Reproducibility Measurements on Two-plate Transverse 

Electromagnetic (TEM) Horn Transmit Antennas 

by Steven Wienecke 

ARL-MR-0732 January 2010 

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Army Research Laboratory 

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ARL-MR-0732 January 2010 

Reproducibility Measurements on Transverse 

Electromagnetic (TEM) Horn Transmit Antennas 

Steven Wienecke 

Sensors and Electron Devices Directorate, ARL 

Approved for public release; distribution unlimited. 
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Reproducibility Measurements on Transverse Electromagnetic (TEM) Horn 

Transmit Antenna 

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Steven Wienecke 

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13. SUPPLEMENTARY NOTES 

14. ABSTRACT 

The performance of the U.S. Army Research Laboratory’s Synchronous Impulse Reconstruction (SIRE) radar is dependent on 

the performance of the antennas and baluns that make up the radar system’s physical aperture. We measured the S11 

parameters of six different antenna configurations to find the most suitable transverse electromagnetic (TEM) antenna for our 

application in the 300 MHz–3 GHz frequency range. In this memorandum report, we show that these TEM transmit antennas 

can be reproduced consistently with a fairly high degree of accuracy, even when constructed by hand. We accomplished this 

by comparing the measurements for two fabrications of six antenna configurations, one fabricated by Gregory Smith and the 

other fabricated by the author. Most of the comparisons between two antenna fabrications are very close. For those antenna 

configurations for which discrepancies exist, we highlight the discrepancy and provide a potential explanation for why the 

difference occurred. 

15. SUBJECT TERMS

Wideband, radar, antenna, TEM horn 

16. SECURITY CLASSIFICATION OF:

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19a. NAME OF RESPONSIBLE PERSON

Steven Wienecke 

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(301) 394-0866 

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iii 

Contents 

List of Figures iv

List of Tables iv

1. Background Information on SIRE 1

2. Construction of Physical Aperture 1

3. Description of Each Antenna 4

4. Comparison of Antennas 6

4.1 Comparison of Antenna #1..............................................................................................6 

4.2 Comparison of Antenna #2..............................................................................................7 

4.3 Comparison of Antenna #3..............................................................................................9 

4.4 Comparison of Antenna #4............................................................................................10 

4.5 Comparison of Antenna #5............................................................................................10 

4.6 Comparison of Antenna #6............................................................................................11 

4.7 All Antenna Measurements ...........................................................................................15 

5. Conclusions 15

Distribution List 16 
iv 

List of Figures 

Figure 1. Basic structure and dimensions of the ARL TEM horn (side view). ..............................2 

Figure 2. Basic structure and dimensions of the ARL TEM horn (rear view). ...............................2 

Figure 3. S11 Comparison of similarly constructed baluns. ...........................................................3 

Figure 4. Dimensions, resistor locations, and values for the resistively loaded plates of 

Antenna #1 and Antenna #5. ......................................................................................................5 

Figure 5. Contiguous resistive ink sheet used for the parallel plates of Antenna #6. .....................6 

Figure 6. Comparison of S11 measurements taken on Antenna #1 (441 Ω resistively loaded 

parallel plate transmission line with 6-cm flares and copper TEM horns faced outwards). ......7 

Figure 7. Comparison of S11 measurements taken on Antenna #2 (copper plate transmission 

line with 6-cm flares and copper TEM horns faced outwards). .................................................8 

Figure 8. Comparison of S11 measurements taken on Antenna #3 (copper plate transmission 

line with 3-cm flares and copper TEM horns faced outwards). .................................................9 

Figure 9. Comparison of S11 measurements taken on Antenna #4 (copper plate transmission 

line, no flares, and copper TEM horns faced outwards). .........................................................10 

Figure 10. Comparison of S11 measurements taken on Antenna #5 (same as Antenna #1 

with copper TEM horns faced inwards). ..................................................................................11 

Figure 11. Comparison of S11 measurements taken on Antenna #6 (contiguous resistive ink 

sheet transmission line, 6-cm flares, and copper TEM horns faced outwards). ......................12 

Figure 12. Comparison of S11 measurements taken on Antenna #6 with differences pointed 

out. ...........................................................................................................................................14 

Figure 13. All of Greg’s S11 measurements plotted together. .....................................................14 

