l o n s t r u c t e d t, h e s ee l e c t r i c a l l ys m a l la n t e n n a sa r eo fn oc o m m e r c i a lv a l u e
whatsoever t o AM b r o a d c a s t e r s s i n c e t h e i n h e r e n t h o r i z o n t a l
commust
ponentoftheradiationrepresentswasted
power,
and
n o ta p p e a ri nt h en u m e r a t o ro ft h er a d i a t i o ne f f i c i e n c y
expression.
This genusofelectricallysmallantennas
would
be o fs i g n i f i c a n tv a l u ea t
low f r e q u e n c i e si ft h e yc o u l d
a l s o be made s e l f - r e s o n a n ta n de x c l u s i v e l yv e r t i c a l l y
p o l a r i z e d ,w h i l ea tt h e
same t i m ee i t h e rr e q u i r i n g
a ground
system no l a r g e r t h a n c o n v e n t i o n a l s h o r t v e r t i c a l t o w e r s , o r
none a t a l l .
The Toroidal H e l i x Approach
i s t ot a k e
a self
One p a r t i c u l a r l yi n t r i g u i n gi d e a
it around i n t o a c l o s e d
resonantnormal
mode h e l i x ,p u l l
t o r u s and l e t t h e r e s u l t i n g s t r u c t u r e
combine t h e t u n i n g and
matching
networks
with
t h er a d i a t i n ge l e m e n ti t s e l f ;( S e e
i s now i n s e r i e sw i t ht h e
Ref.4). The r a d i a t i o nr e s i s t a n c e
c o i li n d u c t a n c e ,
and t h i s combination i s shunted by t h e
The impedance transforming
h e l i xt u r n - t o - t u r nc a p a c i t a n c e .
n a t u r e of t h i s lumped c i r c u i t e q u i v a l e n t i s well known, and
it a l s o h a s t h e a d v a n t a g e o f t r a n s f o r m i n g
a r e l a t i v e l y small
f e e d p o i n tc u r r e n ti n t o
a stepped up currentpassingthrough
t h er a d i a t i o nr e s i s t a n c e .
The c i r c u i t equivalentused t o be
c a l l e d a " c u r r e n ta m p l i f i e r "i nt h eo l d
books on network
theory.
is fairly
The f i e l dt h e o r ya n a l y s i s
of t h ea n t e n n a
is a slow wave s e l f
s t r a i g h t f o r w a r dS. i n c teh se t r u c t u r e
it i s r e a s o n a b l et o
assume a superposed
r e s o n a n th e l i x ,
s i n u s o i d a ld i s t r i b u t i o no fe l e c t r i c
andmagneticcurrent,
where t h e e l e c t r i c c u r r e n t i s given by
t h ec o o r d i n a t e sh a v i n gt h e i ru s u a l
n e t i c c u r r e n t i s found from
where b i s t h e h e l i x r a d i u s
ing.Intheseexpressions,
meanings,
and t h e mag-
and s i s theturn-to-turnspacn i s a mode number f o rt h e
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c u r r e n td i s t r i b u t i o n .
Ref.4 a s :
The r a d i a t e df i e l d sa r e
determined i n
The s u p e r s c r i p t e i n d i c a t e s a f i e l d component a t t r i b u t a b l e
magneticcurrent.
t ot h ee l e c t r i cc u r r e n t
and m t ot h e
J ( x ) is the
usual
Bessel
function
and
Bg
i s the
phase
c g n s t a nat p p r o p r i a t ef o trh eh e l i x .
These f i e l d a
s r en o t
by t h es u p e r p o s i t i o no f
a resonant
unlikethoseproduced
loop anda
c i r c u l a rs l o t .
However, becauseoftheslow
wave
n a t u r e of t h et o r o i d a lh e l i x ,t h ep h y s i c a ls i z eh a sb e e n
considerablyreduced.
S e v e r a lv a r i a t i o n s
of t h eb a s i cc o n f i g u r a t i o na r e
now
possible.
By c o n t r a w i n d i ntghhee l i(xR e f s . 4 , 5 , 6 )t ,h e
azimuthal component of e l e c t r i c c u r r e n t i s cancelled out and
one i s simply l e f t with what i s commonlyknown a s a p o l o i d a l
flow of e l e c t r i c u r r e n tT
. his
i s occasionally
called
a
caduceus
winding.
