880-15063
(88S8-CR-162729) ?XP*lH8E8T»l LOOP 88TE8NIS
FOB 60 KHz TO 200 KHz (Ohio Oniv.) 9 p
HC A02/HP 101 CSCI 176
Onclas
63/04 46679
TECHNICAL MEMORANDUM (NASA) 71
EXPERliVCNTAL LOOP ANTENNAS FOR 60 KHz TO 200 KHz
A series of loop ^termos hove been fabricated
and evaluated f<- oossible use with Loron-C raid
other VLF to LF oand receivers. A companion
low noise and ve'/ high gain prernriplifier cir-
cuit has been devtsed to operate the loop antennas
remote from the receiver. Further work is suggested
on multiple loop antenna systems to provide omni-
directional coverc^e and reduce E-field noise pick-
up in navigation or communications systems.
by
Ralph W. Burhans
Avionics Engineering Center
Deportment of Electrical Engineering
Ohio university
Athens, Ohio 45701
Langley Research Center
Hampton, Virginia
Grant NGR 36-009-017
!•
INTRODUCTION
Some preliminory results with broadband loop mtennos may be of interest. A
primaiy goal has been to investigate some simple systems for p(^ible Loran-C receivers
which require o bandwidth of greater than 20 KHz. A bifilar wound balanced loop system
has been devised which shows considerable promise. The same loop winding con be made
to operate from 60 KHz to 200 KHz with bandwidths of 10 to 100 KHz,depending on the
opplicotion. Designs are presented for a 60 KHz WWVB antenna, several Loron*C
variations, and some 1750 meter band mtennm. Signals hove been received on all
these, including one airborne experiment where a Loron-C receiver gave the correct
time difference reading within 1 microsecond while flying on a straightline course.
An additionol problem with Loron*C is the phase reversal when the direction of travel
changes 180^. This ccn be partially solved by operating pairs of creased loops oriented
90® with respect to each other to obtain on omni-*directional amplitude pattern. The
advantage would be the reduction of electrostatic precipitotion noise in airborne use.
This may also be an advantage in reducing Enfield 60 Hz harmonic noise in urban ground
use of Loran-C# However, with Loron-C there still remains a phase reversal problem
requiring additional receiver processing indeperKlent of amplitude variations. For the
1750 meter bemd communications or time signal use the loop entennas also reduce E-
field noise pickufvond a single loop may be used as a direction finder or to null out
strong interference.
The advantage of a widebond loop is that the main tuning is all done at the
receiver circuit, end the loop may be mounted remote from the receiver location. The
preomplifier circuit devised for use with these loops is capable of summing the output
of several loops in parallel either to provide more sensitivity or omni-directional coverage.
For this multiple loop application, an additional JFET 1st stage amplifier is used for each
loop with a common summing of the current to each JFET by connecting the drain terminals
in porallel. A quad omni-directicmal Lorem-C loop is presently being considered.
Another advantage of this antenna system is the very small size. A 4 to 7 inch
long ferrite rod of 1/2 to 3/4 inch diameter in a suitable electrostatic shield oppears to
provide adequate sensitivity but requires very low noise performance and very high pre-
amplifier gain. A gain of 50 dB is typically required to make this antenno comparable
in sensitivity to a 2 meter E-field whip antenna at the same signal intensity.
The best single reference on loop antenna designs is [1]. The best design reference
on grounded gate low-noise JFET preamplifiers is C2]»
[1] Pettingill, R. C., H. T. Gorl<»id and J. D. Meindl, "Receiving Antenna Design
for Miniature Receivers", IEEE Trans. Ant. and Prop., Vol. AP-25, No. 4,
pp. 528-530, July 1977.
Burwasser, Alex, "Broadbond JFET Ampli iers". Ham Radio, Vol. 12, No. 11,
pp. 13-19, November 1979.