List of Tables 

Table 1. Top resistive sheets. ........................................................................................................12

Table 2. Bottom resistive sheets. ..................................................................................................12
1 

1. Background Information on SIRE 

Detection of concealed weapons and explosives is critical to the Army’s mission in both Iraq and 

Afghanistan. To address this problem, the U.S. Army Research Laboratory has developed the 

Synchronous Impulse Reconstruction (SIRE) radar system, which has wall-, ground-, and 

foliage-penetration capabilities. SIRE is an ultrawideband (UWB) synthetic aperture radar

(SAR) imaging system, which can be mounted on top of a vehicle. The frequency range of 

operation is 300 MHz–3 GHz, which is a low enough frequency range to provide good 

penetration through lossy materials and a wide enough frequency band to achieve high 

downrange resolution. 

The aperture size of the SIRE system is constrained by the size of the vehicle upon which it is 

mounted. This physical aperture, created by placing a horizontal array of receive antennas side 

by side on top of the vehicle, provides the cross-range resolution. A synthetic aperture, formed 

by the forward motion of the vehicle, provides the height resolution. This allows the twodimensional aperture to give the cross-range resolution (from the physical aperture) and the 

height resolution (from the forward motion of the vehicle), resulting in a three-dimensional 

image. The bandwidth provides the downrange resolution (∆r = c/(2*B), where ∆r is the 

downrange resolution, c is the speed of light in free space, and B is the bandwidth). 

2. Construction of Physical Aperture

The performance of the SIRE radar system is dependent on the S11 parameters of the antenna 

array used to create the physical aperture, as the only losses of the system result from the losses 

in the antennas and baluns. The basic structure of these antennas consists of a two-plate 

transverse electromagnetic (TEM) horn that is open lengthwise on two of its four sides, where a 

resistive plate is attached to its emitting end. The feed end is cut short of a perfect point so that 

the TEM horns are electrically divided and the drive wires can be separately attached to the top 

and bottom of the feed. A balun connects the radar transmitter to the antenna. The antenna’s 

characteristic impedance must be 200 Ω in order for it to match the balun and prevent 

undesirable reflections. Figure 1 provides a side view of the basic construction of these antennas 

and figure 2 provides a rear view. 
2 

Figure 1. Basic structure and dimensions of the ARL TEM horn (side view). 

Figure 2. Basic structure and dimensions of the ARL TEM horn (rear view). 

We explored six configurations of this basic antenna structure in order to achieve the best S11 

parameters for our frequency range. In this report, we confirm the reproducibility of these six 

antenna configurations by comparing equivalent measurements for a set of these antennas 

constructed by Greg Smith to those of the same set of antennas constructed by the author. Good 

TEM 

Horn 

Feed width = 

4.6 mm 

Ø ≈9.25° 

202 mm

Feed Height 

= 4.6 mm

Total Length = 910 mm 

Length of Horn = 610 mm 

TEM Horn 

Plates 

Styrofoam 

Encasing Resistive 

Plates 

101 mm

136 mm 
3 

correlation in the measurements suggests that the antenna can be consistently reproduced. For 

those antenna designs for which discrepancies exist over certain frequency bands, a potential 

explanation for the discrepancy will be proposed. All measurements were taken with a Rhode & 

Schwartz ZVB4 Vector Network Analyzer. 

The first five sets of measurements were taken for antennas that shared the same basic physical 

structure, i.e., the same TEM horn printed circuit boards or plates (will be referred to as TEM#1), 

the same Styrofoam encasing, and the same balun (balun#1). Moreover, the only differences in 

these antenna configurations were the resistive plates, the arrangement of the TEM horns (copper 

plate facing inside or outside), and the addition of some external components to the basic 

structure of the antenna. The sixth antenna configuration, however, was made with a different 

TEM horn plate (TEM#2) and balun (balun#2). Both the TEM horn plates and the balun were 

constructed with the intent of making them identical to the TEM horn plates and balun found on 

the first five antennas, but because the intent of this report is to show that these six antennas can 

be consistently reproduced, it is important to point out that the sixth antenna configuration was 

designed on a reproduction of the original basic physical structure of the antenna. Since the 

antenna/balun was handmade, the antenna’s dimensions had an error bound of 3 mm for each 

measurement and the measured S11 parameters of balun#2 differed slightly from that of balun#1. 

