TL;DR
If you have LED headlights that are causing RF interference, ferrites alone will probably not be enough to completely solve the problem: You might need to put the offending switch-mode LED controller in "RF Jail" as described below.
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We live in a world of RF - and an increasing amount of this is from devices that are not intended to produce radio-frequency energy. These devices have proliferated in the past several decades and surround us at home, work and in our cars.
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| Figure 1: The vehicle in question with LED headlights - which are now RF-quiet on all bands. Click on the image for a larger version. |
Generally speaking, modern internal combustion vehicles are "pretty quiet" - especially compared to their electric and hybrid counterparts - and someone in such a vehicle will experience less RF noise when they are "out and about" in a rural-ish area than they will at home. For amateur radio operators, this is a good thing as mobile operation often includes weak signals - whether this is on HF, or on VHF/UHF with weak signals from distant repeaters or during simplex operation, with intervening terrain.
Effective mobile operation is therefore contingent on a vehicle that may be intrinsically "RF-quiet", but this also means that any accessories that you might add to this vehicle also be RF-quiet as well. These days there are any number of things that you might throw in your car that can spoil an otherwise-clean RF environment and the short list includes USB chargers1 , GPS receivers and extra lighting 2, to name but a few.
Real-world case - Aftermarket LED headlights
A friend of mine recently installed LED headlights in his older Honda CRV 3. It took a while to correlate the cause, but he eventually noticed that when the headlights were turned on, he lost significant weak-signal sensitivity on 2 meters : Around town and with stronger repeaters, the effect wasn't really noticeable, but when the repeater was distant - or when using simplex with a weak/distant station - turning on the headlights dramatically reduced received 4 signal quality and range. As it happens, he has more than one VHF/UHF radio in his vehicle and although both were affected, the one with its antenna mounted to the front fender - much closer to the LED headlights - was more severely impacted.
LED headlights typically consist of two modules: The LED itself and the "controller" - which itself is a switch-mode power supply to regulate the current. Sometimes these are combined, but in this case, they were separated with a short cable, the "controller" being separate from the LED module that is mounted in the headlight housing.
Even knowing that it was futile, we tried putting ferrites on the cables to/from the LED modules and its controller, but all that we could manage was a slight reduction in interference that was hard to quantify, taking the problem from being "terrible" to just "awful". This was not unexpected: Under the very best conditions ferrites alone may provide 15-20dB of reduction in conducted energy (2-3 "S" units) but at VHF/UHF getting anywhere near that much attenuation is very difficult - and measurements indicated that even if we did achieve 15dB reduction across the board, the "jamming" of weak signals by the headlights' switch-mode controllers would still be significant: It would be like taking an "10-over S-9" interfering signal down to just "S-8" - still pretty bad!
Methods of filtering
As noted earlier, simply putting ferrite devices on the conductors can reduce the amount of conducted energy, but their effect is typically limited - likely 15-20dB in the best case when this is the only method employed. Ferrite devices - such as beads - simply add inductance (and thus loss or impedance) to RF energy while leaving DC and low-frequency signals alone, but these devices have limitations: Properties such as self-resonance and the permeability of the magnetic material vary wildly with frequency and high levels of attenuation are difficult to attain - particularly at high frequencies (e.g. VHF/UHF) where even short conductors carrying RF currents can radiate with reasonable efficiency - particularly when the noise-generating device and the receive antenna are in close proximity.
As discussed previously on this blog 5 , one sure-fire way to quash such interference is to put the offending device in "jail" - that is, enclose it completely in a metal box and use both inductive and capacitive filtering on each and every conductor to prevent RF energy from being conducted. Done properly, this method can be "completely"effective 6 in preventing interference.
