Hybrid Log Spiral with Loop Antenna 

by Neal Tesny and Marc Litz 

ARL-TR-5106 March 2010 

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Army Research Laboratory 

Adelphi, MD 20783-1197 

ARL-TR-5106 March 2010 

Hybrid Log Spiral with Loop Antenna 

Neal Tesny and Marc Litz 

Sensors and Electron Devices Directorate, ARL 

Approved for public release; distribution unlimited. 
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4. TITLE AND SUBTITLE 

Hybrid Log Spiral with Loop Antenna 

5a. CONTRACT NUMBER 

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6. AUTHOR(S) 

Neal Tesny and Marc Litz 

5d. PROJECT NUMBER 

5e. TASK NUMBER 

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7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

U.S. Army Research Laboratory 

ATTN: RDRL-SER-M 

2800 Powder Mill Road 

Adelphi, MD 20783-1197

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ARL-TR-5106

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Approved for public release; distribution unlimited. 

13. SUPPLEMENTARY NOTES 

14. ABSTRACT 

We fabricated a hybrid antenna that consists of a logarithmic spiral and loop antenna. The antenna was designed to detect 

electromagnetic (EM) noise and be broadband in the range of 1 MHz to 1 GHz. This style of antenna can be located in 

buildings and passageways in inaccessible areas for the purpose of shielding integrity surveillance and life cycle shielding 

degradation monitoring, and can be used as permanent fixtures that detect critical threshold levels of unwanted EM radiation. 

The antenna structures were modeled with the FEKO EM numerical code and were also experimentally characterized in the 

laboratory. The measurements compare well with the predictions from the modeling. This report presents the detailed results 

of the design, and the FEKO analysis and the measured results. 

15. SUBJECT TERMS

Hybrid antenna, spiral antenna, loop antenna, wideband 

16. SECURITY CLASSIFICATION OF:

17. LIMITATION 

OF ABSTRACT 

UU 

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OF PAGES 

22 

19a. NAME OF RESPONSIBLE PERSON

Neal Tesny 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified 

c. THIS PAGE 

Unclassified 

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(301) 394-5559 

Standard Form 298 (Rev. 8/98) 

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iii 

Contents 

List of Figures iv

List of Tables iv

1. Objective 1

2. Approach 1

3. Design Goals 1

4. Log Spiral Fabrication 3

5. Log Spiral Results 4

5.1 Modeled ...........................................................................................................................4

5.2 Measured .........................................................................................................................6

5.2.1 Balun ...................................................................................................................6

5.2.2 Experimental Test Results ...................................................................................7

6. Loop Antenna Results 10

7. Conclusions 11

7.1 Future Work ..................................................................................................................11

8. References 13 

Distribution List 14 
iv 

List of Figures 

Figure 1. A use for the low profile hybrid antenna. ........................................................................3

Figure 2. Modeled log spiral antenna, with approximately 4000 method of moment 

elements. ....................................................................................................................................4

Figure 3. Fabricated log spiral antenna. ..........................................................................................4

Figure 4. Modeled antenna gain for ideal and 50 Ω source. ...........................................................5

Figure 5. Modeled input impedance magnitude. ............................................................................5

Figure 6. Modeled complex input impedance of log spiral antenna. ..............................................6

Figure 7. Balun attached to log spiral feed. ....................................................................................6

Figure 8. Design of wideband balun. ..............................................................................................7

Figure 9. Measured gain of log spiral antenna, with and without balun. ........................................7

Figure 10. Measured gain of log spiral antenna, expanded linear scale. ........................................8

Figure 11. Input impedance of log spiral antenna, with and without balun. ...................................8

Figure 12. Reflection coefficient of log spiral antenna, with and without balun. ...........................9

Figure 13. Simulated and measured gains of the log spiral antenna. ..............................................9

Figure 14. Hybrid log spiral with loop antenna. ...........................................................................10

Figure 15. Modeled and measured gains of loop antenna. ...........................................................11

Figure 16. Skin depth versus frequency for various metals. .........................................................12

List of Tables 

Table 1. Mean simulated and measured gains in frequency region of interest. .............................9
1 

1. Objective 

Shielding integrity surveillance and life cycle shielding degradation monitoring are often 

performed in buildings and passageways which are inaccessible with the large antennas needed 

for coverage of the appropriate bandwidths. Hence, we desired to develop a thin, wideband 

antenna for use in confined, narrow spaces for the use of wideband measurements. Our desire 

was to have an antenna with a frequency range of interest of 10 kHz to 1 GHz and with an 

unpolarized response. 