The S11 parameters for balun#1 and balun#2 are compared in figure 3. 

Figure 3. S11 Comparison of similarly constructed baluns. 
4 

3. Description of Each Antenna

The dimensions of each antenna configuration (except the sixth whose dimensions were slightly 

off as explained previously) are given by figures 1 and 2. 

The characteristics of each antenna are as follows: 

• Antenna #1: The first antenna measured had a 441 Ω resistively loaded parallel plate 

transmission line (shown in figure 4) connected to each TEM horn, as well as a 6-cm 

radius, 180° cylindrical flare. The TEM horn plates were ascribed with the copper surface 

on the outside of the antenna (the other side simply being the dielectric substrate). 

• Antenna #2: In the second antenna measured, the same 6-cm radius, 180° cylindrical flares 

was kept, but the resistive plates were replaced with copper plates, which only had a 

resistance of 0.2 Ω. The copper surface of the TEM horns faced outwards. 

• Antenna #3: In the third antenna, the copper plates remained, but the 6-cm cylindrical flare 

was replaced by a 3-cm, 180° cylindrical flare. The copper surface of the TEM horns faced 

outwards. 

• Antenna #4: The fourth antenna only contained the copper plates, and all cylindrical flares 

were removed. The copper surface of the antennas faced outwards. 

• Antenna #5: The fifth antenna had the 441 Ω resistively loaded parallel plate transmission 

line, as well as the same 180°, 6-cm radius flares, but in this antenna the copper surface of 

the TEM horns faced inwards and the dielectric substrate faced outwards. 

• Antenna #6: It is important to reiterate that the final measurement was taken on the second 

antenna structure using balun #2. In this antenna, the parallel plate resistive sheets were 

replaced by the three contiguous resistive sheets shown in figure 5. These contiguous 

resistive sheets were created using a special dielectric ink spread over top each of the 

plates. The thickness and concentration of this ink determined the actual resistive values 

obtained. Because there was no uniform way to spread the ink and control the thickness of 

each coat, the resistive values of the plates were not uniform (although the concentration of 

the ink used was consistent). 
5 

441 Ω Resistively Loaded Parallel Plate Transmission Line 

Figure 4. Dimensions, resistor locations, and values for the resistively loaded plates of Antenna #1 and Antenna #5. 

47.625mm 46.228mm 37.592mm 23.114mm 14.478mm 8.636mm 

3.175m 

374Ω 249Ω 154Ω 90.9Ω

61.9Ω

Total Resistance = 441Ω

Aperture 

(In/Out) 

To 

Triangular 

Plates 
6 

Figure 5. Contiguous resistive ink sheet used for the parallel plates of Antenna #6. 

4. Comparison of Antennas

This section compares the measurements made by Greg on each particular antenna to the 

measurements I made on the same antenna. Both measurements were taken with a Rhode & 

Schwartz ZVB4 Vector Network Analyzer in the same room, under generally the same 

conditions. The only difference in our measurements was that Greg took 400 samples of data 

within the 300–30000 MHz frequency range, whereas I used 800 samples. 

4.1 Comparison of Antenna #1 

Figure 6 shows the comparison between Greg’s and my measurements for Antenna #1. As can 

be seen, the two plots are nearly identical, and thus it is safe to conclude that Antenna #1 can be 

consistently reproduced. 

103mm 

29mm 67mm 

≈185Ω

≈12Ω ≈15Ω

Rear Sheet 

Middle 

Sheet Front Sheet 

Aperture 

(In/Out) 

To 

Triangular 

Plates 
7 

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Frequency (GHz)

S11 (db)

Greg's Data

Verification

Measurements

Pink graph's file All_components_attached_cu_outside_tem#4_balun#4_441ohms_circle_6cmradius_tipwidth_5.4mm_tipheight_4.6mm_

withfeed_db

Blue graph's file tem_4_balun4_441_res_6cm_rad_5.4w_4.6h_feed_complex Both are on L drive, in Steven_Wienecke directorate

Antenna #1

`

Figure 6. Comparison of S11 measurements taken on Antenna #1 (441 Ω resistively loaded parallel plate 

transmission line with 6-cm flares and copper TEM horns faced outwards).