[ 2 ]
II. LOOP DESIGN
A. Bolonced Loop. To minimize E-field pickup, a symmetrical loop winding is
desirable. This can be obtained by using a bifilar coil wound with parallel insulated
transmission line such as low-pcwer audio speaker cable. Opposite sides of the winding
ore grounded at each end, resulting in a symmetry with respect to a ground plane or shield
trough. The ungrounded opposite ends then become the loop terminals. The inductance
of a single winding of one winding of the pair will be l/4th the total inductance of the
two pairs in series. The series common ground connection is a center-tap for the loop.
The output power from one end is 4 times the output power available from a single wind-
ing, but the e active number of turns for sensitivity is twice that of a single winding.
In other words, two, 50 turn wirKlings become 100 turns for sensitivity computations with
respect to the H-field, but the inductance is 4 times the inductance of a single 50 turn
winding of the same length. (See Figure 1.)
(595 pH)
(.595niH)
Ferrite Gore
6" X 5/8”
PALOMAR ENGINEERS
U = 125 material
tANOINGS
1-2, 3-4
in sdries
with CT ground
loop mounted with
^stic cable
clanps
trough
electrostatic
shield
S’htS'hc?"
t<^ edge of coil
flush with shield
mount in plastic
box for weather
shield with
preamp
Figure 1. Broadband Balanced VLF Loop Antenna Desi^.
III. LOOP CALCULATIONS
For optimum coupling of loop winding to H-field use single layer winding over the
entire length of the ferrite rode. For wideband performance use a low L - high C, resonant
circuit with relatively large wire size.
- 2 -
Q
u
unloaded Q of loop indjctance with capacitor at operating frequency
with no load or only a very high Z scope connection f'>r measurement.
- loaded Q of loop antenna in operoting circuit.
R = effective parallel resistance of loop winding on core
R|^ - load resistance of circuit - •
QuRl
Solve for = Q X. «
= Q X, •
u L
u L L
note that If then ^ (approx.) and - 1 implies a
recKonably wideband circuit.
Choose input R|^ circuit for very low noise^ but low input impedance such as a
grounded gate JFET ciro'it.
R. = 1/G for grounded gate JFET, typical 2N5457 JFET will have G ^ .001
mho5, then the inpuT Rj^ • about 1000 ohms.
Ou^ut circuit of JFET will effect overall bandwidth. Use a wideband transformer
coupled from first stage to second stage with LC loading across transformer to control output
bandwidth. Stagger tune loop slightly with respect to transformer output loading to adjust
final bandwidth desired. At VLF to LF range, a 600 ohm line-*to-line transformer in sub-
miniature size is suitoble. Note thot high frequency roll-off of transformer will effect
shape of overall response, particularly at high end of range like 200 KHz.
be:
A. Effective Height.
The sensitivity of a loop antenna will be quite low and will
H
e
2tt nAU ,F
rod o
X
A -wavelength (usually in meters)
where F^ “ averaging factor of coil and rod (typically 0.5 to 0.7) and n * number of turns
total, and A = cross sectional orea of one turn. Be sure to tjse numbers all in the same
units, such as meters.
A typical 6” (15cm) ferrite rod will have an effective height H^ ~ 3 millimeters,
which requires a very high gain and low-noise preamplifier circuit. A 35 to 50 dB gain
system is suggested such as a single grounded gate JFET driving a differential amplifier
with the output transformer coupled to a transmission line back to the receiver.
For wideband systems, the maximum number of turns on low U ferrite material
spread out over the entire core appears to offer more se«isitivity than a high U material
because the inductance of the coil will usually be too high for a reasonable sensitivity
- 3 -
number (He)« A compact' multiple turn coil centered on a long core rod may achieve very
high Q but will suffer poor sensitivity compared to a long solenoid of the some Q. (See
Figure 2.)
LOOP AMTENML
5/mvvm-i.
OIPP AMP
GA3053
ferrite core |
1 nnnrnnrvn( 2
' ’
Cl,, loop tuning cep
polyst 3 rrene or mice type
parallel to achieve resonance
T s Monser TLD16 600CT to 600CT
C.output loading line to line trana.
cap
RPC s loading inductor
coax cable to receiver
Receiver Input
50 to 500 ohm
level
up to 200kHz
Gain s aOdb or more
depending on
loading of
input stage
n2V DC regulated
(amplifier will operate from 't-10 to ♦IS?)
