Coax Cable for Satellite Receiver: DC on the Same Conductor
Quick answer / TL;DR: Short answer: use RG6 with a copper conductor and good shielding, keep the run as short as the installation allows, and fit compression connectors rather than twist-on types. Check the DC voltage at the LNB, since a long run with a poor connector can drop enough to make the LNB behave unpredictably. Where DiSEqC switches are used, keep the chain simple and label every port.
Three coaxial families: BNC for CCTV, F-type for TV, N for RF – each with its impedance.A satellite receiver sends power and control signals up the same coax that carries the signal down, which makes the cable's DC resistance as important as its loss.
The LNB at the dish draws current through the cable, and a long run with a poor connector drops voltage as well as signal.
This article covers the loss and DC budget, the connector practice that keeps water out, and the switching arrangements that confuse a receiver.
The receiver supplies a nominal voltage up the cable to select polarisation and band, and the LNB converts that into current draw.
Every ohm of resistance between the receiver and the LNB reduces the voltage that arrives, and connectors are a significant part of that resistance when they are poorly fitted.
The signal path and the DC path share the same centre conductor and shield, so a fault affects both.
Electrical Budget
| Element | Effect | Note |
|---|
| RG6 loss at 2 GHz | About 10 dB per 30 m | Higher than at 1 GHz |
| DC resistance | Voltage drop | Copper centre conductor preferred |
| Twist-on connector | High and variable resistance | Avoid |
| Compression connector | Low, stable | Preferred |
| Moisture ingress | Corrosion | The main long-term failure |
| DiSEqC switch | Insertion loss plus DC drop | One per hop |
The moisture row explains why satellite installations fail after a few seasons rather than immediately. Water enters at the connector and corrodes the shield.
A self-amalgamating tape wrap over a compression connector is the cheapest long-term protection available.
Switching and Control
A single receiver with a single LNB is the simplest arrangement and the one least likely to produce puzzling faults.
Adding a DiSEqC switch lets one receiver select between several LNBs, and each switch adds insertion loss plus a DC drop.
Where two receivers share a dish, a multiswitch is required rather than a splitter, because a splitter cannot route different polarisations to different receivers.
Label the switch ports and record which satellite is on which port, because a mislabelled port produces a symptom that looks like a failed LNB.
Installation Practice
- Use compression connectors and seal them against water
- Keep the run short and avoid unnecessary joints
- Check the DC voltage at the dish end
- Record the switch configuration
- Support the cable so its weight is not carried by the connector
Real-World Applications
A receiver loses some transponders but not others. The voltage at the LNB is low, and a corroded connector is the cause.
A dish serves two receivers through a splitter. Neither can select independently, and a multiswitch is required.
A DiSEqC switch is added and one satellite disappears. The port is mislabelled, and correcting the assignment restores it.
Reception degrades after two winters. Water has entered a connector, and replacing it with a sealed compression type resolves the fault.
Standards and Verification
Coax loss is published per frequency by the cable manufacturer, and DiSEqC signalling is defined by the DiSEqC specification maintained by Eutelsat.
Verification is a signal level and quality reading at the receiver plus a DC voltage measurement at the LNB.
Measure at both ends. The difference between them is what the installation is losing.
Signal Quality Versus Strength
A receiver reports both strength and quality, and the two are not the same measurement.
Strength indicates level, while quality indicates the error rate after demodulation.
A high strength with poor quality points to interference or a misaligned dish rather than to low signal.
A low strength with good quality points to loss in the cable or connectors, and the system may still work.
Reading both figures separates the two problems before any hardware is changed.
Where quality fluctuates with weather, the alignment is usually marginal and no cable change will fix it.
Where strength falls gradually over months, moisture in a connector is the likely cause.
Record both readings at commissioning so that a later change can be compared against a baseline.
Without a baseline, a gradual degradation is indistinguishable from an installation that was always marginal.
Multiswitch and Distribution
A multiswitch takes the satellite feeds and distributes them to several receivers, each able to select independently.
The switch requires a power supply and introduces insertion loss, and both belong in the loss budget.
Where the multiswitch is added to an existing installation, the previous splitter arrangement has to be removed rather than left in series.
Record which input of the multiswitch receives which satellite and polarisation, because the labelling is not standardised.
A receiver that sees the wrong satellite after a switch replacement is a port assignment issue rather than a dish issue.
Test each receiver in turn after any change, since a fault on one port affects only the receiver connected to it.
Where the installation has grown over years, drawing the current topology is often the fastest way to find the fault.
An undocumented chain of splitters and switches is the most common reason a satellite fault takes hours to resolve.
Related reading: Coax cable signal loss — blogs/news/coax-cable-signal-loss
Related reading: Coax cable bend radius — blogs/news/coax-cable-bend-radius
Related reading: Coaxial cable splitter — blogs/news/coaxial-cable-splitter
Frequently Asked Questions
Why do some transponders fail?
Often low voltage at the LNB, caused by a corroded or poorly fitted connector.
Can I split a dish feed to two receivers?
Not with a passive splitter. A multiswitch is required for independent selection.
Do I need RG6 or RG59?
RG6. RG59 loses more at satellite frequencies.
How do I stop water ingress?
Compression connectors plus self-amalgamating tape over the joint.
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