A Low-Loss VHF/UHF Bias Tee

A simple circuit lets your transmission line do double duty.

By Tom Cefalo Jr, W1EX

hen I became interested in bias tee allows dc power to be super- band of operation. To meet these re- satellite communications, I imposed onto a transmission line with- quirements, a ceramic-chip Wdecided to operate the popu- out altering the RF characteristics, was selected. These have lar 2-m/70-cm mode. To reduce the thereby allowing both signals to share low dissipation factors, little series system noise overall, low-noise ampli- a single cable. This article describes a inductance, high breakdown fiers were installed at the antennas. low-loss bias tee that has been opti- and those with larger packages will To avoid running additional cables mized for each band. safely transfer the transmitted power. from the station up to the antennas to The RF choke has a dual role. First, power and control the low-noise ampli- Theory of Operation it provides a means to inject dc power fiers, I decided to use the RF coaxial Fig 1 shows a typical bias tee and the onto the center conductor of the co- cable as the medium to carry out this component values for the two bands of axial cable. Second, it presents high task. This method provides a low- operation. The bias tee has three impedance to the RF signal. This pro- resistance path and eliminates addi- ports: a dc port, an RF port and a vides isolation between the RF signal tional cabling. RF+dc port. The operation of the cir- and the dc port. In other words, it pre- To perform this function, a device cuit is very simple. The series capaci- vents the RF from flowing through the called a dc injector—more commonly tor provides a dc block to stop the dc dc port. As a general rule of thumb, the known as a “bias tee”—is required. A from appearing at the RF port. inductive reactance should be at least This capacitor must have low-loss 10 times the system impedance (the characteristics so it can safely pass the characteristic impedance of the co- 51 Oak St transmitted RF power. In addition to axial transmission line). However, the Winchester, MA 01890 this, the capacitor’s self-resonant fre- self-resonant frequency of the induc- [email protected] quency must be above the desired tor must be above the band of opera-

52 May/June 2002 tion. The must also be able to voltage is blocked from reaching the BNC connectors were used. The pass the dc current without altering by the series capacitor. The feedthrough capacitor can be mounted its characteristics. The inductance dc voltage is extracted from the coaxial at any convenient location on the box value must be carefully selected. If the transmission line via the dc port of the away from the RF. The inductor is above inductor criteria are not met, bias tee, thus providing power to the soldered between the feedthrough the performance of the bias tee will be low-noise . capacitor and the transmission line greatly degraded. using the wire leads to support the With an RF signal applied to the RF Construction inductor. The windings around the fer- port and a dc voltage applied to the dc A bias tee should be constructed in a rite core should be evenly spaced. Re- port, the output port will contain both small metal enclosure such as a member that each time the wire of these signals. The coaxial trans- Pomona box. RF path lengths should passes through the center of the core, mission line conveys both of the com- be minimized to reduce losses; there- it is counted as a turn. ponents. The dc return path is the fore the connectors should be placed on Since the series-blocking capacitor shield of the coaxial transmission line. the narrow ends of the box. It is essen- is a chip component, it was mounted Fig 2 shows a typical bias-tee instal- tial that N-type connectors be used, on a double-sided PC board between lation. Notice that the bias tee at the especially for UHF operation. The 50-Ω microstrip lines. The lines can be antenna end is installed with the RF electrical performance (SWR and in- made using a sharp hobby knife to cut port connected to the antenna. The dc sertion loss) would suffer if SO-239 or and remove the excess copper foil. The bottom of the PC board is a solid ground plane of copper. Table 1 lists Table 1—Microstrip Widths for some 50-Ω line widths for several com- various Board Materials and mon PC board materials. There are Thicknesses also several programs available on the Web to calculate a line width if a dif- Board Material H W ferent board material is used. Rogers 4003 0.032 0.073 It is very important to insure a good GML 1000 0.030 0.071 ground between the connectors and FR4 0.031 0.055 the ground plane of the PC board. Fig 3 shows one method of transition

Fig 1—A schematic diagram of the bias tee. L1—An RF choke wound on a Micrometals T25-6 ferrite core. For VHF, use 16 turns of #28 AWG wire. For UHF, use 9 turns of #28 AWG wire. C1—Dielectric Labs chip capacitor. For VHF, 1000 pF (#C22AH102J7PXL). For UHF, 330 pF (#C22AH331JAPXL). Fig 2—A block diagram of a typical station setup.

Fig 3—Construction details.

May/June 2002 53 Fig 4—VHF bias tee return (1) and insertion (2) loss. Fig 5—VHF bias tee RF-to-DC port isolation.

Fig 6—UHF bias tee return (1) and insertion (2) loss. Fig 7—UHF bias tee RF-to-DC port isolation.

between the connector and the PC Fig 4 shows the VHF bias tee’s insertion As seen from the measured data, the board. A right-angle bracket made of loss and return loss. The average inser- bias tee has low losses, excellent SWR brass can be formed to the correct di- tion loss was 0.02 dB and the minimum and isolation. The measured data mensions. The bracket is soldered to return loss was 35 dB, corresponding to were unaltered when the units were the ground plane and attached to the an SWR of 1.036:1. Fig 5 shows the iso- tested with a bias voltage of 28 V at connector via the connector’s two bot- lation between the RF and dc ports to be 1 A. The bias tee has been operating at tom mounting screws. 60 dB. Figs 6 and 7 are the measured the 100-W level with no problems. A data for the UHF bias tee. The average single bias tee could have been de- Measured Data insertion loss was 0.04 dB and the re- signed to cover both bands but the goal The following data were measured on turn loss was 30 dB (SWR 1.065:1). The of low-loss characteristics would not an HP scalar network analyzer. isolation was also 60 dB. have been achieved.

54 May/June 2002