4.2 Comparison of Antenna #2 

Figure 7 shows the comparison for Antenna #2, where the 6-cm radius flares were kept, but the 

resistively loaded plates of figure 4 were replaced by copper plates with a resistance of 0.2 Ω. 

There were a few minor deviations between our recorded data, but as the figure shows, the two 

sets of data are extremely close to one another. Where differences did exist, my measurements 

were consistently higher than Greg’s measurements. The places in the graph where at least four 

points on Greg’s plot differed from the corresponding eight points of my plot by at least 1 dB are 

marked with ellipses. The rest of the comparisons throughout this report hold to the same 

criteria. 
8 

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Frequency (GHz)

S11 (db)

Greg's

Data

Verification

Data

Antenna #2

Pink file 2nd_verify_no_resistive_plates_cu_outside_tem#4_balun#4_circle_radius6cm_tipwidth_5.4mm_tipheight_4.6mm_withfeed_db

Blue file TEM_4_200ohm_6cm_rad_no_res_5.4mm_wx4.6mm_h_feed_complex Both in L drive, in Steven_Wienecke directorate

Figure 7. Comparison of S11 measurements taken on Antenna #2 (copper plate transmission line with 6-cm 

flares and copper TEM horns faced outwards).

All but one of the circled differences for the data on Antenna #2 were less than or equal to 

2.5 dB. The largest difference (in terms of dB) occurred around 1.5 GHz, with an 8-dB 

difference in our measurements. However, both measurements at this frequency had an S11 less 

than –17 dB, indicating that this relatively large dB difference corresponds to a very small 

difference in terms of actual percentage of power reflected at 1.5 GHz (roughly a difference of 

1.5% of the total supplied power). 

The small differences seen in our graphs may be attributed to a number of different factors, one 

of which being the sensitivity of the Vector Network Analyzer. Greg and I assumed that any 

reflections from the room would produce negligible differences in the S11 data, so we did not 

time gate the measurements. However, of course, reflections from the room did occur. 

Moreover, the measurements were performed on different days, so it is conceivable that certain 

objects in the room may have been moved around during the time lapse from whence the two 

measurements were taken. Thus it is possible that differences seen at the lower dB values (since 

at the lower dB values a larger difference in dB corresponds to a smaller percentage of total 

reflected power than at higher dB values) may have been caused by the differences in the 

reflections received from the room as measured by the Vector Network Analyzer. This is a 

likely explanation of the deviation seen at 1.5 GHz. 
9 

Additionally, any time the coaxial cables are bent to attach/detach the cable to/from the balun of 

the antenna, the tension and compression exerted on the dielectric in the cable will change the 

impedance of the cable. This can cause a slight impedance mismatch between the cable and the 

balun of the antenna, resulting in a higher value of the measured S11. Thus, if Greg and I were 

to have made the measurements of the antenna with the cable oriented in different positions, 

there is a strong possibility the measured S11 parameter would be different due to different 

degrees of mismatch between the cable and balun. Furthermore, before conducting any of my 

own measurements, I detached and subsequently reattached the two TEM horns to the balun, and 

in the process slightly changed the feed height from what Greg had. This difference, coupled 

with the variation in the amount of solder used in the antenna/balun configuration, may have 

been a contributing factor as well. 

Despite these minor disparities, the fact that our plots have identical shape and very close dB 

values verifies the reproducibility of Antenna #2. As will be seen, this holds true for Antennas 

#3–#5 as well. Because the causes of the small differences in these antennas are the same as for 

Antenna #2, the proposed explanation for these discrepancies is omitted for Antennas #3–#5 in 

the coming comparisons. 

4.3 Comparison of Antenna #3 

Figure 8 shows the comparison for Antenna #3, where the copper plates were kept, but the 6-cm 

radius flares were replaced by 3-cm radius, 180° cylindrical flares. This graph clearly indicates 

the reproducibility of Antenna #3. 

