Power over Coax: How PoC Works & Where It’s Used

Power over Coax (PoC) is a technology that delivers DC electrical power to a remote device using the same coaxial cable that carries the data signal, eliminating the need for a separate power cable. In a PoC system, a power inserter combines DC voltage with the RF signal at the headend, and a power extractor at the device separates the two, providing clean power to the equipment. This article explains the principles behind PoC, its common applications, and how it compares to alternatives like Power over Ethernet (PoE).

Formula

The calculator above estimates the current draw and voltage drop for a given cable length, system voltage, and device power. The formulas are:

• Current (I) = Device Power (P) ÷ System Voltage (V), in amperes (A).
• Voltage Drop (V_drop) = I × R_cable, where R_cable is the total resistance of the coax cable. For typical RG-6 coax, the loop resistance is about 0.02 Ω/m (20 Ω per 1000 m). The calculator uses 0.02 × length to get the total loop resistance.
• Voltage at Device (V_dev) = System Voltage (V) − Voltage Drop (V_drop).

All variables are defined with their units: length in meters (m), voltage in volts (V), power in watts (W), current in amperes (A), and resistance in ohms (Ω).

Worked Example

Using the default values: cable length = 100 m, system voltage = 48 V, device power = 10 W.

1. Current: I = 10 W ÷ 48 V = 0.2083 A ≈ 0.21 A.
2. Cable resistance: R = 0.02 Ω/m × 100 m = 2 Ω.
3. Voltage drop: V_drop = 0.2083 A × 2 Ω = 0.4167 V ≈ 0.42 V.
4. Voltage at device: V_dev = 48 V − 0.42 V = 47.58 V ≈ 47.58 V.

The calculator rounds the current to 0.21 A, voltage drop to 0.42 V, and voltage at device to 47.58 V.

Background: Why Power over Coax?

Coaxial cable has been used for decades to carry RF signals for television, satellite, and CCTV. Originally, each device needed its own power adapter, which often meant running a separate power line or installing a local outlet near the device. PoC was developed to simplify installation, reduce cost, and allow devices to be placed in locations where AC power is not readily available.

The concept is not new—it has been used in satellite TV LNB (low-noise block downconverter) powering for years, where the receiver sends a DC voltage up the coax to power the LNB. However, modern PoC systems are more sophisticated, using higher voltages and more robust filtering to support IP cameras, wireless access points, and other networked devices.

How Power over Coax Works

A PoC system consists of three main components:

• Power Inserter (or PSU): Located at the headend (e.g., in a network closet or DVR room), it combines DC power (typically 12V, 24V, or 48V) with the data signal onto the coax cable. It often includes a DC-to-DC converter and a bias tee.
• Coaxial Cable: The same cable carries both the RF/data signal and the DC power. The DC power is superimposed on the cable’s center conductor, while the shield acts as the return path.
• Power Extractor (or Bias Tee): At the device end, this circuit separates the DC power from the data signal, feeding power to the device and passing the data to the device’s network port.

The key principle is that the DC power and the high-frequency data signal occupy different frequency ranges. The bias tee uses capacitors to block DC and pass AC/RF, and inductors (or ferrite beads) to pass DC and block AC/RF. This allows both to travel on the same conductor without interfering with each other.

Where Power over Coax Is Used

PoC is most commonly used in the following applications:

• CCTV and IP Security Cameras: Many analog and IP cameras support PoC, allowing a single coax cable (typically RG-59 or RG-6) to provide both video/data and power. This is especially useful for retrofitting older coax installations.
• Satellite TV Systems: The LNB at the satellite dish is powered via the coax from the receiver, using a DC voltage (13V or 18V) to also switch polarization.
• Wireless Access Points and Bridges: Some outdoor Wi-Fi and point-to-point wireless devices use PoC to avoid running AC power to the antenna location.
• Remote Radio Heads (RRH): In some distributed antenna systems (DAS) and small cell installations, PoC is used to power remote radio units over the existing coax.
• Legacy Coax Networks: In buildings already wired with coax for cable TV, PoC can be used to power IP cameras or other devices without rewiring.

