Bias teeFrom Wikipedia, the free encyclopedia

A bias tee is a three port network used for setting the DC bias point of some electronic components without disturbing other components. The bias tee is a diplexer. The low frequency port is used to set the bias; the high frequency port passes the radio frequency signals but blocks the biasing levels; the combined port connects to the device, which sees both the bias and RF. It is called a tee because the 3 ports are often arranged in the shape of a T.

Contents

 [hide] 

1 Design 2 Application 3 Construction

o 3.1 A particular construction 3.1.1 Capacitor 3.1.2 Coil 3.1.3 Oscillations

4 See also 5 References 6 External links

[edit] Design

Equivalent circuit of a bias tee.

Conceptually, the bias tee can be viewed as an ideal capacitor that allows AC through but blocks the DC bias and an ideal inductor that blocks AC but allows DC. Although some bias tees can be made with a simple inductor and capacitor, wideband bias tees are considerably more complicated because practical components have parasitic elements.

Bias tees are designed for transmission line environments. Typically, the characteristic impedance Z0 will be 50 ohms or 75 ohms. The impedance of the capacitor (XC) is chosen to be much less than Z0, and the impedance of the inductor (XL) is chosen to be much greater than Z0.

Where ω is the frequency in radians per second and f is the frequency in hertz.

Bias tees are designed to operate over a range of signal frequencies. The reactances are chosen to have minimal impact at the lowest frequency.

For wide range bias tees, the inductor must be large at the lowest frequency. A large inductor will have a stray capacitance (which creates its self-resonant frequency). At a high enough frequency, the stray capacitance presents a low impedance shunt path for the signal, and the bias tee becomes ineffective. Practical wide band bias tees must use circuit topologies that avoid the shunt path. For example, a Picosecond Pulse Labs 5580 works from 10 kHz to 15 GHz. (Andrews 2000, p. 3) Consequently, the simple design would need an inductance of at least 800 μH (XL about j500 ohms at 10 kHz), and that inductor must still look like an inductor at 15 GHz. However, a commercial 820 μH inductor has a self-resonant frequency of only 1.8 MHz -- four orders of magnitude too low.[1]

[edit] Application

A bias tee is used to insert DC power into an AC signal to power remote antenna amplifiers or other devices. It is usually positioned at the receiving end of the coaxial cable to pass DC power from an external source to the coaxial cable running to powered device. A bias “T” consists of a feed inductor to deliver DC to a connector on the device side and a blocking capacitor to keep DC from passing through to the receiver. The RF signal is connected directly from one connector to the other with only the blocking capacitor in series. The internal blocking diode prevents damage to the bias “T” if reverse supply voltage is applied.

Biasing for photodiodes (vacuum and solid state), Microchannel plate detectors, transistors, and triodes. High frequencies are not leaking into a common power supply rail and noise from the power supply does not appear on the signal line. Bias "T" has been used in a variety of applications, but is generally used to provide an RF signal and power to a remote device where running two separate cables would not be advantageous.[2] Examples of this are: Power over Ethernet [3] [4] , active antennas, low-noise amplifiers, and down converters[5]

[edit] Construction

There are several bias tee designs.

[edit] A particular construction

The construction of the horizontal bar of the T is based on the rigid coaxial cable with air as dielectric. The radius is chosen to be as large as possible without allowing higher modes. The design of a bias "T" is based upon power going out to the remote device, but not being seen by the base station or receiver. It does this by using a capacitor on the RF output terminal, effectively creating an open circuit for the DC current.[6] The incoming RF signal, or the one from the antenna, is the output for the DC power. This front-end of a bias "T" typically consists of a bandpass filter, a low noise amplifier, and a mixer coupled to a local oscillator.[6]

