Going Mobile, Part 2 – Solving RFI Problems

In a previous feature, I described the mobile installation in my truck, providing a few general tenets of mobile station installation and using my truck station as an example of each. I’ve had many HF contacts now using that mobile station and I’ve received excellent signal reports from DX stations ranging from Germany to Chile to Australia, as well as from coast-to-coast in North America. However, I experienced a few problems with my transmitted and received signals on some bands at first, so I promised to issue this Going Mobile Part 2 piece to elaborate a little about basic noise and interference problem solving with mobile stations.  It is not possible to thoroughly treat all the topics and issues of resolving mobile installation RF interference problems in a single short article, so I’ll attempt to hit some big picture concepts and potential fixes to some of the most commonly encountered issues.

[Video of this mobile station in operation during CQ Worldwide DX Contest, Oct 2014.]

Mobile Station RFI: Egress and Ingress

Unless you’re going horse buggy or bicycle mobile, the RF environment in your vehicle is packed with signals from many different potential sources.  First, of course, is your amateur transmitter, but RF signals also emanate from your vehicle ignition system, fuel pump, fans, electric motors, onboard computers, and possibly many other sources! Especially in your car, you are swimming in a broadband slurry of RF emissions.  It makes you want to shower after just a quick jaunt to the corner grocer, doesn’t it?

RFI Environment

The mobile RFI environment is loaded with signals. Your station may receive signals from your vehicle’s noisy emitters such as ignition sparks, motors, or other electric devices. Your own transmissions, emitted very nearby from your mobile antenna, can also create RFI issues. Electrical ground loops and “common mode” currents can impose noise on your signals, both received and transmitted.

Radio frequency interference (RFI) of some sort with a new mobile station is nearly a sure bet.  Mobile RFI can be categorized broadly as either egress or ingress, relative to the vehicle.  Egress interference is interference from your vehicle systems to your mobile station.  Ingress interference is from your amateur station to your automobile systems.  For example, a common form of egress interference with mobile stations is ignition noise from the vehicle on the receive audio of the amateur station, creating a regular popping or crackling noise.  An example of the much less commonly experienced ingress interference would be amateur radio transmissions impacting the performance of an engine control computer, perhaps causing acceleration hiccups with certain frequency transmissions.  “QSL, but stand by old man, the traffic light is turning yellow.”

We will consider primarily egress RFI here – the case where automobile system’s emissions interfere with your amateur station – as it is far more common than ingress RFI.  But, the fixes described for egress can often help resolve the occasional ingress RFI issue as well.

Egress RFI is typically radiated by your vehicle systems to be received by your amateur station antenna, or less commonly it may be conducted from the RF sources through wires to your transceiver.  The type of fix you try first may depend on whether the noise is received RF emissions or RF conduction, so here’s a quick test you can conduct to find out which type of interference you are experiencing.

Remove your antenna from the vehicle and then crank up the engine to hear if the noise is still prominent on the receive audio.  If the noise is reduced markedly or eliminated without the antenna, you are almost certainly receiving vehicle-emitted RFI via your antenna. You’ll probably want to first try some of the ground plane and bonding solutions described next.  If the noise remains prominent without the antenna connected, you likely have a conduction issue that may be more difficult to rectify.  Refer to the ground loop and choke fixes further along in this article, and reference the sources listed at the end of the article for more in-depth guidance on curing such problems.

RFI, Antenna Ground Plane, and Bonding

The body of your vehicle essentially serves as a ground plane for your mobile vertical antenna.  The ground plane provides for return currents to the antenna, and it may be considered a substitute for the “other half” of a half-wave antenna, such as a dipole.  It is an integral part of the antenna AC electric circuit.  If the ground plane imposes significant losses (and it will in a car), the performance of your antenna will suffer due to reduced current flow.  Ground plane losses only exacerbate losses that may already be imposed by a coil-loaded, physically shortened antenna typical of most mobile stations, so it is prudent to do what you can to minimize ground losses by enhancing the ground plane.

Signal to Noise

Noise of all types combine to produce a noise floor above which received signal strength must reach to be clearly detected by the receiver. Greater desired signal strength or lower noise levels make signals more discernible, with a greater resultant signal-to-noise ratio.