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| Figure 2: Simplified diagram of the method of filtering. The inductance - provided by the ferrite beads ("L") provide "choking" impedance to the RF currents being carried on the wires from the LED controller (the "noisy device") before they connect to the feedthrough capacitors ("FT") in order to maximize their efficacy. Click on the image for a larger version. |
Figure 2 shows a very effective method of dealing with this problem and it involves inductance ("L") and capacitance in the form of "feedthrough" capacitors (marked "FT"). The inductance is in the form of a ferrite bead installed on each of the conductors between the "noisy device" and the capacitors that increase the impedance at radio frequencies on that conductor. The capacitors are then used to shunt the remaining RF energy to the local "ground" which, in this case, is the partition on which the feedthrough capacitors are mounted (this will be discussed shortly) and also the metal enclosure in which the noisy device is mounted.
The intent here is to prevent RF currents flowing through to the "external connections" where the wires themselves will act as antennas to radiate the RFI generated by the noisy device. By shunting RF to the partition - and the metal box itself - the remaining RF energy will be minimal and confined within the enclosure. The above configuration is capable of attenuating RF energy from HF through UHF by 30dB or better (50-60dB at some frequency) - a value far higher than ferrite alone.
The metal box containing the electronics and filtering offer another important benefit: As even short conductors a few inches/cm long can radiate at VHF/UHF, placing the noisy device and its conductors within the metal enclosure will prevent this.
An important feature of the design is that the inductances ("L") are located between the noisy device and the feedthrough capacitors. As these inductances (ferrite beads) offer 10s to 100s of Ohms of impedance to the RF signal, this allows the very low impedance of the feedthrough capacitors at those same frequencies (which is likely an Ohm or less at higher frequencies) to more-effectively shunt that energy. If the ferrite beads had not been installed the shunting of the low-impedance RF energy from the noisy device would have been far less effective.
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| Figure 3: An assortment of feedthrough capacitors. The top two rows are of the "screw-in" type, typically mounted to chassis- walls and bulkheads while those on the bottom row are the "solder-in" type as used in the partition in this project. Click on the image for a larger version. |
In this case, we were preventing RF from a noisy device from leaving the enclosure - but if we were trying to protect a device FROM RF energy (e.g. a transmitter) we would place the inductances on the conductors coming from the outside world as well to allow the capacitors to better-perform their function.
Feedthrough capacitors and the partition
A bit more needs to be said about "feedthrough capacitors". Even if you are "into" electronics, you may not have seen these devices for the simple reason that they are a bit esoteric - and, perhaps, they are not quite as prominent as they have been in the past. Figure 3 shows an assortment of feedthrough capacitors: The top two rows are chassis-mounted types that are held in a pre-drilled hole by a nut while the three on the bottom row are of the "solder-in" type.
A feedthrough capacitors has a wire that passes through its center with the "capacitance" surrounding this wire over the length of the of its body and the other "plate" of this capacitor is the body of the feedthrough capacitor itself. By being constructed this way, there are no wires or leads between the "capacitor" part and either the signal or ground wires and as such, any series inductance - which would reduce the efficacy of the capacitor - is minimized.
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| Figure 4: Solder-in type feedthrough capacitors soldered to the brass partition. This large sheet of metal provides a low-impedance RF path to the common "ground" (e.g. case) to contain RF entirely within the metal case. Click on the image for a larger version. |
Figure 4 shows nine feedthrough capacitors soldered to a brass partition (the solder is on the opposite side) and as can be seen from the photo, these are capacitors that have a wire that runs through them. As such, they have no "ground lead" aside from the outside of the body of the device and all of them are tied together on the same piece of metal. This method assures a low impedance RF path between all of the capacitors and since the partition itself is bolted to the aluminum case (see Figure 5), it, too, is well-bonded.
Putting it in the box
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| Figure 5: The LED controller in the box w/filtering. By containing RF currents within the box, both common-mode and differential RF currents on the leads are reduced to near zero. Click on the image for a larger version. |
Toward the top of the image we see the switch-mode controller for the LED headlights, bolted inside the case (which also helps dissipate heat) and farther down we see that all nine wires (three for power, the remaining six to the LED module itself) connect to the feedthrough capacitors on the partition. Each of these wires has its own ferrite bead and these wires go directly to their respective feedthrough capacitors on the brass partition, which is held in the case with screws.