The logarithmic spiral antenna, first described by Dyson in 1959, would be specified entirely by 

angles. The performance would then be independent of wavelength, with the limit of the lowest 

frequency transmitted being dependent on the length of the spiral arms. Antennas of this sort 

were constructed with bandwidths of over 20:1 (1). 

Curtis found that for an Archimedean spiral, the high-frequency limit is determined by the feed 

configuration, and that the low-frequency limit occurs when the outside diameter is a little 

greater than a half wavelength (2). 

2. Approach 

The approach was to design a hybrid antenna that would consist of two antenna portions. The 

two portions would be 1) a printed logarithmic spiral to cover the upper range of frequencies 

from 20 MHz to 1 GHz, and 2) a square loop to cover the lower range of frequencies from 

10 kHz to 20 MHz. 

A log spiral antenna was designed by Polun (3) and manufactured at the U.S. Army Research 

Laboratory (ARL). 

Models of the antenna were then fabricated at ARL. We modeled the antenna using FEKO 

modeling software (4). We then performed anechoic chamber tests to characterize the fabricated 

antenna. 

3. Design Goals 

The design goals of the loop antenna portion include the following: 

 Frequency Range: 10 kHz–20 MHz 
2 

 Flat low-profile footprint 

 1.5 Decade Bandwidth or Greater 

 Gain >3 dBi 

 Impedance match to ~50 Ω

The design goals of the log spiral antenna portion include the following: 

 Frequency Range: 10 MHz to >1.0 GHz 

 Physically flat, low-profile footprint 

 1.5 Decade Bandwidth or Greater 

 Gain >3 dBi 

 Impedance match to ~50 Ω

In addition to the above performance requirements, the antenna would be versatile in the 

following ways: 

 Inexpensive 

 High power levels (100 W) as sensor 

 CW and Pulse Applications 

 Use in confined spaces 

 Provides Circular Polarization 

Its applications would include the measurement of electromagnetic (EM) Shielding Effectiveness 

in confined spaces and as facility-embedded EM sensors for receive antennas, as in the 

environment shown in figure 1. 
3 

Figure 1. A use for the low profile hybrid antenna. 

4. Log Spiral Fabrication 

The log spiral portion was CAD-designed, and Gerber files were generated for the fabrication 

process using a circuit board. It was fabricated in-house at ARL. The dimensions of the 

fabricated log spiral antenna are 97 x 76 cm (38 x 30 in). 

The equations, in polar coordinates, are (3) 

0.1103

0.1103 0.1823

0.1103

0.1103 0.1823

Inner right spiral: 0.5

Outer right spiral: 0.5

Inner left spiral: 0.5

Outer left spiral: 0.5

r e

r e

r e

r e











 



 





 

 

where 

r is radius, 

θ is polar angle, in radians. 

The modeled and fabricated log spiral antennas are shown in figures 2 and 3. 
4 

Figure 2. Modeled log spiral antenna, with approximately 

4000 method of moment elements. 

Figure 3. Fabricated log spiral antenna. 

5. Log Spiral Results 

5.1 Modeled 

The modeled gain for an ideal, matched source and a 50  source are shown in figure 4. The 

ideal source showed a peak gain of approximately 6 dBi at 473 MHz. The gain levels off at 

around 2 dB below 50 MHz. 

For the 50  source, the peak gain was approximately 2 dBi and was relatively flat at 

frequencies over 100 MHz. The gain drops off significantly below 100 MHz. 
5 

Figure 4. Modeled antenna gain for ideal and 50 Ω source. 

The modeled input impedance is shown in figure 5. In the100 MHz–1 GHz region of interest for 

the spiral, the input impedance was shown to be 200–300 . 