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Frequency (GHz)

S11 (db)

Greg's

Data

Verification

Data

Antenna #3

Pink file 2nd_verify_no_resistive_plates_cu_outside_tem#4_balun#4_circle_3cmradius_tipwidth_5.4mm_tipheight_4.6mm_withfeed_db

Blue file TEM_4_200ohm_3cm_rad_no_res_5mm_wx4.6mm_h_feed Both in L drive, in Steven_Wienecke directorate

Figure 8. Comparison of S11 measurements taken on Antenna #3 (copper plate transmission line with 3-cm 

flares and copper TEM horns faced outwards).
10 

4.4 Comparison of Antenna #4 

Figure 9 is a comparison of Antenna #4, where the copper plates were kept, but all cylindrical 

flares were removed. Again, the graph confirms the reproducibility of this antenna. 

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Frequency (GHz)

S11 (db)

Greg's Data

Verification

Data

Antenna #4

Pink graph's file 2nd_verify_no_resistive_plates_cu_outside_tem#4_balun#4_tipwidth_5.4mm_tipheight_4.6mm_withfeed_db

Blue graph's fileTEM_4_200ohm_no_res_5.4mm_wx4.6mm_h_feed_complex Both in L drive, in Steven_Wienecke directorate

Figure 9. Comparison of S11 measurements taken on Antenna #4 (copper plate transmission line, no flares, and 

copper TEM horns faced outwards).

4.5 Comparison of Antenna #5 

Figure 10 is a comparison of our measurements for Antenna #5, which was very similar to the 

first, in that it had the 441 Ω resistively loaded parallel plates, as well as the 180°, 6-cm radius 

cylindrical flares. However, in this antenna the copper face of the TEM horns faced downward. 

The graph verifies the reproducibility of Antenna #5.
11 

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Frequency (GHz)

S11 (db)

Greg's Data

Verification

Data

Antenna #5

Pink graph's file verify_cu_inside_tem#4_balun#4_441ohms_circle_6cmradius_tipwidth_5.4mm_tipheight_4.6mm_withfeed_complex 

Blue graph's file tem_4_balun4_441_res_6cm_cu_inside_rad_5.4w_4.6h_feed_complex

Figure 10. Comparison of S11 measurements taken on Antenna #5 (same as Antenna #1 with copper TEM 

horns faced inwards). 

4.6 Comparison of Antenna #6 

Figure 11 shows the comparison for Antenna #6, where three contiguous resistive sheets were 

attached to each TEM horn in place of either the copper plates or the 441 Ω parallel resistive 

plates. Although Greg and I both used the same antenna structure (Styrofoam encasing, TEM 

horns, and balun) to take our measurements, it should be noted that the antenna structure and 

balun used were not exactly the same as those used in the previous measurements, as explained 

in section 3. 
12 

Antenna #6

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Frequency (GHz)

S11 (d b )

Greg's

Measurements

Verification

Measurements

file is TEM#3, balun#3, Ink plates, no circles in L drive under Steven_Wienecke directorate

Figure 11. Comparison of S11 measurements taken on Antenna #6 (contiguous resistive ink sheet 

transmission line, 6-cm flares, and copper TEM horns faced outwards).

Perhaps more importantly, it must be stated that the resistive values used for the contiguous 

resistive sheets in the antenna I measured were not the same as those used by Greg. As 

mentioned previously, there was no uniform way for us to spread the special dielectric ink and 

control the thickness of each coating, making it difficult to create resistive plates with the desired 

resistance. Table 1 compares the resistances of the sheets Greg used with the values I used for 

the top-side of the antenna. Table 2 gives the values for the resistive sheets used on the bottom of 

the antenna. There was a difference in our data at the lowest part of the frequency range, but our 

data were almost the same at frequencies higher than 450 MHz. 

Table 1. Top resistive sheets. 

 My Resistive Sheets Greg’s Resistive Sheets 

Rearmost sheet 183.5 Ω 189.5 Ω

Middle sheet 9.7 Ω 12 Ω

Front sheet 12.9 Ω 15.6 Ω

Table 2. Bottom resistive sheets. 

