RF PA voltage drop can reduce output even when the power supply display still reads 28 V. The missing condition is often the voltage measured directly at the amplifier input while the RF PA is producing the required output power.
An undersized or overly long DC cable can create enough resistance to lower the module-input voltage. Connectors, fuse holders, relays, distribution boards, terminals, and the negative return path can add further loss.
This article focuses specifically on the DC cable and connection path between the power supply and the RF power amplifier. Current reserve, ripple, startup timing, grounding architecture, antenna mismatch, and thermal derating require separate checks.
1. Why Is 28 V at the Supply Not 28 V at the PA?
The voltage shown on a power supply is measured at the supply terminals. It does not prove that the RF PA receives the same voltage.
When current flows through a cable, resistance creates voltage loss. The actual amplifier input voltage is therefore:
PA input voltage = supply voltage − total DC path voltage drop
The complete path includes:
- Positive cable
- Negative return cable
- Connectors and terminals
- Fuse holders
- Relays and switches
- Distribution boards
- PCB or copper-bus connections
A power supply may continue to display 28 V while the amplifier input falls to 26 V, 25.5 V, or lower under RF load.
Whether that value is acceptable depends on the amplifier’s specified input range, required RF output, operating temperature, duty cycle, and protection settings. A measured 25.8 V is not automatically a pass or fail.
2. How to Calculate DC Cable Voltage Drop for an RF PA
DC cable sizing should be based on maximum operating current and complete loop resistance, not only the cable’s rated ampacity.
Use:
Vdrop = Iload × Rloop
VPA = Vsupply − Vdrop
Pcable = Iload² × Rloop
Where:
- Vdrop is the total DC path voltage drop
- Iload is the actual amplifier current under the required RF condition
- Rloop is the resistance of the complete positive and negative path
- VPA is the voltage reaching the RF PA terminals
- Pcable is the power converted into heat inside the DC path
The loop length must include both directions.
For example, a power supply located 3 m from the amplifier does not create a 3 m electrical path. The positive and negative conductors create approximately 6 m of total cable length before connector and terminal resistance is added.
Use the cable manufacturer’s resistance data in Ω/m or mΩ/m whenever possible. Cable gauge alone is not enough because conductor material, strand construction, temperature, and manufacturing tolerance affect resistance.
DC Cable Voltage-Drop Worksheet
| Item | Required value |
|---|---|
| Supply voltage | Voltage at the supply terminals |
| Required PA input voltage | Minimum voltage allowed at the target output |
| Maximum load current | Full-load or worst-case current |
| One-way cable length | Physical distance from supply to PA |
| Total loop length | Positive and negative conductors combined |
| Cable resistance | Manufacturer value in Ω/m or mΩ/m |
| Connection resistance | Connectors, fuse, relay, switch, terminals |
| Calculated voltage drop | Current × total loop resistance |
| Measured PA input voltage | Voltage at the PA during RF operation |
| Cable and terminal temperature | Recorded after thermal stabilization |
| Acceptance result | Pass or fail against the defined condition |
A cable is not approved simply because it can carry the current without immediate damage. It must also keep voltage drop and temperature rise within the required operating boundary.
3. What RF Symptoms Can Low PA Input Voltage Create?
Low amplifier input voltage does not always cause an immediate shutdown. It may first appear as unstable or reduced RF performance.
Possible symptoms include:
- Lower output power at high drive
- Earlier output compression
- Output reduction after several minutes
- Different behavior between cold and hot operation
- Uneven performance between channels
- Current-limit activation
- Repeated protection alarms
- Controller or auxiliary-device resets
- Cable or connector heating
These symptoms can be mistaken for an RF transistor problem, poor gain flatness, insufficient cooling, or antenna mismatch.
Do not diagnose the amplifier from RF output alone. Record at least:
- Supply-terminal voltage
- PA-terminal voltage
- DC current
- RF input power
- RF output power
- Reflected power
- Temperature
- Alarm state
The current response may vary between amplifier designs. A lower input voltage may change current draw, cause earlier current limiting, or trigger protection. The result must be verified through measurement rather than assumed.
For a broader review of DC-source capacity, ripple, shared rails, and startup behavior, check RF PA output drops from DC power issues.
4. What Resistance Must Be Included in the DC Path?
Cable resistance is only one part of the power path.
A thick conductor can still produce excessive voltage loss when the path contains:
- Poorly crimped terminals
- Loose threaded connections
- Undersized connector contacts
- Oxidized contact surfaces
- Weak fuse holders
- High-resistance relays
- Thin distribution-board traces
- Inadequate negative-return connections
- Too many adapters or transition points
A connection may pass a continuity test and still fail at high current. Milliohms of added resistance can become significant when an RF PA draws tens of amperes.
For example, 10 mΩ of unwanted resistance creates:
- 0.2 V drop at 20 A
- 0.4 V drop at 40 A
- 16 W of local heat at 40 A
That heat may be concentrated inside one connector or terminal rather than distributed along the cable.
