Abstract— This study characterizes 5.8 GHz backscatter radio

links in a transient, high-voltage power line environment. The

measured results demonstrate how increased RF carrier

frequency provides additional resistance to the noise,

interference, and corona shielding of communication antennas

that operate near high-voltage lines. The results lead to

important design rules for low-powered wireless sensor

applications deployed in the future smart grid.

I. INTRODUCTION

It has been known for years that high-voltage transmission

systems produce above average levels of radio interference,

providing an extra challenge for radio frequency (RF) system

designers [1]. These links must be resilient in the harsh,

charged-particle and impulsive-noise power systems

environment in order to provide efficient communication. For

instance, Wang et al. [2] have measured the performance of an

802.11b link in a high-voltage environment and determined

that the link’s data rate is reduced 80% by the electromagnetic

interference generated by the breakdown of sulfur

hexafluoride (SF6) gas, a common dielectric for high-voltage

applications. Due to this unreliability and the short lifetimes

associated with sensor power supplies, these types of systems

have been largely avoided by the power industry.

The benefits of using radio links in high-voltage

environments are pivotal to the successful implementation of a

“smart power grid” [3] [4]. The principle motivating

application for the links study in this paper is remote fault

detection at power substations. For example, capacitor banks

at wind farms such as in Fig. 1 are dynamically switched in

order to balance the ever-changing power factor – a scenario

that requires careful equipment monitoring and sensing.

Outfitting these substations with a 5.8 GHz backscatter radio

current-sensing system would provide the information

necessary to efficiently and effectively manage the power

factor with robust, battery-less radio sensors [5].

The work reported in this paper was sponsored in part by Southern States,

LLC and the National Science Foundation (NSF) Career Grant #0546955.

Remote fault detection is also pivotal in avoiding

catastrophic substation failures. These failures, possibly

resulting in fires and explosions, can be caused by sudden

equipment malfunctions, switching transients, and lightning

strikes [6]. Wired monitoring solutions are difficult and

unsafe across the high-voltage insulation gap due to dielectric

breakdown; current transformer (CT) tanks that perform a

wired measurement of high voltage lines inside an insulating

oil tank are expensive and themselves present a failure point

and fire hazard. 5.8 GHz backscatter is well-suited for this

high-voltage environment because it is extremely low power

(instant turn-on, which is required for fault detection), high-

frequency (resistant to corona shielding), and secure (short-

range, difficult to jam/spoof/intercept).

Reliable radio links along with low-power sensors would

allow real-time monitoring of a variety of power line

conditions, including fault detection, temperature, and line

Backscatter Channel Measurements at 5.8 GHz

Across High-Voltage Corona

Christopher R. Valenta, Patrick A. Graf, Matthew S. Trotter, Gregory A. Koo, and Gregory D. Durgin

School of Electrical and Computer Engineering, Georgia Institute of Technology

777 Atlantic Dr., Atlanta, Georgia 30332-0250

Email: cvalenta3@gatech.edu, pgraf3@gatech.edu, mtrotter@gatech.edu, gkoo3@gatech.edu,

durgin@ece.gatech.edu

Bradley J. Schafer

Southern States, LLC.

30 Georgia Ave., Hampton, Georgia 30228

Email: b.schafer@southernstatesllc.com

Fig. 1. Image of high-voltage switches for capacitor banks at a wind farm in

Illinois. Due to the constantly changing power factor that results from the use

of wind turbines, capacitors must be switched into and out of the grid to cancel the effects of reactive power on the lines.

sag. Sensors using backscatter radio, particularly at 5.8 GHz,

allow many of the aforementioned problems to be avoided [7].

This paper first presents background on the radio

interference of high-voltage environments and SF6 gas in

particular. Next, a description of the RF testbed is discussed.

The radio interference corona on a 5.8 GHz backscatter link is

then measured in the following section. Finally, a list of

design rules for backscatter sensors or RF devices operating in

high-voltage environments is presented.

II. DESCRIPTION OF HIGH-VOLTAGE ENVIRONMENT

The high-voltage environment exhibits several key

behaviors that affect physical layer RF channels, many of

which differ from conventional wireless systems. These

attributes include additional corona noise, impulsive noise,

and plasma shielding on line-potential antennas. These

attributes result from the characteristics of the strong electric

fields that build up around high-voltage power lines. Corona

generation and arc flashovers, both impulsive and steady state,

may also be present in this type of environment. Collectively,

these physical effects lead to channels that are far more data-

hostile than conventional wireless links.