[edit] Capacitor

At one point a small slice is cut out of the center conductor, therefore a capacitor is formed and low frequencies are blocked. This kind of capacitor has the advantage that it is nearly invisible to higher frequencies. To pass frequencies down to 1 MHz the capacitance has to be increased. A dielectric like NPO multiplies the capacitance by a factor of 65. The thickness of the capacitor has to be minimal without leading to electric breakdown in the dielectric, this means to avoid any peaks in the electric field and this means smooth electrodes with rounded edges and a dielectric protruding between the electrodes (doorknob design). A stack of capacitors can be used, but every capacitor needs access to the surface of the inner conductor, because if it's hidden behind another capacitor the high frequencies won't see it, because the electric field needs a lot of time to travel through a dielectric with a high dielectric constant

[edit] Coil

A small coil made of fine wire with an air core or MnFeZn-core connects the inner conductor of one of the sides of the capacitor with the a port in the outer conductor leading down the T. Frequencies above 1 GHz hit the coil from the side and apply an equal electric field to the whole coil. Therefore no higher modes are excited within the coil. Because of the inductiveness of the coil almost no current leaks from the center conductor to the port. Frequencies between 1 MHz and 1 GHz do leak into this port, so there is a second coil with a cone shaped core outside of the outer conductor, but inside of a housing to avoid interference with other components. This cone acts like a tapered transmission line transformer. It starts with a high impedance, so a lot of power will be reflected, but the rest will travel down the coil and there is some leakage into the low frequency port.

[edit] Oscillations

Any oscillations in the capacitor or the coil or the composed LC circuit are damped by the dielectric and the core. Also the small coil should have about 10 ohm resistance to further damp oscillations and avoid ripple on the transmitted spectrum.

SMA, Type N, and 7/16's Bias T's

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These bias t-s (aka bias tee's) allow a dc voltage to be inserted into an rf path with a minimal loss to the rf. Key specs are loss, isolation, frequency, power handkling, 7 connector type.

SB3000BIAS T 10MHZ-3GHZ VSWR 1.5SMA-F'S & PIN 3AMPSSpecs (pdf):

SB3000.PDF

Price (1-9):

$228.15

Price $218.0

SB6000BIAS T .1-6GHZ VSWR 1.5SMA-F'S & PIN 500maSpecs (pdf):

SB6000.PDF

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$256.04

Price (10- $245.4

SB6050BIAS T .01-6GHZ 50VSMA-SMC-SMA 100maSpecs (pdf):

SB6050.PDF

Price (1-9): $194.59Price (10-24): $184.35Price (25-49): $179.22Qty On Hand:

4

SB18000BIAS T .1-18GHZSMA-F'S & PIN 700maSpecs (pdf):

SB18000A.PDF

Price (1-9): $435.26Price (10-24): $419.90Price (25-49): $409.66Qty On Hand:

7

(10-24): 1Price (25-49):

$207.87

Qty On Hand:

17

24): 9Price (25-49):

$236.67

Qty On Hand:

25

SB4000BIAS T 30KHZ-40GHZ VSWR 1.82.92-F'S & PIN 500MASpecs (pdf): n/

a Price (1-9):

$1115.40

Price (10-24):

$1064.70

Price (25-49):

$1024.14

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1

SB65000BIAS T 30KHZ-65GHZ VSWR 1.81.85-F'S & PIN 500MASpecs (pdf): n/a Price (1-9):

$1977.30

Price (10-24):

$1926.60

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$1875.90

Qty On Hand:

2

SB3021BIAS T 10MHZ-3GHZ VSWR 1.2N M/F/F 1.5AMPSSpecs (pdf): n/a Price (1-9): $162.24Price (10-24): $157.17Price (25-49): $152.10Qty On Hand:

8

SB3023BIAS T 10MHZ-3GHZ VSWR 1.2N M/F/PIN 1.5AMPSSpecs (pdf): SB3023.PD

FPrice (1-9): $157.17Price (10-24): $152.10Price (25-49): $147.03Qty On Hand:

23

SB4200BIAS T 10MHZ-

SB4203BIAS T 10MHZ-

SB3500BIAS-T TYPE N'S

SB4500BIAS-T 7-16'S BNC-F

4.2GHZ VSWR 1.25N M/F/F 1.5AMPSSpecs (pdf): n/

a Price (1-9):