Interestingly, improving the ground plane conditions can also resolve or reduce some types of RFI problems.  One reason why it helps is due to the fundamental concept of signal-to-noise ratio.  Your car systems are producing RF noise that at least partially masks the RF signal you wish to receive from another station – a low signal-to-noise ratio due to big egress RFI noise.  If you can increase the receive signal strength the signal-to-noise ratio improves, and you will hear the desired station more clearly.  An improved ground plane with reduced losses (greater current flow) helps to increase the received signal strength, and thus, increases the signal-to-noise ratio that your receiver has to deal with.

Another way an improved ground plane can help signal-to-noise ratio is by reducing the egress RFI noise.  Here’s one example that’s very common to illustrate the point.  The exhaust system of many vehicles is poorly connected electrically to the rest of the vehicle structure.  There is poor conductivity, or the conductive connection is primarily at the engine end of the system.  The lengthy run of pipe routing stinky fumes to the rear of your vehicle can act like a pretty good antenna (especially on some HF bands), receiving your own transmitted signals or receiving signals from any of the myriad noise sources in the vehicle environment, and then re-radiating them or conducting them through the vehicle and to your receiver.  However, if you incorporate the exhaust system into the ground plane by improving the conductive connections along its length, the re-radiated or conducted RF from the exhaust system can be mostly extinguished!  Noise is reduced and the receiver signal-to-noise ratio is improved.

So, how do you improve this ground plane, you ask?  Take the 007 approach:  Bond.  Frame bond.

Tinned Braid

Tinned braid is a flexible, low impedance conductor suitable for bonding various components of your vehicle for enhanced ground plane performance.

Kit

A sample of exhaust pipe bonding kit materials, courtesy DX Engineering.

Bonding is the process of making low impedance connections among the various conductive parts of your vehicle.  The “low impedance” part comes from using conductive strap, such as tinned braid, and solidly attaching it between frame and body elements of the vehicle.  For instance, using tinned braid you may electrically bond your exhaust pipe to the vehicle frame at three or more points along its length, thereby integrating the accidental antenna into the rolling ground plane.  Solidly attach the braid to the pipe using ring clamps, and use screws with star or tooth lock washers and soldered ring connectors to ensure a good connection to the car body or frame.  Many amateur radio and electronics distributors sell ready-to-install kits or parts specifically for exhaust system grounding, or you can homebrew your own.  [One kit example from DX Engineering.]

Trunk Lid Bonding Strap

The hinges of trunks and hoods may not provide adequate electrical connectivity for effective ground plane integration. Photo courtesy KØBG.com.

Beyond bonding the exhaust system you may find it helpful for RFI noise reduction and improved antenna efficiency to bond horizontal body surfaces together, such as bonding the hood and trunk to the rest of the vehicle body.  Usually this is done by attaching short segments of braid across the hinge connections of the hood and trunk.  Often, the hinges do not provide a good electrically conductive connection for the hood or trunk, so the braid connection helps to electrically integrate these surfaces into a single massive ground plane.  For pickup trucks you may also improve the ground plane by bonding the bed to the cab, using braid segments beneath the truck and frequently using existing stock bolts and connection points.  Bumpers are also common targets for bonding to the vehicle frame.  Remember, the goal is to maximize the extent of the ground plane under the antenna and to improve the conductivity at RF frequencies.  Don’t assume that just because your multimeter show a low DC resistance across body and frame components that the impedance at RF frequencies will be similarly low!

And one related note on this topic regarding antenna mounting:  If you do not have a solid connection between the antenna ground side (coaxial shield side) and your now-solidly-bonded ground plane, you are mostly wasting your time with the tinned braid bondings.  Be sure that your antenna mount provides solid continuity with the ground plane.  “Permanently” mounted antennas using bolts and washers are usually best for electrical continuity, as long as they are mounted to a portion of the vehicle that has been incorporated into the overall ground plane.  Magnetic mounted antennas will frequently result in poor grounding continuity for the antenna, so performance may be degraded, especially for HF ops.  Provide an additional ground strap at the very least for HF mag-mounts.  Further, as a general rule, mount your antenna as high as feasible on the vehicle and with as much metal mass (ground plane) below the antenna as possible.  This will generally improve your antenna performance.

Ground Loops and Common Mode Currents

Ground loops and common mode currents can produce noise on your receive audio as well as on your transmit signal.  The bonding and mounting methods described above can help to reduce noise resulting from ground loops or common mode currents, but additional measures may be necessary.

Chassis Ground

Grounding station component chassis to the vehicle body can help alleviate ground loops and stray currents. A wide, low impedance conductor is best, and while copper strap may be used it can weaken over time from vibration and motion and may crack or break eventually. Here, a seat mounting bolt is used as a single ground point for multiple component ground straps.