Figure 5. Modeled input impedance magnitude. 

The modeled input impedance for the resistive and reactive components (known as real and 

imaginary in mathematics) is shown in figure 6. Above 100 MHz, the reactive response is 

primarily greater than 0, which indicates inductive reactance in the 100 MHz–1 GHz region of 

interest. The resistive component dominates above 80 MHz. 

Input Impedance (Magn)

0.00

200.00

400.00

600.00

800.00

1000.00

1200.00

1400.00

1600.00

1.00E+07 1.00E+08 1.00E+09 1.00E+10

Frequency (Hz)

Impedance (Ohms)

-50

-40

-30

-20

-10

0

10

1.00E+07 1.00E+08 1.00E+09 1.00E+10

Gain (dBi)

Frequency (Hz)

Log-Spiral Gain, modeled

Matched Source

50-Ohm Source
6 

Figure 6. Modeled complex input impedance of log spiral antenna. 

5.2 Measured 

5.2.1 Balun 

A balun was used in the measurements of the log spiral antenna. It is shown in figure 7, and its 

design is shown in figure 8. It is a 2:1 wideband coplanar, stripline balun mounted on the logspiral feed. Its dimensions are 40 mm by 30 mm by 14 mm. The balun was designed to match a 

100 Ω load to a 50 Ω load. 

Figure 7. Balun attached to log spiral feed. 

Input Impedance (Complex)

-1500.00

-1000.00

-500.00

0.00

500.00

1000.00

1500.00

1.00E+07 1.00E+08 1.00E+09 1.00E+10

Frequency (Hz)

Impedance (Ohms)

Real

Imag
7 

Figure 8. Design of wideband balun. 

5.2.2 Experimental Test Results 

The log spiral antenna gain was measured in an anechoic chamber. The results of the antenna 

with and without a balun are shown in figure 9. The plot illustrates that the balun provided a 

much smoother gain curve. 

Figure 9. Measured gain of log spiral antenna, with and without balun. 

The gain shown with an expanded frequency scale in main range of interest is shown in 

figure 10. From the plot, there is a 0–2 dBi gain in range of interest with the balun. 

Log-Spiral Gain, measured

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

1.00E+07 1.00E+08 1.00E+09 1.00E+10

Frequency (Hz)

Gain (dBi)

with balun

w/o balun
8 

Figure 10. Measured gain of log spiral antenna, expanded linear scale. 

The magnitude of the measured Input Impedance with and without the balun is shown in 

figure 11. In the frequency range of interest, the mean impedance without the balun was 77 Ω. 

However, the large ringing in this region shows large variations in impedance, while the balun 

provides much more stable impedance along the frequency range of interest, which is sufficient 

for the application. 

Figure 11. Input impedance of log spiral antenna, with and without balun. 

This is also seen in the measurement of the reflection coefficient S11, which is shown in 

figure 12. The reflection coefficient is better above 100 MHz with the balun in place. 

Log-Spiral Gains, measured

-25

-20

-15

-10

-5

0

5

10

0.0E+00 2.0E+08 4.0E+08 6.0E+08 8.0E+08 1.0E+09

Frequency (Hz)

Gain (dBi)

with balun

w/o balun

Input Impedance, magnitude, measured

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

400.0

450.0

500.0

1.00E+07 1.00E+08 1.00E+09 1.00E+10

Frequency (Hz)

Impedance, mag ()

with Balun

w/o Balun
9 

Figure 12. Reflection coefficient of log spiral antenna, with and without balun. 

A comparison of the simulated and measured gain of the log spiral antenna is shown in figure 13. 

Table 1 shows the average gain over the frequency range of interest. This shows generally good 

agreement (<1 dB) between measured and simulated gain. 

Figure 13. Simulated and measured gains of the log spiral antenna. 

Table 1. Mean simulated and measured gains 

in frequency region of interest. 