 My Resistive Sheets Greg’s Resistive Sheets 

Rearmost sheet 178.9 Ω 189.5 Ω

Middle sheet 10.3 Ω 12.8 Ω

Front sheet 14.1 Ω 15.6 Ω
13 

The major difference that occurs in this graph is between 100 MHz and 450 MHz, where Greg’s 

data were 1–3 dB worse than mine. Though it is only a few dB different, the difference covers a 

larger frequency range than any of the other previous comparisons, suggesting its cause may be 

different as well. Because the only disparity between the antennas we measured were the exact 

values of the ink plates, it seems reasonable to believe that they were the cause. In selecting the 

resistance values for our ink plates, we tried to match the values to those used in a paper written 

by Kurt L. Shlager, Glenn S. Smith, and James G. Maloney1. Although the paper states that they 

were restricted by the resistive values of the ink plates available to them, they were able to 

compute the optimal resistance value for their antenna using the finite-difference time-domain 

(FDTD) method. 

The problem with using their values is that both the resistance and dimensions of our TEM horn 

antenna differ from the TEM horn antenna that they used. This means that the values we used 

were not the optimal ones for our antenna. 

Although my measurements were only a few dB better than Greg’s (and this difference was 

restricted to the lowest frequency range), finding the best resistance value for our ink plates may 

be worth exploring. For one thing, the performance of the balun is a limiting factor to the overall 

performance of the antenna, but looking back, figure 3 shows that our balun performs very well 

in this frequency range. This indicates that there is room for improvement, in this frequency 

range, to be made through modifications to the rest of the antenna. Another reason for 

optimization is the fact that within the desired frequency range of SIRE (300 MHz–3 GHz) our 

antenna currently performs the worst between 300 and 400 MHz, and thus it is especially 

important to improve our antennas performance in this area. Finally, overall, Antenna #6 

performed the best out of all the antennas (as shown in figures 12 and 13), and thus more time 

and effort should be devoted to optimizing this antenna. Moreover, once the correct values are 

found, a better method must be found for uniformly spreading the dielectric ink so that a 

consistent resistance value of the sheets will be obtained, in turn increasing the reproducibility of 

the antenna. 

 

1 Shlager, K. L.; Smith, G. S.; Maloney, J. G. TEM Horn Antenna for Pulse Radiation. Microwave and Optical 

Technology Letters Dec 2 1996, 12 (2), 86-90.
14 

Comparison of All Greg's Data

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Frequency (GHz)

S11 (d b )

Antenna #1

Antenna #2

Antenna #3

Antenna #4

Antenna #5

Antenna #6

Figure 12. Comparison of S11 measurements taken on Antenna #6 with differences pointed out.

Comparisons of Verification Data

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Frequency (GHz)

S 11 (d b )

Antenna #1

Antenna #2

Antenna #3

Antenna #4

Antenna #5

Antenna #6

Figure 13. All of Greg’s S11 measurements plotted together. 
15 

It is common to define an S11 of –10 dB at a certain frequency as the highest S11 allowed for 

that specific frequency to be considered within the bandwidth of the antenna. For many 

applications, an S11 higher than this is not sufficient. The Antenna #6 that I constructed has an 

S11 ≤ –10 dB from 300 MHz on up, indicating that this antenna operates adequately or better at 

every frequency within SIRE’s frequency range. However, the antenna Greg constructed does 

not consistently have an S11 ≤ –10 dB until ≈430 MHz, meaning his antenna would not perform 

adequately over SIRE’s frequency range. This suggests, at least when spreading the dielectric 

ink in a similar fashion to how Greg and I did, that some reproductions of the antenna may 

perform adequately over SIRE’s frequency range, while others may not. Thus, unless a better 

method for spreading the dielectric ink is introduced, one cannot say with full confidence that 

this antenna can be properly reproduced 100% of the time. 

4.7 All Antenna Measurements 

Figure 12 displays all of the measurements Greg made on one graph, and figure 13 shows all the 

measurements I made. 

5. Conclusions

After comparing the measurements taken for the first five antennas, it can be seen that antennas 

#1–#5 can be consistently reproduced. 

For Antenna #6, the S11 improvement seen at the lower frequencies produced by a change in the 

values of the resistive ink plates suggests it may be worth computing the optimal values of the 

contiguous resistive plates in order to improve the antenna’s performance in the 300–450 MHz 

range. As Antenna #6 performed the best out of all the antennas, it seems logical that more time 

be spent in optimizing it, in comparison to the other antennas. Moreover, once it is optimized, a 

better method must be found to spread the dielectric ink in order to obtain consistent resistance 

values for the resistive sheets, and in turn increase the reproducibility of Antenna #6. 
16 

NO. OF 

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