$263.64

Price (10-24):

$258.57

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4.2GHZ VSWR 1.25N M/F/F 1.5AMPSSpecs (pdf): n/a Price (1-9):

$263.64

Price (10-24):

$258.57

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$253.50

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10

BNC-F700-2700 MHZ VSWR 1.35 100 WATTSSpecs (pdf):

SB3500.PDF

Price (1-9): $223.08Price (10-24): $218.01Price (25-49): $212.94Qty On Hand:

7

700-2700 MHZ VSWR 1.35 175 WATTSSpecs (pdf): SB4500.PD

FPrice (1-9): $302.88Price (10-24): $292.02Price (25-49): $287.68Qty On Hand:

6

Home Adapters Attenuators Bias T's Cable Cable Assemblies Custom Cable Assemblies Cal Kits Etc Connectors Couplers DC Blocks Dust Caps Fiber Optic Filters Hybrids Imp Match Interface Guide Isolators, Circulators Low Pim Kits Open Shorts Phase Shifters

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Feedback Firesale! Order Info/Terms RMA form

Bias-T Design Considerations for the LWABrian Hicks and Bill EricksonMay 21, 2008The strawman design document [1] for the LWA suggests that the Front End Electronics (FEE)could be powered through the use of a circuit known as a bias-T. We will discuss how a bias-Toperates and present a low cost design optimized for performance within the LWA frequency

range of 20 to 80 MHz. We highly recommend the use of bias-T based on discrete componentsincorporated directly into the LWA FEE and ARX subsystems.

I. IntroductionA bias-T is a three port network designed to provide power to remote devices, such asamplifiers, over the same coaxial cable that RF signals are conveyed. The basictopology, and means of operation, of a bias-T network suitable for LWA applications isgiven in Figure 1.Capacitor Passes RF andBlocks DC and LowFrequency (60 Hz) ACInductor Blocks RF andPasses DC and LowFrequency (60 Hz) ACRF ONLY RF + PowerShunt Capacitor Routes anyremaining RF leakage to Ground,Increasing Isolation between RFPorts and DC Supply Port.DC Power SupplyC1C2L1

Figure 1 – Basic ‘Inductive’ Bias-TCommercially available bias-Ts are available in both “connectorized” and surface mountversions. These units are typically expensive ($40 to $100) and, although designed to bewideband, often suffer in performance at frequencies below 50 MHz.By concentrating on an LWA specific bias-T we can achieve a significant cost savings.We will focus on presenting a bias-T circuit that is optimized for operation below 100MHz and has the ability to supply a single polarization of an LWA FEE (230 mA). Wewill also deliver a circuit with a compact printed circuit board (PCB) footprint.2II. LWA Bias-T Design ConsiderationsConsisting of one inductor and one capacitor the bias-T circuit is simple, but particularconsideration must be given to component selection. Although not a part of the genericbias-T circuit, the shunt capacitor (C2) on the DC port should not be considered optional(Figure 1). Addition of this capacitance substantially increases isolation between the RFports and the DC supply connection by routing any remaining RF leakage on the supplyside of the inductor to ground (Figures 11, 12, and 13).