Ground loops are undesired currents flowing on your gear as a result of a voltage differential between two points in a ground or power connection.  A ground loop may often result in alternator whine on the transmitted signal, but other manifestations are also possible.  To minimize ground loops you should make your station power connections directly to the battery terminals of the vehicle, avoiding all automobile wiring and avoiding the use of the vehicle chassis as a power ground return connection.  Go direct with power, as indicated in Going Mobile, Part 1.  Further, ground your station equipment chassis to the vehicle body or frame using a separate low impedance ground connection for each component to a common point.  Wide tinned braid is a good low impedance option for station chassis ground connections.  This will provide a common ground level voltage for your equipment to help eliminate any stray currents that may arise.

Common mode currents are undesired currents flowing on wires and most particularly on the antenna coaxial cable shield.  Common mode currents may be induced on your coax by RF from your noisy vehicle emitters or imposed on your coaxial cable by your own amateur station transmissions.  A mobile station will almost always have some common mode currents flowing, and the currents will result in RF radiation (or “re-radiation”) from the cable, thereby producing RFI problems.  One result of the common mode problem is reports of “RF on your transmit signal.”  This will cause your signal to sound distorted or garbled to other stations.

As noted above, RFI problems are made worse by poor antenna mounting and high ground losses.  Common mode RFI problems are no exception.  So, after ensuring that you have properly mounted your antenna, bonded your vehicle components together, and grounded your station chassis, you may need to implement a choke on your coaxial cable to alleviate common mode RFI, particularly for the HF bands.

Coaxial Choke

The Mix 31 ferrite bead, 6-loop choke for 10m – 80m HF operations, as advocated by Alan KØBG. The coax loops are about 3″ in diameter, each loop through the bead orifice. Photo courtesy KØBG.com.

A choke is a type of filter that blocks the passage of RF currents.  A very effective choke may be implemented on your coaxial feedline to stem the flow of common mode RF currents on the coax shield.  Various methods of implementing chokes are widely used, but one of the most effective is the use of ferrite snap-on beads.  A specific solution advocated by Alan Applegate KØBG on his expansive mobile operator web site is a “mix 31” split ferrite bead with a ¾ inch inner diameter with 6 or 7 loops of RG-58 or RG-8X cable passing through the bead with approximately 3 inch diameter of the loops.  The choke should be inserted in the feedline as close to the base of the antenna as possible, and not at all inside the vehicle.  This option is claimed to be adequate for 10 meter to 80 meter band common mode currents.  DX Engineering is again one supplier of mix 31 beads of this type.

Additional Considerations

As I noted from the outset, this article covers only the basics of mobile station RFI problems and resolutions.  Sometimes a vehicle’s systems will produce interference that is very difficult to squelch and more elaborate efforts may have to be undertaken involving corrections applied to the vehicle systems.  For instance, a malfunctioning computer module may need to be replaced, leaky ignition system wires may need to be replaced with RF suppressive wires, an alternator diode may need to be replaced, a fuel pump may need to be grounded, or other electrical or ignition system corrections may be required to eliminate noise sources.  Tracking down such sources can be a difficult, even maddening enterprise, and a knowledgeable elmer is mighty handy in such cases!

Although it is a patch for real noise fixes, your transceiver’s noise blanker may be used to filter regular patterned noise sources such as that typical of ignition system noise.  Yes, the noise blanker will reduce receive performance to a small extent, but it may be a good compromise until you can track down a stubborn noise source.  Check the user’s manual of your radio for instructions on activating and adjusting the noise blanker and see if it reduces or eliminates pulsing noise sources in your receive audio.

Eliminating RFI in a mobile station often requires a shotgun approach, as there may be multiple sources of noise and infrequently a lone magic bullet that will kill it all at once.  Be patient, be meticulous, and don’t give up!  Consider it a challenge and a learning opportunity, and once you’ve got your mobile station transmitting and receiving flawlessly you will be well prepared to provide advice to other hams with a hankering to go mobile!

Good luck, and 73!
~WØSTU

[Video of this mobile station in operation during CQ Worldwide DX Contest, Oct 2014.]

Recommended Resources:

KØBG Mobile Amateur Radio Web Site:  http://www.k0bg.com/

The ARRL Handbook for Radio Communications:  http://www.arrl.org/shop/2014-Handbook-Hardcover-Centennial-Edition

The ARRL Mobile Station Page: http://www.arrl.org/mobile-stations