Figure 2. Active Loop Preomplifier System Mounted at Antenna.
- 4 -
IV. LOOP ANTENNA DESIGN (See Figures 3 and 4)
Frequency
Range
Loop Tuning
Capocitor
1st Stage
Load RFC
1st Stage
Load Cap.
Bond
Width
Reactance
^Loop
1750 Meters
(A)
1400 pf
1 MH
none
75KHz
500 ohms
1750 Meters
(B)
1400 pf
1 MH
270 pf
65KHz
500 ohms
Loran-C
4200 pf
5 MH
none
36KHz
360 ohms
Loran-C
4000 pf
2.5 MH
430 pf
29KHz
360 ohms
Loran-C
4200 pf
1 MH
2200 pf
21 KHz
360 ohms
V/WVB60KHZ
10,000 pf
10 MH
none
12 KHz
240 ohms
1
Table 1.
A. Loop Anfenno Winding .
Wire = 2 conductor No. 24 solid insulafed poir speoker cable (Radio Shack Cat. No. 278-1509).
Width of one turn - opprox. 0.12".
Thickness of winding = approx. 0.0625" (1/16).
Winding length =6.0" = (b) (single layer solenoid).
Effective diameter of one turn = 0.6562" = (a).
Number of turns of pair = 50 (100 turns total both wires).
Core = 6" length of Palomar engineers U ~ 125 rod (originally 5/8" x ").
Effective permeability of rod = 38. 4 =
Length of wire used = approx. 10 feet.
Inductance of single winding = 148 microhenries.
Inductance end-to-end with opposite ends common ground (^X4) = 595 microhenries.
- 5 -
RESPDNBE
B. Formulas.
Inductance of solenoid oir core ~ L
(single layer)
2 2
0.2 o n
3q + 9b
Inductance of loop on core - L, ~ L . x U ,
loop Oir rod
Rod Permeability - -
^ferrite
D demagnetizarton factor for rod * 0.37m
-1.44
m - length to dicaneter ratio for rod - b/o
a ^ diom inches
b = length inches
n - number of turns
V. SUMMARY
Some design data on low-frequency loop antenna systems is presented. Loop antennas
may be desirable for airborne and mobile Loran-C receivers to reduce E-field noise pickup.
However^ the phose reversal of the signal from the antenna for a direction change of 180®
creates on additionol problem for the receiver processor. An envelope manipulating receiver
which averages the phase code from the Loran-C signals might be made to work with a crossed
poir of loops or a quod loop combining circuit. Another approach suggested In the early
literature [3] is to square the signal at a low level and add the result from 90® oriented loop
pairs. This results in a processing signal at twice the signal frequency or 200 KHz for a
100 KHz Loran-C pulse. Squaring also further delays the third cycle rise time of the pulse
envelope. Thus an entirely different type of Loron-C receiver circuit would be required
using sc|uc.. methods to eliminate the phase reversal from loop antennas. Alternatively,
the receiver nc 'igation processor and a direction sensor on the vehicle could be used to
reverse the phase or switch loop polarity. However, this requires a much more complex
and more expensive receiver processor for Loran-C.
For other applications, such as communications or time signal reception, this loop
antenna can improve the performance of receivers by reducing E-field and 60 Hz harmonic
noise pickup. Additionally, the loop antenna provides a very simple direction finder or
null circuit for reducing strong interference.
VI. ACKNOV/LEDGEMENTS
This work has been supported by NASA Langley Research Center, Grant NGR-36
009-017. The help of James Irvine and Daryl McCall is appreciated in collecting preliminary
airborne test data on experimental Loran-C loop antennas.
[3] Cheng, D. K., and R. A. Galbraith, ** Stagger-Tuned Loop Antennas for Wide-
Band Low-Frequency Reception*®, Proc. IRF Vol. 41, pp. 1024-1031, August 1953.
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