 100 MHz–1 GHz 

Meas w/o Balun –3.5 

Meas with Balun –0.2 

Simulation 0.7 

-50

-40

-30

-20

-10

0

10

1.00E+07 1.00E+08 1.00E+09 1.00E+10

Gain (dBi)

Frequency (Hz)

Log-Spiral Gain, modeled

Modeled

Measured

S11, magnitude, measured

0

0.2

0.4

0.6

0.8

1

1.2

1.00E+07 1.00E+08 1.00E+09 1.00E+10

Frequency (Hz)

S11, mag ()

with Balun

w/o Balun
10 

6. Loop Antenna Results 

The log spiral antenna with the loop is shown in figure 14. The dimensions of the loop are 97 x 

76 x 15 cm (38 x 30 x 6 in). It was constructed from 0.5-in copper tubing to provide durability 

and minimize resistance. 

Figure 14. Hybrid log spiral with loop antenna. 

The modeled and measured gains of the loop antenna are shown in figure 15. These are along 

the coplanar axis of the loop. The measured range was from 10 kHz to 20 MHz, and the 

modeled range was from 10 kHz to 1 GHz. The results show good agreement between measured 

and modeled loop gains. 

Loop on 

perimeter 

97 X 76 X 15 cm 

Log Spiral feed 

Loop feed 
11 

Figure 15. Modeled and measured gains of loop antenna. 

7. Conclusions 

A hybrid loop-spiral was designed and evaluated. The loop portion covers the frequency range 

of 10 kHz to 20 MHz. The spiral portion covers the frequency range of 100 MHz to 1 GHz. 

The hybrid antenna has a wide bandwidth capability and is used extensively on field tests. It is 

lightweight and has a thin profile. The gains are approximately 0 to 2 dBi, as compared to a 

standard log periodic antenna of about 6 dBi. 

The input impedance of the log spiral portion is roughly 100 ; a balun, however, corrects this 

to roughly 50 . 

In practice, the hybrid antenna provides good performance. Though, deficiencies exist from 

10 kHz to 100 kHz and from ~5 MHz to ~50 MHz. 

These hybrid antennas are installed in buildings and passageways, and in inaccessible areas for 

the purpose of shielding integrity surveillance and life cycle shielding degradation monitoring. 

They are useful as permanent fixtures that detect critical threshold levels of unwanted EM 

radiation. 

7.1 Future Work 

Future work includes 

 Improve balun for better broadband characteristics 

Measured vs. Modeled Gain of Loop

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08 1.0E+09 Frequency (Hz)

Gain (dBi)

Measured

Modeled
12 

 Develop low-loss, cross-over network between antennas 

 Fabricate a two-turn Loop for Higher Sensitivity 

 Increase copper print-thickness (skin depth) on spiral to get higher gain [10 MHz to 

100 MHz] 

An additional method for extending the low frequency performance involves terminating the 

ends of the spiral arms and is described in (5), in which bandwidths of 100:1 are achieved. A 

version of this technique could also be applied to the log spiral for additional gain at low 

frequencies. 

Thicker copper printed on the board would allow lower frequency content, according to the 

following equation and plotted in figure 16. For our application, the skin depth of copper at 

10 MHz would be 20.6 μm. Having a rule of thumb of 5 skin depths would necessitate a board 

thickness of 103 μm. 

Figure 16. Skin depth versus frequency for various metals. 
13 

8. References 

1. Dyson, John D. The Equiangular Spiral Antenna. IRE Transactions on Antennas and 

Propagation, April 1959, 181. 

2. Curtis, Walter L. Spiral Antennas. IRE Transactions on Antennas and Propagation, May, 

1960, 298. 

3. Polun, Max. Feasibility Studies of New High Altitude Electromagnetic Pulse Test Materials; 

ARL-TR-3677; U.S. Army Research Laboratory: Adelphi, MD, November 2005. 

4. Copyright © 2000-2010 EM Software & Systems, http://www.feko.info/ (accessed January 

2010). 

5. Numberger, M. W.; Abdelmoneum, M. A.; Volakis, J. L. New Techniques for Extremely 

Broadband Planar Slot Spiral Antennas. Antennas and Propagation Society International 

Symposium, IEEE July 1999, 4, 2690–2693. 
14 

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