A. Radio Interference Sources

Corona describes the particle or dielectric breakdown of the

medium surrounding a conductor, usually air or SF6. It begins

in areas of high electric field gradients, such as sharp points,

corners, wires, and water droplets. Corona generation occurs

when the electric field intensity exceeds that of the dielectric

strength of the surrounding material.

Two types of corona are possible, but similarly affect the

radio link. Around the cathode (negative corona), freed

electrons create secondary avalanches of additional electrons,

which are generated as a result of the photoelectric effect from

the cathode. This process will continue if the voltage is

maintained until it reaches a steady state of ion formation and

recombination. Around the anode (positive corona), corona

occurs due to the same process, but the secondary avalanche is

generated by the gas surrounding the plasma and not the anode

itself. [8] The ion clouds are conductive and will produce

radio interference upon charge recombination, sputtering

broadband RF interference across all bands and potentially

leading to plasma shielding of entire devices of any material

and their antennas. Other potential effects of corona include

generation of visual light, audible noise, energy loss,

mechanical vibrations, and chemical reactions (ozone and

nitrogen oxide generation) [9].

B. Radio Channel Effects

Corona and arc flashover events add both broadband and

impulsive noise that result from the time-dependent transition

times of the positive and negative corona and the flashover

discharge time [10]. While the majority of spectral energy for

these pulses exists at frequencies less than 200 kHz, there are

significant contributions through the 100’s of MHz as

measured by [11]. As expected and measured by Sporn and

Monteith [1], this energy is directly proportional to line

voltage. Moreover, the energy at high frequencies is inversely

proportional to the transition time. Therefore, a 5.8 GHz

system should experience little effect from time-dependent

noise sources.

When in direct proximity to the corona source, a

phenomenon known as plasma shielding may occur. At high-

voltages, the air around the conductor becomes plasma. The

volume of this plasma is directly proportional to the line

voltage and dependent on the system geometry [8]. The

electrons in this cloud create a conductive medium that

prohibits wave propagation beneath a certain cutoff frequency

[11]. The communication failure can be attributed to the large

electron density in the proximity of the corona source; these

charge clouds act as an invisible Faraday cage that surrounds a

communicating antenna at line potential. This occurrence is

the same phenomenon that occurs when spacecraft re-enter the

Earth’s atmosphere. In this case, it is the intense heat of re-

entry that forms electrical plasma around the spacecraft that

effectively shields all radio communication below a certain

frequency. This critical frequency ( in Hz) is related to the

number of electrons per cubic meter ( ) and is listed

below as Equation 1 [12].

(1)

The use of higher microwave frequencies is particularly

effective at penetrating into these plasmas, although there are

still upper-limit thresholds of charge density that can shield

the RF tag from communications.

C. Use of SF6 Gas

The first industrial use of SF6 gas for power switching was

performed by Westinghouse in the early 1950’s [13]. SF6 gas

was identified as having excellent thermal and dielectric

properties that helped to overcome the technical challenges of

extinguishing high-voltage arcs. The high dielectric strength

allows smaller gaps between high-voltage components

reducing the overall size and weight of devices. The thermal

properties of SF6 allow the gas to better cool arcs during

switching operations. While the choice of this gas helps to

improve equipment performance these qualities also cause

shorter arc transition times (on the order of nanoseconds) that

produce higher frequency electromagnetic emissions.

Therefore, devices that use this dielectric may potentially

generate more harmful, high frequency radio interference than

devices that use air gaps.

III. MICROWAVE 5.8 GHZ TESTBED AND RF TAG

The 5.8 GHz testbed consisted of a transceiver in a bi-static

antenna arrangement shown in Fig. 2, a signal generator, and

PC used for sampling and data processing. The system was

assembled as shown in Fig. 3 [14]. The direct-conversion

receiver operates in the range of 5725-5850 MHz in the ISM

band. It then converts the in-phase and quadrature elements of

the received signal to DC where it is sampled by an analog-to-

digital converter and processed [15]. A signal generator is

used to produce the 5.79 GHz carrier wave for the transmitted

signal and also provides a frequency reference for the receiver

down conversion. The receiver also has the capability of

providing an automatic gain control. This gain is controlled by

a PC, which uses an algorithm to adjust the DC offset of the

receiver.