It is especially important that the inductor be rated for the necessary current and shouldoptimally have a minimum self resonant frequency (SRF) that is above the highest LWAfrequencies. Throughout the entire LWA band, the inductor must present highimpedance, and both capacitors must present low impedance; resonances must beavoided.Based on research and experience, we have selected the following components forconsideration and testing:Capacitors (C1, C2): 0.1 μF Ceramic Capacitor (SMT 1208), Panasonic, ECJ-3VB1E104KInductor (L1): 4.7 μH Wirewound Inductor (SMT 1008), Delevan, 1008-472JIII. LWA Bias-T PrototypingWe have produced a PCB layout to enable us to reliably evaluate the performance of ourbias-T with a variety of components (Figure 2). The SMA connectors featured here areonly included as a convenience for testing. The basic orthogonal arrangement of L1 andC1 can be readily incorporated into the FEE and ARX regardless of the connectors thesesubsystems utilize.Figure 2 – Bias-T Evaluation BoardThe circuit was evaluated both as a single unit and in the intended configuration with twobias-Ts connected together and transferring power. In the configurations with two bias-Ts, we supplied 15 VDC at ~240 mA to a power-resistor load (~3.6 Watts dissipated)throughout the test (Figure 3). It was our intention to evaluate the performance of theinductors in the circuit while operating under load conditions representative of a G250Rbalun (FEE).3Figure 3 – Characterization of the Bias-T Operating under LoadIn the single unit configurations we placed an SMA “short” on the DC port of the bias-T.It was our intention here to capture the performance of the single bias-T when connectedto a DC power supply with very low output impedance.An Agilent N3383A vector network analyzer was used to characterize insertion loss

(S21), return loss (S11 and S22), and isolation (S21 between RF and DC ports). Acomplete set of data is included in the measurements section of this report.IV. Observations and RecommendationsThe performance of the bias-T presented here is comparable to designs that we havealready fielded and found to be entirely successful in operation. Within the LWA band,the performance of this circuit compares favorably to costly commercial units. Theinsertion loss of two of these cascaded bias-Ts was found to be less than 0.2 dB, and theaggregate return loss was found to be least -20 dB.RF coupling from one dipole to another through the DC power network is a seriousconcern and potential source of indirect mutual coupling. It is important that significantattenuation be presented between the RF ports and the DC supply node of the bias-T.Direct mutual coupling between elements at a spacing of 4m has been demonstrated to beapproximately -20 dB, declining ~5 dB for each 2m increase in spacing [2]. After 20meters the mutual coupling between elements is approximately -60 dB and only graduallydiminishes with additional spacing [2]. Consequently, we recommend that feed systemcoupling be kept well below -80 dB.The most expedient means of accomplishing this goal is the introduction of a capacitiveshunt on the DC side of the bias-T. This can be seen on the schematic (Figure 14) ascapacitor C2. It is typically a part of good engineering practice to incorporate “bypasscapacitors” (such as C2) at DC supply nodes; we provide measurements here toemphasize the consequences of omitting this component. The isolation between the RFport and DC port of the bias-T without the capacitive shunt is seen to be inadequate inFigure 11. A definite improvement in isolation is seen when a 0.1 μF shunt is added at4the DC supply node (Figure 12). With the shunt, at least 60 dB of attenuation between

the RF ports and the DC supply port is achieved. An RF signal must run the gauntlet oftwo of networks consisting of L1 and C2 to traverse the DC feed ports of two bias-Ts andget from one dipole signal chain to another. Greater than 100 dB of attenuation isobserved in this configuration (Figure 13).The prototype circuit fits within an 8.3 x 10 mm square on the circuit board; PCB spaceconstraints should not be problematic.We recommend the use of a high-capacity DC power supply to supply power to the FEEsvia a bias-T arrangement such as discussed here. This method of power distribution willspare the cost of installing and maintaining a separate power distribution network andeliminate unnecessary complexity in station installations. Because DC power is sourcedvia the center conductor of shielded coaxial cable, the resulting power network iseffectively shielded at no additional cost. Candidate connectors for LWA RF cabling willlikely incorporate conductive backshells which will act to further inhibit the introductionof RFI via the power distribution network.By incorporating a bias-T into the ARX, the option of allowing power to the antennastands to be under control of the LWA Monitor and Control System (MCS) becomesinexpensive and straightforward to implement. This feature could prove valuable fordiagnostics, maintenance, and station commissioning.V. Summary TableOperating Frequency 15 to 115 MHzIsolation 60 dB MinimumInsertion Loss 0.2 dB MaximumVSWR 1.2:1 MaximumMaximum DC Voltage 25 VDCMaximum DC Current 334 mASize PCB Footprint Approximately 8.3 x 10 mmParts Cost $0.925VI. Measurements: -30 dBm Power Level for RF Stimulus1. Measurements of a Single Bias-T

