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Full text of "NASA Technical Reports Server (NTRS) 19800006804: Experimental loop antennas for 60 KHz to 200 KHz"

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(88S8-CR-162729) ?XP*lH8E8T»l LOOP 88TE8NIS 
FOB 60 KHz TO 200 KHz (Ohio Oniv.) 9 p 
HC A02/HP 101 CSCI 176 


63/04 46679 



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. 


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 



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 ] 


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) 

Ferrite Gore 
6" X 5/8” 

U = 125 material 

1-2, 3-4 
in sdries 
with CT ground 

loop mounted with 
^stic cable 





t<^ edge of coil 
flush with shield 

mount in plastic 
box for weather 
shield with 

Figure 1. Broadband Balanced VLF Loop Antenna Desi^. 


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. 

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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 - • 


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. 


A. Effective Height. 

The sensitivity of a loop antenna will be quite low and will 



2tt nAU ,F 
rod o 


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 

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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.) 




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. 


RPC s loading inductor 
coax cable to receiver 

Receiver Input 
50 to 500 ohm 

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. 

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IV. LOOP ANTENNA DESIGN (See Figures 3 and 4) 



Loop Tuning 

1st Stage 
Load RFC 

1st Stage 
Load Cap. 





1750 Meters 

1400 pf 

1 MH 



500 ohms 

1750 Meters 

1400 pf 

1 MH 

270 pf 


500 ohms 


4200 pf 

5 MH 



360 ohms 


4000 pf 

2.5 MH 

430 pf 


360 ohms 


4200 pf 

1 MH 

2200 pf 

21 KHz 

360 ohms 


10,000 pf 

10 MH 


12 KHz 

240 ohms 


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 - 


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 - - 


D demagnetizarton factor for rod * 0.37m 


m - length to dicaneter ratio for rod - b/o 

a ^ diom inches 
b = length inches 
n - number of turns 


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. 


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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