The RF tag in Fig. 4, made from two-layer FR4 printed

circuit board, is capable of being reprogrammed to transmit

any variety of digital signal code and modified to accept any

number of external sensor inputs. These signals can then be

backscattered back to the receiver for processing. A 3-volt

coin cell battery powers the tag’s electronics, but does not

contribute to the tag’s data transmission.

For these measurements, the RF tag repeated a 500 kHz

pseudo-random (PN) sequence with a length of 31 bits. This

sequence was programmed in the C-language via a 6-pin

header to USB cable. This spread spectrum sequence allowed

the communication system to spread its frequency content

along a range of frequencies making it less prone to

interference and narrowband jamming. To reclaim the tag’s

data, the receiver performs a cross-correlation of the down-

converted received signal with an identical, locally generated

PN sequence. This cross-correlated data was then normalized

with the auto-correlated, locally generated PN sequence. The

normalized data was then time-averaged 16 times to produce a

reliable complex channel coefficient that measures signal

levels on the in-phase and quadrature backscatter components.

This measurement technique is discussed exhaustively by

Griffin in [13].

The PN sequence was generated on the RF tag by using a

Microchip PIC microcontroller (PIC18F2520). This

microcontroller allows for clock speeds up to 40 MHz and

micro-watt operation [15]. The 31-bit sequence can be

generated by using a 5-bit shift register. The outputs of the 2nd

and 5th

registers were XOR-ed together and fed into the input

of the 1st register in order to generate this sequence. As shown

in Fig. 5, this microcontroller was connected to a microwave

gallium arsenide, pseudo-morphic, high electron mobility

transistor (PHEMT) switch (M/A-Com MASW-007207 V2).

This switch allowed the RF tag to change the 5.79 GHz patch

antenna’s load between an open and short-circuited state.

These opposite impedances were chosen to modulate the

incoming signal with maximum backscatter power.

A. Experimental Design

To measure the effects of corona noise on backscatter

communication, the channel coefficient of a 5.8 GHz

backscatter communication system was measured for various

voltage levels. This channel coefficient describes how a

signal’s amplitude and phase are affected by the operating

environment. The channel coefficient is also proportional to

the signal power which in turn gives path loss.

Fig. 2. Image of the 5.8 GHz transceiver with internal boards connected. The

direct conversion receiver uses two, four-layer, FR4 printed circuit boards, an RF front-end and a baseband amplification board [14].

Fig. 3. Block diagram of the 5.8 GHz communication system in bi-static

antenna configuration. BBI and BBQ represent the base-band in-phase (BBI)

and quadrature (BBQ) signals, respectively. BB1 and BB2 connect two internal boards in the receiver.

Fig. 5. Circuit diagram of the 5.8 GHz RF tag, modified from [16].

Fig. 4. Image of 5.8 GHz RF tag. The RF tag is printed on a 2 layer FR-4

PCB and contains a 5.8 GHz slot antenna, switching transistor to change the

antenna’s impedance between and open- and short-circuit antenna loads, and

a microcontroller to produce the PN sequence.

A high-voltage production test set, which uses a transformer

to create high AC voltage, was used to generate voltages from

0 V to 250 kVrms line-to-ground (0 V to 425 kVrms line-to-

line) along a 16 gauge aluminum wire which will act as the

corona source. Corona is generated at points at which the

voltage gradient is the greatest. Thus, this very thin wire

represents a worst-case scenario for the occurrence of corona.

Commercial power lines would never be this small, so the

corona generated by this wire at these voltages will be more

disruptive than that of any power line at similar voltages.

A semi-passive RF tag was placed on a movable platform

near the corona source as seen in Fig. 6. In this way, space

averaging could be performed in order to cancel out the effects

of multipath interference and still allow for corona to exist in

the proximity of the tag.

B. Calibration

A calibration procedure was followed to ensure that the

recorded data could be normalized to a known set. A step

attenuator was placed in series with the receive antenna and

the receiver to simulate signal attenuation. With the high-

voltage set off and the tag repeating its PN code, the channel

coefficient was measured for various values of attenuation.