Power over Coax vs. Power over Ethernet

PoC and Power over Ethernet (PoE) both deliver power over a data cable, but they differ in cable type, distance, and standards.

• Cable Type: PoC uses coaxial cable (e.g., RG-59, RG-6) while PoE uses twisted-pair Ethernet (Cat5e/6).
• Distance: PoC can often transmit power over longer distances than standard PoE. While PoE is limited to 100 meters (328 ft) for data, PoC can sometimes reach 300 meters or more, depending on voltage and cable gauge. However, higher voltage drop over long runs must be considered.
• Standards: PoE has IEEE standards (802.3af, 802.3at, 802.3bt) that define power levels and negotiation. PoC has no universal standard; implementations vary by manufacturer, though some use the 12V or 24V DC common in CCTV.
• Data Rate: Coax can support high data rates for video (e.g., HD-TVI, AHD, or IP over coax), but Ethernet typically offers higher and more standardized data rates.

In practice, PoC is often chosen for existing coax infrastructure, while PoE is preferred for new Ethernet installations.

Choosing the Right PoC System

When selecting a PoC solution, consider the following factors:

• Voltage and Power: Match the voltage and power rating of the inserter to the device’s requirements. Common voltages are 12V, 24V, and 48V. Ensure the inserter can supply enough current for the device plus margin for voltage drop.
• Cable Type and Length: Use a good quality coax with low resistance (RG-6 is better than RG-59 for longer runs). Calculate voltage drop to ensure the device receives sufficient voltage (most devices have a tolerance range).
• Surge Protection: Outdoor installations should include surge protectors to guard against lightning-induced surges.
• Compatibility: Verify that the camera or device explicitly supports PoC. Some devices require a specific polarity or voltage, and using the wrong one can damage the equipment.

Common Problems and Troubleshooting

PoC systems are generally reliable, but issues can arise:

• Voltage Drop: Over long cable runs, voltage drop can cause the device to underpower or reset. Solution: use a higher voltage inserter, thicker cable, or a local power supply.
• Signal Interference: Poor shielding or loose connectors can cause data errors. Ensure all connections are tight and use quality connectors.
• Overload: Connecting too many devices to one inserter can exceed its current rating. Check the total power draw and use a higher-capacity inserter.
• Polarity Reversal: Some devices are sensitive to polarity. Double-check the wiring if the device does not power up.

Safety Considerations

Working with PoC involves DC voltages that, while typically low (12-48V), can still cause shocks under certain conditions. Always disconnect power before handling cables. For outdoor installations, proper grounding of the coax shield is essential to prevent damage from lightning strikes. Use surge protectors and follow local electrical codes. If you are unsure about any aspect, consult a qualified technician.

Frequently Asked Questions

Can I use Power over Coax with any camera?

No. The camera must explicitly support PoC. Many modern IP cameras and some analog HD cameras have PoC built-in, but older cameras may not. Check the camera’s datasheet or manual for PoC support.

What is the maximum distance for Power over Coax?

There is no fixed maximum; it depends on the cable type, voltage, and power draw. With RG-6 and 48V, distances of 300 meters (about 1000 ft) are possible for low-power devices, but you must calculate voltage drop to ensure the device gets enough voltage.

Is Power over Coax the same as PoE?

No. PoC uses coaxial cable, while PoE uses Ethernet twisted-pair. They are different technologies with different standards and applications.

Can I convert a PoC camera to PoE?

Yes, but you need a converter or a different camera. Some cameras support both, but if not, you can use a PoC-to-Ethernet converter or replace the camera with a PoE model.

Does Power over Coax affect video quality?

When properly implemented, PoC should not affect video quality because the DC power is filtered out and does not interfere with the high-frequency video signal. Poor quality cables or connectors can introduce noise, but that is true for any coax system.

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in t | Cable length (m) | m | 100 | 0 | 1000
in v | System voltage (V) | V | 48 | 5 | 60
in p | Device power (W) | W | 10 | 0.1 | 100
out i | Current (A) | A | p / v | 2
out vdrop | Voltage drop (V) | V | i * 0.02 * t | 2
out veff | Voltage at device (V) | V | v - vdrop | 2