If conductor size is correct but the voltage still falls, inspect RF power amplifier harness crimping and measure the voltage across individual connection points while current is flowing.
When the negative return, chassis reference, and RF ground are unclear, review RF amplifier grounding separately. They should not be treated as interchangeable connections.
5. How to Size DC Cables for Multiple RF PA Modules
A cable that works for one amplifier may fail when several modules operate together.
The DC path should be checked against the maximum realistic simultaneous load, including:
- Number of active RF PA modules
- Output power required from each module
- Module efficiency
- Shared controllers and cooling devices
- Startup or switching current
- Continuous-duty operating time
- Hot-state current
- Unequal branch-cable lengths
Do not approve a shared DC bus using the sum of nominal current values alone. Confirm whether all channels can transmit simultaneously and whether the supply, main cable, distribution point, branch cables, connectors, and return path can support that condition.
Where possible, use individual branch protection and measure the voltage at each amplifier input.
A common-bus measurement can hide one weak branch. One module may receive 27.2 V while another receives 25.6 V because of cable length, terminal resistance, or an uneven distribution path.
The acceptance test should therefore record voltage and current by module or by branch, not only at the main power supply.
6. How to Measure PA Input Voltage Under Full RF Load
Voltage-drop testing should be performed while the amplifier is operating under the condition that matters.
Recommended measurement sequence
- Measure the power-supply output voltage.
- Measure voltage directly across the RF PA positive and negative input terminals.
- Apply the required RF input signal.
- Increase output to the specified operating level.
- Record PA input voltage, DC current, RF output, reflected power, and temperature.
- Continue until the system reaches thermal stability.
- Repeat at the highest expected simultaneous-channel load.
- Inspect the cable, terminals, fuse holders, and connectors for abnormal heating.
Use short measurement leads and place the probes at the actual amplifier input terminals. Measuring only at the distribution board may miss loss in the final cable and connector.
To locate a weak connection, measure the voltage across that individual point while current is flowing. A measurable voltage difference across a connector, relay, fuse holder, or crimp indicates resistance at that location.
Infrared images can support the diagnosis, but temperature alone is not enough. A hot connection should be confirmed with electrical measurements and then retested after repair.
7. What Defines a Pass or Fail Result?
There is no universal cable-gauge answer for every RF PA.
The correct cable depends on:
- Supply voltage
- Required PA input voltage
- Maximum current
- Peak current
- Loop length
- Conductor resistance
- Connector count
- Ambient temperature
- Cable bundling
- Duty cycle
- Number of active modules
- Allowed voltage drop
- Required RF output
A proper acceptance limit should connect the DC condition to RF performance.
A useful requirement is:
At the specified RF output, duty cycle, ambient temperature, and simultaneous-channel load, the voltage measured at each RF PA input shall remain within the approved operating range without abnormal output reduction, current limiting, protection alarms, or excessive cable and connector temperature rise.
The test record should include the actual cable type, conductor size, length, connector path, supply voltage, module voltage, current, RF output, test duration, and temperature.
An unloaded voltage check or brief low-power bench test is not enough.
What DC Cable Evidence Should Be Confirmed Before RFQ?
Before quotation or system integration, confirm:
- RF frequency range
- Target RF output power
- Required module-input voltage
- Maximum continuous current
- Peak or startup current
- One-way cable length
- Positive and negative conductor size
- Cable resistance per metre
- Connector, fuse, relay, and switch path
- Number of simultaneously active modules
- Duty cycle
- Ambient temperature
- Cooling method
- Control interface
- VSWR boundary
- Required test duration
- S/N-linked test-report requirements
| RFQ item | Why it matters |
|---|---|
| Required PA input voltage | Defines the allowable path loss |
| Maximum and peak current | Determines conductor and connector stress |
| Complete loop length | Prevents one-way length errors |
| Connector and fuse path | Captures resistance outside the cable |
| Simultaneous channel count | Defines the real shared-bus load |
| Duty cycle and ambient temperature | Affects resistance and thermal rise |
| Acceptance evidence | Defines how the installed path will be approved |
Projects that require a custom RF power amplifier module should define the DC path at the same time as the RF requirements. Selecting the amplifier first and checking the cable later can leave the system unable to reproduce rated output after installation.
Conclusion
DC cable size affects more than wiring safety. It determines how much of the power-supply voltage reaches the RF PA during real operation.
The correct check is not whether the supply display shows 28 V. It is whether the amplifier terminals remain within the approved voltage range while the required RF output, duty cycle, temperature, and simultaneous-channel load are present.
Calculate the complete loop resistance, include every connector and switching point, measure voltage at the PA terminals, and verify the result after thermal stabilization.
Send our RF engineering team your frequency range, target RF output, required module-input voltage, maximum and peak current, cable length, conductor size, connector path, duty cycle, simultaneous channel count, cooling method, control interface, VSWR boundary, and test-report requirements.
RF SKYPOWER will review whether the selected RF PA module and the complete DC path can support the same full-load operating condition before final RFQ approval.