The r a d i a t e d
f i e l da
sre
then
given
by
onlyexpressions(3c)
and ( 3 d ) .F u r t h e r ,
and t h i s i s impor4 o r more segtant,if
one d i v i d e st h eT o r o i d a lH e l i xi n t o
ments it canbe
fedasSmith'scloverleafantenna,so.fami1i a r from FM b r o a d c a s t i n g(.S eR
e efs.4,7).
The r e s u l t a n t
m a g n e t i cc u r r e n td i s t r i b u t i o n
w i l l beuniform,
( n = 0 ) , and
i s describedsimply by e q u a t i o n ( 3 c ) .
theradiatedfield
We now have a Low P r o f i l e s, l o w
wave, v e r t i c a l l y
larized, selfresonant,omni-directional
( i n theazimuthal
r a d i a t o rw i t h
a s u b s t a n t i a fl e e d - p o i n t
impedance.
Consequently,thestructurehasconsiderabledesirabilityas
an E l e c t r i c a l l y SmallAntenna
a tf r e q u e n c i e s
wheregroundwave propagation or ground e f f e c t s a r e i m p o r t a n t .
b
834
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One doesnotgetsomethingfornothing,
however.Chu's
Q of
fundamental l i m i t , which r e l a t e st h el o w e s ta c h i e v a b l e
a l o s s l e s s E l e c t r i c a l l y SmallAntenna
t o i t s maximum physiThe ToroidalHelix
is a
c a l dimension, i s s t i l l i nf o r c e .
remarkable structure,
b u t it h a s beenbought
at thepriceof
reducedbandwidth.
O f course, one may
make
a trade-offbetween
o p e r a t i o n abl a n d w i d t h
and
e f f i c i e n c yi,o
f ne
so
desires.
L a s t l y s, e v e r a el x p e r i m e n t a l l ym e a s u r e de l e c t r i c a l
p r o p e r t i e ss h o u l d be reported. A t y p i c a ls t r u c t u r ec o n s i s t i n 9o f
3 2 contrawoundrings1/60
th of a w a v e l t ~ g t hi n
d l a m e t e ra,r r a n g eidn
a t o r u s of . r a d i u s 1 / 2 1
of a
wavelengthhad
a resonantfeed-point
impedance on t h eo r d e r
of
1500
ohms ( p u r e l yr e s i s t i v e ) .
The Toroidal
Helix
produced p u r e l y
v e r t i c apl o l a r i z a t i o n .
(The h o r i z o n t a l component was a t l e a s t 35 dB, o r more, down from t h ev e r t i c a l
of
35,
as
component.)
The s t r u c t u r eh a d
a Q on t h eo r d e r
determined from impedance measurements.
The measured f i e l d
i n t e n s i t y was within 3 dB of a q u a r t e r wave monopole above
36 r a d i a l s one quarter-wavelengthlong.
REFERENCES
1. "SmallAntennas",
by H.A. Wheeler, ( c h a p t e r 6 i n Antenna
Engineering Handbook, H . J a s i k , 2nd e d i t i o n ,1 9 8 4 ) .C o n s u l t
t h e many r e f e r e n c e s l i s t e d .
2.
"Small
Antenna
Location
Using
C h a r a c t e r i s t i c Modes",
by
Newman, I E E E Trans. on A n t . andProp.,Vol.
AP-27, No.
4, J u l y 1979, PP. 530-531.
E.H.
3 . "The C a l c u l a t i o n of Ground-Wave F i e l dI n t e n s i t y
Over a
F i n i t e l y ConductingSphericalEarth",
by K.A. Norton,Proc.
IRE, Vol. 29, Dec., 1941, PP. 623-639.
4. "Toroidal
Antenna",
J . F . Corum,
U.S.
Patent
#4,623,558;
C a n a d i a nP a t e n#t 1 , 1 8 6 , 0 4 9A
; u s t r a l i a nP a t e n#t 5 4 8 , 5 4 1 .
Other Patents Pending.
5.
"Modified
Contrawound
H e l i x C u r r e n t sf o r
High
Power
Traveling Wave Tubes", by C . K . B i r d s a l l and T . E . Everhart,
IRE Trans. on ElectronDevices,
ED-3, O c t . 1956, Pg. 190.
7. "Cloverleaf Antenna f o r FM Broadcasting", by P . H .
Proc. IRE, Vol.35,
Dec. 1947, PP. 1556-1563.
Smith,
835
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