Figure 4 – Insertion Loss (S21) through a Single Bias-T with DC Port ShortedFigure 5 – Return Loss (S11 and S22) into a Single Bias-T with DC Port Shorted6Figure 6 – Return Loss (S11 and S22) into a Single Bias-T with DC Port Shorted (Smith)2. Measurements of a Two Bias-Ts in Cascade Delivering 240 mA to a LoadFigure 7 – Insertion Loss through Two Bias-T Supplying 240 mA to a Resistive Load7Figure 8 – Return Loss (S11 and S22) into Two Cascaded Bias-Ts Delivering 240 mA

FigureBias-T Design Considerations for the LWABrian Hicks and Bill EricksonMay 21, 2008The strawman design document [1] for the LWA suggests that the Front End Electronics (FEE)could be powered through the use of a circuit known as a bias-T. We will discuss how a bias-Toperates and present a low cost design optimized for performance within the LWA frequencyrange of 20 to 80 MHz. We highly recommend the use of bias-T based on discrete componentsincorporated directly into the LWA FEE and ARX subsystems.

I. IntroductionA bias-T is a three port network designed to provide power to remote devices, such asamplifiers, over the same coaxial cable that RF signals are conveyed. The basictopology, and means of operation, of a bias-T network suitable for LWA applications isgiven in Figure 1.Capacitor Passes RF andBlocks DC and LowFrequency (60 Hz) ACInductor Blocks RF andPasses DC and LowFrequency (60 Hz) ACRF ONLY RF + PowerShunt Capacitor Routes anyremaining RF leakage to Ground,Increasing Isolation between RFPorts and DC Supply Port.DC Power SupplyC1C2L1

Figure 1 – Basic ‘Inductive’ Bias-TCommercially available bias-Ts are available in both “connectorized” and surface mountversions. These units are typically expensive ($40 to $100) and, although designed to bewideband, often suffer in performance at frequencies below 50 MHz.By concentrating on an LWA specific bias-T we can achieve a significant cost savings.We will focus on presenting a bias-T circuit that is optimized for operation below 100

MHz and has the ability to supply a single polarization of an LWA FEE (230 mA). Wewill also deliver a circuit with a compact printed circuit board (PCB) footprint.2II. LWA Bias-T Design ConsiderationsConsisting of one inductor and one capacitor the bias-T circuit is simple, but particularconsideration must be given to component selection. Although not a part of the genericbias-T circuit, the shunt capacitor (C2) on the DC port should not be considered optional(Figure 1). Addition of this capacitance substantially increases isolation between the RFports and the DC supply connection by routing any remaining RF leakage on the supplyside of the inductor to ground (Figures 11, 12, and 13).It is especially important that the inductor be rated for the necessary current and shouldoptimally have a minimum self resonant frequency (SRF) that is above the highest LWAfrequencies. Throughout the entire LWA band, the inductor must present highimpedance, and both capacitors must present low impedance; resonances must beavoided.Based on research and experience, we have selected the following components forconsideration and testing:Capacitors (C1, C2): 0.1 μF Ceramic Capacitor (SMT 1208), Panasonic, ECJ-3VB1E104KInductor (L1): 4.7 μH Wirewound Inductor (SMT 1008), Delevan, 1008-472JIII. LWA Bias-T PrototypingWe have produced a PCB layout to enable us to reliably evaluate the performance of ourbias-T with a variety of components (Figure 2). The SMA connectors featured here areonly included as a convenience for testing. The basic orthogonal arrangement of L1 andC1 can be readily incorporated into the FEE and ARX regardless of the connectors thesesubsystems utilize.Figure 2 – Bias-T Evaluation BoardThe circuit was evaluated both as a single unit and in the intended configuration with twobias-Ts connected together and transferring power. In the configurations with two bias-