Below the partition are the wires that connect to the outside world: On the right are the three wires that go to the power supply (e.g. the original connector to the headlights) while the gray cable on the left goes to the LED module. The original LED retrofit had very short leads - on the order of 5" (13cm) for the headlight connector and another set of similar length to the LED module - and this made the installation a bit challenging as there was just enough wire to make the connection between the controller and the capacitors on the partition.
The box containing everything is die-cast aluminum and it's a bit larger than necessary - but it was the only size for which I had two identical cases and also large enough to accommodate the LED controller and the filtering. As this box is quite a bit larger than the original controller, rather long wires had to be used to allow it to be placed where there was room in the rather crowded engine compartment, somewhat away from the headlights.
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| Figure 6: As the original cables were very short (about 5", 13cm) longer wires had to be spliced to allow placement of the large die- cast box. This shows the male headlight connector and the LED module spliced to the cable and covered with nylon web. Click on the image for a larger version. |
At the opposite end of the wires it was necessary to splice the added cable to the LED unit's headlight connector and LED module and this was carefully done using soldered connections insulated with head-shrinkable tubing, all of which was covered by woven nylon braid for protection and a neater appearance.
The result
With the added length of the cables, there were nooks and crannies into which the die-cast box with the LED controller could be placed within the engine compartment of the Honda CRV.
The real test came when a distant 2-meter repeater was keyed up to cause it to send its ID: The return signal was very weak and noisy - as expected - but there was no difference in the way that it sounded when the lights were switched on and off. While admittedly unscientific, this test tells us pretty much everything that we needed to know: Whatever RF interference there is that might be escaping the box and its filtering is well below the level at which it can be detected and the problem is considered to be solved!
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Footnotes:
- The topic of "very noisy" plug-in USB chargers effectively "jamming" VHF/UHF reception was discussed on this blog several years ago - see: "How USB car power power adapters can ruin 2 meter mobile reception" - link and its follow-up article: "A 'quiet' 5 volt USB car power supply" - link.
- This same friend frequently volunteers in public service events involving runners and cyclists on roads where it is required that yellow/amber lights be used to minimize hazards. Certain makes/models of these lights have been observed to produce tremendous amounts of RF energy that effectively quashed all 2 meter reception,so they were sent back to the seller until he found a unit that was "quiet".
- Check your local regulations regarding retrofitting of headlights with equipment other than that of the type provided by the original manufacturer.
- Of course, a low-level increase in the noise floor in the proximity of the vehicle would have absolutely no effect on the transmitted signals, but the result of this interference is that the station in his vehicle became an "alligator" - all mouth, no ears - meaning that he was able to "talk" much farther than he could hear.
- Whereas simple capacitor (shunt) or inductor (series - and this includes ferrite devices) may reliably attenuate an offending signal by 15-20dB or so at best (very generally speaking) combining both types of reactance - "L" (inductor) and "C" (capacitor) - appropriately can provide many 10s of dB of attenuation if done properly - easily 30-60dB for simple circuits. This greater amount of attenuation is far more likely to be able to put the interference from the device well below the noise floor of the receive system. This is the technique used in footnote #1 (above) and explained in some detail in the blog entry "Completely containing Switching Power Supply RFI" link.
- "Completely" eliminating is actually impossible, but reducing it by 30-60dB is likely to attenuation it below the level of detection.
- The use of surface-mount components - like capacitors - with their lower parasitic reactance than their counterparts with leads - can be used very effectively to filter RF, but several cascaded stages of such capacitors and inductors - and careful layout of a PC board - are likely to be required to obtain sufficient attenuation. "Feedthrough"-type surface-mount capacitors are also available - which have excellent performance - but these, too, require a properly-designed PC board.
- The ferrite beads used in this project were Fair-Rite 2643000801. These use "43 mix" ferrite and are 0.295" O.D., 0.297" long and 0.094 I.D. (7.5x7.55x2.375mm) and are able to accommodate the wires + insulation of the conductors from the LED controller. This material has a typical impedance of about 94 ohms at 100 MHz and cost about $0.24 each in single quantity at the time of writing. I used them primarily because they were on hand.
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This page stolen from ka7oei.blogspot.com
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