Reducing QRM (interference) from a Renogy 200 watt portable solar panel system

• The wires connecting to the load.  Typically a battery being charged - which can be connected to other things (e.g. vehicle, inverter, etc.)  The wires connecting the panel to these other things - and those devices themselves - act as part of the "antenna" that potentially radiates noise.
  

• The solar panel itself.  The solar panel consists of large plates of metal - not only the silicon of the panel, but any metal frame and wiring.
  

The "load" and solar panel constitute two different parts of the charge controller:  The panel is connected to the INPUT of the PWM circuitry while the wiring is connected to the OUTPUT of the PWM circuitry, effectively forming a dipole antenna.  To a degree, the electrical lengths of these two conductors - which can include power cords or even a vehicle - overall can broadly resonate, affecting certain frequency ranges more than others.

The reason for the generation of the interference is due to the fact that the PWM circuitry (which is operating at a frequency of 10s or 100s of kHz) uses square waves, rich in harmonics.  As the voltage input (from the panel) and the output (to the battery/load) are different parts of the PWM circuit, they necessarily have different waveforms on them.

Figure 3: Charge controller with additional filtering showing added bifilar-wound chokes on both the input and output leads. Click on the image for a larger version.

While this device does have some of filtering to provide a degree of input impedance reduction (fairly high capacitance) and smoothing of the PWM waveform of the output (more capacitors and likely some inductance) the degree to which this filtering is implemented is suitable for the purpose of providing clean DC power to the load and maximize power conversion efficiency, this filtering - and likely the controller's circuit board itself - was likely not intended to provide the high degree of RF suppression needed to make it quiet enough to avoid the conduction of RF energy onto its conductors which is then picked up by a nearby receiver.

Ferrite alone is NOT the answer!

One may presume that the answer to this problem is the implementation of RF device such as snap-on or toroidal ferrite devices - and you would be partially correct.  Any practical inductor - such as that formed by the introduction of a ferrite device - will have rather limited efficacy in quashing RF currents.

Snap-on devices (e.g. those through which a wire passes) have very limited usefulness at HF frequencies (<30 MHz) - especially on the lower bands - as they simply cannot impart a significant amount of reactance in the conductor onto which they are installed.  At higher frequencies (VHF, UHF)they can have a greater effect - but their efficacy will be disappointing at HF.

A device that can accommodate multiple turns through its center such as a toroid (or even a larger snap-on device) it may be possible to get up to a few hundred ohms of reactance on a conductor across a fairly wide frequency range - but even this will be capable of reducing the amount of RF by 10-20 dB (2-3 "S" units) at most:  Depending on the intensity of the RFI from the solar controller, this may not be enough to quash the interfering energy to inaudibility.

To be sure, it's worth trying just the ferrite devices by themselves to see if - in your situation - it reduces the RF interference from the controller to your satisfaction but remember that the location where you are likely to be using this panel is likely far quieter (RF-wise) than your home QTH:  It may seem quiet enough at home but still be noisy in the middle of nowhere.

The addition of capacitors to the circuit can improve the efficacy over ferrite alone by orders of magnitude.  Consider the diagram below:

Figure 4: Diagram, including additional filtering.  L1 and L2 are the bifilar chokes seen in Figure 3, above while the capacitors (C1a, C1b, C1c and C2a, C2b and C2c) and their implementation are described below. Click on the image for a larger version.

A glimpse of what was done may be seen in Figure 3.  Some 14 AWG paired copper wire (red/black) was wound on two FT140-43 ferrite toroids - about 6 bifilar turns in this case:  Individual wires could have been used other than "zip" cord - just be sure that the two parallel conductors are laid in parallel to maximize the effectiveness of the bifilar configuration.  Two of these wire/bifilar devices were constructed - one for the DC from the panel and the other for the output to the battery/load.  "Spade" lugs were installed on one end of the red/black wires - two lugs per wire/bifilar assembly.  (FT240-43 or FT240-31 toroids could also have been used, but the FT140-43 is a fraction of the cost, half the diameter, and perfectly suitable for this application.  The FT240 size may be more appropriate if such a filter network is constructed for a higher-current system with larger-gauge wire.)

On the solar controller itself, small 0.1uF, 50 volt monolithic capacitors were installed (C2a, C2b, C2c) to form part of the filter circuitry:  Minimal lead length is important for maximum effectiveness.  While monolithic capacitors are preferred because they are small (and will fit more easily in tight spaces) and have very low ESR (Effective Series Resistance) one could use disk ceramic capacitors instead.  Film/plastic capacitors are less effective at higher frequencies.

Figure 6: Terminal strip with capacitors C1a, C1b and C1c. As described, these capacitors do much of the "bypassing" of RF differential currents between the input and output. Click on the image for a larger version.

As can be seen from this picture, the terminals are the "clamp" type and are connected in the same manner as the lugs on the cable on the bifilar toroid assembly. - and also note that this "modification" is completely reversible as nothing at all was changed on the controller itself.

Spectrum analysis plots

Using a Tiny SA Ultra spectrum analyzer, I coupled the supplied telescoping antenna to the output (battery/load) cable by holding it in parallel with it.  While inductive coupling would have been preferable - and more repeatable and sensitive - this quick test gives a general indication of the nature of the energy being emitted by the charge controller and the reduction afforded by the added filtering.

Take a look at the "before" trace with no filtering:

Figure 7: "Before" (no filtering) analysis plot with the telescoping antenna of the analyzer held against the DC output cord. Click on the image for a larger version.