Using this data, it could be determined what ‘optimal’

conditions were and where the noise floor existed. This data

could then be compared with data from the corona tests to see

if there were any deviations.

C. Measured Results

It was discovered that a backscatter link at 5.8 GHz is

largely resistant to the effects of corona noise at a distance of

approximately 140 cm. As shown in Fig. 7, the signal power

was unaffected by the presence of corona noise.

One can see the signal power remains above the noise floor

caused by the corona noise for all values of voltage up to 250

kV line-to-ground (425 kV line-to-line). In fact, channel levels

remain relatively unchanged from ideal levels while in the

presence of corona. The small variations on the order of 2-3

dB in the power can be attributed to thermal noise and

measurement error.

Ideal values were calculated by measuring the signal power

with the RF tag in the absence of corona. Likewise,

background noise was measured in the absence of corona and

the forward transmitted signal.

After seeing the resistance of a 5.8 GHz backscatter radio

link at distance of 140 cm to a corona source, the RF tag was

moved 10 cm away from the corona source as shown in Fig. 8.

In this case, the backscattered signal should be more affected

by the corona noise. The same testing procedure was repeated

as previously discussed.

In this case, the backscatter link failed above 150 kVrms

line-to-ground (255 kVrms line-to-line) as shown in Fig. 9.

While at 50 kVrms line-to-ground (85 kVrms line-to-line), the

link remains unchanged from the ideal case. However, when

voltage is increased to 100 kVrms line-to-ground (170 kVrms

line-to-line), the signal power decreases by 5dB, but this value

remains above the noise floor, so communication is still

possible. As the voltage continues to increase, the corona

charge density increases as well until it effectively shields

communication at values above 150 kVrms line-to-ground

(255 kVrms line-to-line).

Fig. 8. Image of tag positioning 10 cm from corona source (tip of wire). This

wire was then attached to the high-voltage production set. The tag was

attached to a wooden pole using electrical tape.

Fig. 7. Corona effects on signal power including noise floor and ideal values,

and curve fits. ‘w/ tag on’ refers to data taken when the RF tag was functional

and corona was present. ‘w/ tag off’ refers to data taken when the RF tag was

not functional and only corona was present.

Fig. 6. Diagram of corona noise measurement test setup. The tag was

positioned on a moveable ceramic platform near the corona source so that

space averaging could be used to reduce multipath.

IV. CONCLUSION

The 5.8 GHz backscatter system worked well in the high-

voltage power environment, revealing several interesting

design parameters for radio systems in this area. While the

system was not impervious to all corona and impulsive-noise

conditions, it has been shown that through rudimentary

hardware modifications and communication layer protocol

implementations, these problems may be overcome.

Based on the findings presented in this paper, as well as

other portions of the high-voltage microwave link

environment, the following design rules for designing

microwave backscatter sensors and RF devices in general are

suggested [5] [17].

1. Keep digital symbols larger than 1 microsecond and add

forward error correction or error detection to maintain

reliability during arc-over events. This will prevent

lightning and other arc-over events from making burst

errors in the communication link.

2. Add low-pass filters with break points around 250 MHz

to receiver antennas to filter out the bulk of air-gap arc

over, SF6 spark noise, and broadband corona noise.

3. Small, resonant antenna types with less than 3%

bandwidth (like the patch) are preferred for use as

transmit, receiver, or backscatter antennas since they

provide an extra level of filtering for the broadband

noise sources of the high-voltage environment.

4. Extra shielding is needed around all sensor and

transceiver electronics – baseband circuits as well as RF

devices – to keep impulsive noise from resetting logic

circuits and corrupting signal lines.

5. Higher carrier frequencies for in-line sensors should be

used as the line voltage increases, as higher UHF and

microwave frequencies more effectively penetrate

corona plasmas. The additional RF noise due to corona

also drops sharply at higher, microwave frequencies.

6. Avoid sharp points and corners in the design of antennas

and physical layout to diminish the build-up of

additional corona.

ACKNOWLEDGMENT

The authors would like to thank Rusty Ortkiese of Southern

States, LLC for his assistance with the high-voltage

measurements and insight into high-voltage environments.

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Fig. 9. Close range corona effects on signal power. Note how with increasing

voltage, the signal power decreases corresponding to a negative effect on the

communication between the tag and the receiver.