Ts, we supplied 15 VDC at ~240 mA to a power-resistor load (~3.6 Watts dissipated)throughout the test (Figure 3). It was our intention to evaluate the performance of theinductors in the circuit while operating under load conditions representative of a G250Rbalun (FEE).3Figure 3 – Characterization of the Bias-T Operating under LoadIn the single unit configurations we placed an SMA “short” on the DC port of the bias-T.It was our intention here to capture the performance of the single bias-T when connectedto a DC power supply with very low output impedance.An Agilent N3383A vector network analyzer was used to characterize insertion loss(S21), return loss (S11 and S22), and isolation (S21 between RF and DC ports). Acomplete set of data is included in the measurements section of this report.IV. Observations and RecommendationsThe performance of the bias-T presented here is comparable to designs that we havealready fielded and found to be entirely successful in operation. Within the LWA band,the performance of this circuit compares favorably to costly commercial units. Theinsertion loss of two of these cascaded bias-Ts was found to be less than 0.2 dB, and theaggregate return loss was found to be least -20 dB.RF coupling from one dipole to another through the DC power network is a seriousconcern and potential source of indirect mutual coupling. It is important that significantattenuation be presented between the RF ports and the DC supply node of the bias-T.Direct mutual coupling between elements at a spacing of 4m has been demonstrated to beapproximately -20 dB, declining ~5 dB for each 2m increase in spacing [2]. After 20meters the mutual coupling between elements is approximately -60 dB and only graduallydiminishes with additional spacing [2]. Consequently, we recommend that feed systemcoupling be kept well below -80 dB.The most expedient means of accomplishing this goal is the introduction of a capacitive

shunt on the DC side of the bias-T. This can be seen on the schematic (Figure 14) ascapacitor C2. It is typically a part of good engineering practice to incorporate “bypasscapacitors” (such as C2) at DC supply nodes; we provide measurements here toemphasize the consequences of omitting this component. The isolation between the RFport and DC port of the bias-T without the capacitive shunt is seen to be inadequate inFigure 11. A definite improvement in isolation is seen when a 0.1 μF shunt is added at4the DC supply node (Figure 12). With the shunt, at least 60 dB of attenuation betweenthe RF ports and the DC supply port is achieved. An RF signal must run the gauntlet oftwo of networks consisting of L1 and C2 to traverse the DC feed ports of two bias-Ts andget from one dipole signal chain to another. Greater than 100 dB of attenuation isobserved in this configuration (Figure 13).The prototype circuit fits within an 8.3 x 10 mm square on the circuit board; PCB spaceconstraints should not be problematic.We recommend the use of a high-capacity DC power supply to supply power to the FEEsvia a bias-T arrangement such as discussed here. This method of power distribution willspare the cost of installing and maintaining a separate power distribution network andeliminate unnecessary complexity in station installations. Because DC power is sourcedvia the center conductor of shielded coaxial cable, the resulting power network iseffectively shielded at no additional cost. Candidate connectors for LWA RF cabling willlikely incorporate conductive backshells which will act to further inhibit the introductionof RFI via the power distribution network.By incorporating a bias-T into the ARX, the option of allowing power to the antennastands to be under control of the LWA Monitor and Control System (MCS) becomesinexpensive and straightforward to implement. This feature could prove valuable for

diagnostics, maintenance, and station commissioning.V. Summary TableOperating Frequency 15 to 115 MHzIsolation 60 dB MinimumInsertion Loss 0.2 dB MaximumVSWR 1.2:1 MaximumMaximum DC Voltage 25 VDCMaximum DC Current 334 mASize PCB Footprint Approximately 8.3 x 10 mmParts Cost $0.925VI. Measurements: -30 dBm Power Level for RF Stimulus1. Measurements of a Single Bias-TFigure 4 – Insertion Loss (S21) through a Single Bias-T with DC Port ShortedFigure 5 – Return Loss (S11 and S22) into a Single Bias-T with DC Port Shorted6Figure 6 – Return Loss (S11 and S22) into a Single Bias-T with DC Port Shorted (Smith)2. Measurements of a Two Bias-Ts in Cascade Delivering 240 mA to a LoadFigure 7 – Insertion Loss through Two Bias-T Supplying 240 mA to a Resistive Load7Figure 8 – Return Loss (S11 and S22) into Two Cascaded Bias-Ts Delivering 240 mA

FigureBias-T Design Considerations for the LWABrian Hicks and Bill EricksonMay 21, 2008The strawman design document [1] for the LWA suggests that the Front End Electronics (FEE)could be powered through the use of a circuit known as a bias-T. We will discuss how a bias-Toperates and present a low cost design optimized for performance within the LWA frequencyrange of 20 to 80 MHz. We highly recommend the use of bias-T based on discrete componentsincorporated directly into the LWA FEE and ARX subsystems.

I. IntroductionA bias-T is a three port network designed to provide power to remote devices, such asamplifiers, over the same coaxial cable that RF signals are conveyed. The basictopology, and means of operation, of a bias-T network suitable for LWA applications isgiven in Figure 1.Capacitor Passes RF andBlocks DC and LowFrequency (60 Hz) ACInductor Blocks RF andPasses DC and LowFrequency (60 Hz) ACRF ONLY RF + PowerShunt Capacitor Routes anyremaining RF leakage to Ground,

Increasing Isolation between RFPorts and DC Supply Port.DC Power SupplyC1C2L1

Figure 1 – Basic ‘Inductive’ Bias-TCommercially available bias-Ts are available in both “connectorized” and surface mountversions. These units are typically expensive ($40 to $100) and, although designed to bewideband, often suffer in performance at frequencies below 50 MHz.By concentrating on an LWA specific bias-T we can achieve a significant cost savings.We will focus on presenting a bias-T circuit that is optimized for operation below 100MHz and has the ability to supply a single polarization of an LWA FEE (230 mA). Wewill also deliver a circuit with a compact printed circuit board (PCB) footprint.2II. LWA Bias-T Design ConsiderationsConsisting of one inductor and one capacitor the bias-T circuit is simple, but particularconsideration must be given to component selection. Although not a part of the genericbias-T circuit, the shunt capacitor (C2) on the DC port should not be considered optional(Figure 1). Addition of this capacitance substantially increases isolation between the RFports and the DC supply connection by routing any remaining RF leakage on the supplyside of the inductor to ground (Figures 11, 12, and 13).It is especially important that the inductor be rated for the necessary current and shouldoptimally have a minimum self resonant frequency (SRF) that is above the highest LWAfrequencies. Throughout the entire LWA band, the inductor must present highimpedance, and both capacitors must present low impedance; resonances must beavoided.Based on research and experience, we have selected the following components forconsideration and testing:Capacitors (C1, C2): 0.1 μF Ceramic Capacitor (SMT 1208), Panasonic, ECJ-3VB1E104KInductor (L1): 4.7 μH Wirewound Inductor (SMT 1008), Delevan, 1008-472JIII. LWA Bias-T Prototyping

We have produced a PCB layout to enable us to reliably evaluate the performance of ourbias-T with a variety of components (Figure 2). The SMA connectors featured here areonly included as a convenience for testing. The basic orthogonal arrangement of L1 andC1 can be readily incorporated into the FEE and ARX regardless of the connectors thesesubsystems utilize.Figure 2 – Bias-T Evaluation BoardThe circuit was evaluated both as a single unit and in the intended configuration with twobias-Ts connected together and transferring power. In the configurations with two bias-Ts, we supplied 15 VDC at ~240 mA to a power-resistor load (~3.6 Watts dissipated)throughout the test (Figure 3). It was our intention to evaluate the performance of theinductors in the circuit while operating under load conditions representative of a G250Rbalun (FEE).3Figure 3 – Characterization of the Bias-T Operating under LoadIn the single unit configurations we placed an SMA “short” on the DC port of the bias-T.It was our intention here to capture the performance of the single bias-T when connectedto a DC power supply with very low output impedance.An Agilent N3383A vector network analyzer was used to characterize insertion loss(S21), return loss (S11 and S22), and isolation (S21 between RF and DC ports). Acomplete set of data is included in the measurements section of this report.IV. Observations and RecommendationsThe performance of the bias-T presented here is comparable to designs that we havealready fielded and found to be entirely successful in operation. Within the LWA band,the performance of this circuit compares favorably to costly commercial units. Theinsertion loss of two of these cascaded bias-Ts was found to be less than 0.2 dB, and theaggregate return loss was found to be least -20 dB.RF coupling from one dipole to another through the DC power network is a serious

concern and potential source of indirect mutual coupling. It is important that significantattenuation be presented between the RF ports and the DC supply node of the bias-T.Direct mutual coupling between elements at a spacing of 4m has been demonstrated to beapproximately -20 dB, declining ~5 dB for each 2m increase in spacing [2]. After 20meters the mutual coupling between elements is approximately -60 dB and only graduallydiminishes with additional spacing [2]. Consequently, we recommend that feed systemcoupling be kept well below -80 dB.The most expedient means of accomplishing this goal is the introduction of a capacitiveshunt on the DC side of the bias-T. This can be seen on the schematic (Figure 14) ascapacitor C2. It is typically a part of good engineering practice to incorporate “bypasscapacitors” (such as C2) at DC supply nodes; we provide measurements here toemphasize the consequences of omitting this component. The isolation between the RFport and DC port of the bias-T without the capacitive shunt is seen to be inadequate inFigure 11. A definite improvement in isolation is seen when a 0.1 μF shunt is added at4the DC supply node (Figure 12). With the shunt, at least 60 dB of attenuation betweenthe RF ports and the DC supply port is achieved. An RF signal must run the gauntlet oftwo of networks consisting of L1 and C2 to traverse the DC feed ports of two bias-Ts andget from one dipole signal chain to another. Greater than 100 dB of attenuation isobserved in this configuration (Figure 13).The prototype circuit fits within an 8.3 x 10 mm square on the circuit board; PCB spaceconstraints should not be problematic.We recommend the use of a high-capacity DC power supply to supply power to the FEEsvia a bias-T arrangement such as discussed here. This method of power distribution willspare the cost of installing and maintaining a separate power distribution network and

eliminate unnecessary complexity in station installations. Because DC power is sourcedvia the center conductor of shielded coaxial cable, the resulting power network iseffectively shielded at no additional cost. Candidate connectors for LWA RF cabling willlikely incorporate conductive backshells which will act to further inhibit the introductionof RFI via the power distribution network.By incorporating a bias-T into the ARX, the option of allowing power to the antennastands to be under control of the LWA Monitor and Control System (MCS) becomesinexpensive and straightforward to implement. This feature could prove valuable fordiagnostics, maintenance, and station commissioning.V. Summary TableOperating Frequency 15 to 115 MHzIsolation 60 dB MinimumInsertion Loss 0.2 dB MaximumVSWR 1.2:1 MaximumMaximum DC Voltage 25 VDCMaximum DC Current 334 mASize PCB Footprint Approximately 8.3 x 10 mmParts Cost $0.925VI. Measurements: -30 dBm Power Level for RF Stimulus1. Measurements of a Single Bias-TFigure 4 – Insertion Loss (S21) through a Single Bias-T with DC Port ShortedFigure 5 – Return Loss (S11 and S22) into a Single Bias-T with DC Port Shorted6Figure 6 – Return Loss (S11 and S22) into a Single Bias-T with DC Port Shorted (Smith)2. Measurements of a Two Bias-Ts in Cascade Delivering 240 mA to a LoadFigure 7 – Insertion Loss through Two Bias-T Supplying 240 mA to a Resistive Load7Figure 8 – Return Loss (S11 and S22) into Two Cascaded Bias-Ts Delivering 240 mA

Figure