RFI & EMI

EMI & RFI  (by NU8Z)

i-am-experiencing-interferenceLiving in the city I guess one has to accept the fact that there will always be some degree of interference caused by the numerous power lines, appliances, and electronic devices that will surround your station.  That being said, I have experienced a number of electrical interference issues that significantly impacted my radio operation to the point where they needed to be identified and resolved. To date all major issues have been tracked down and resolved. Also, the other side of this coin is that living in the city increases the likelihood that your station may interfere with some other device(s). I have been both the recipient and the alleged cause of interference. The bottom line is that most problems are the result of poor equipment design in terms of providing adequate incoming and outgoing EMI suppression. Regardless of where the problem is, the issues need to be solved if you want to have and effective DX station.  I have supplied a number of links to websites and documents that discuss finding and resolving interference issues. Some of these are written by folks with a higher level of knowledge than me and I hope you will find them helpful. I have included some of my own suggestions below that you may find helpful.

Interference can be a problem for any amateur radio installation. As mentioned above, Power poleInterference problems can be experienced on both the transmitting and receiving modes. I guess it is best to think of these as:

  1. Interference that is caused by your station; and
  2. Interference that is caused others or devices that affects the reception of received signals at your station.

 

RF Interference: First a quick mention of interference that may be caused by your station. First I will make the assumption that “all is well” with your transmitted signal. That being said there are a number of things that can be done in your shack to prevent interference with other devices. Some of these actions include the following:

  1. Be sure that your equipment is well grounded. Ground all equipment to a single point. (Avoid Ground loops)
  2. The path to ground should be as short a possible. The ground wire can radiate if long enough in comparison to the transmitting frequency.  This is not a good thing. I use to have a shack in a second story room and had all sorts of problems with ground wire radiation and the resulting interference. These problems can be minimized by changing the length or tuning the ground wire. You can also shield a ground wire by using the center of RG-213 as a ground conductor and bypassing one end to the shield with capacitors. This will minimize the ground wire radiation.
  3. Use a good high quality coax cable

 

Interference on Received Signals:  With regard to interference to the received signals at your station, I have the following general observations and suggestions:

  1.  ”Wall Wart” switching power supplies can be a major cause of in shack interference to received signals. Many are of poor quality and emit EMI. The problem can be solved by one or more of the following approaches.
    1. Use clamp on ferrite chokes on the output of the wall warts. Get as many turns as possible. The attenuation level is proportional to the square of the number of turns.
    2. Select a new Wall wart. All Wall warts are not created equal!
    3. Replace all wall warts with a linear power supply. One supply can be used to power many of your in shack DC powered devices. Properly sized resistors can be used to make small voltage reductions if needed. (Ie.. 12 to 9 volt) Size resistor for proper voltage drop and power dissipation.
  2. Switching power supplies used on many new LED lighting systems suck!
  3. Use clamp on ferrite chokes on all lines in and out of computer. This is especially true for keyboard and monitor lines.
  4. Use a LCD monitor. CRT monitors are noise machines.
  5. Interference can be radiated through the air from the device or power cable acting as an antenna, or interference can be sent through the interconnecting wiring to other devices.

 

RFI chart

Tracking Down Interference: I guess the first and likely the easiest step in process is to recognize that a interference problem is EMI related. This first and most fundamental step is followed by the process of analyzing the nature of the interference, locating its source, and last but not least, resolving the problem.

Gathering and Analyzing EMI Information: Before you go after a problem there is a lot of information that you can collect that can help you zero in on the source. Here are some examples of what can be done:

Use your stations directional antennas to determine the general direction of the interference.

Are there certain times that the interference occurs and other times it is absent?

Does the interference occur during Day or Night (or both)?

Is the interference constant or intermittent?

Is the interference occurring on a consistent schedule (same time everyday – maybe on a timer circuit or photo cell switch?)

Does it get better or worse after rain storm? If better -may indicate dirty insulators – arc tracking. If worse, it may indicate marginal spacing to ground path. If better, it could also indicate marginal non ground path connection (improved by rain)

Is there any repeatable pattern on bands? If noise moves up and down in frequency, or is found every X number of KC’s it is likely caused by a electrical device (Not power line noise)

Is the interference just pure broad band noise or is it modulated in any fashion? If broad banded  un-modulated noise, it is likely power line noise (buzz). If it is modulated (changes sound), it is likely some electrical device  problem.

Don’t rule out the possibility of multiple sources. All the above are indicators that may be helpful in zeroing in on the source.

 

DF AntennaLocating the problem: Hopefully the analyzing portion of this process gets you headed in the right direction.  Within your shack or home one of the quickest ways to ID the culprit is to systematically evaluate possible sources. A good first step is to get a battery to power you rig and pull the main breaker on the distribution system for your home. You could also shut off all the circuits one at a time.  If the interference subsides you know it is coming from something in your house. Bring the power back on one circuit at a time and note if the interference re-appears. Once you narrow it down to the circuit you can zero in on it by removing the power from each device and determining if there is any improvement.   Again, remember that you may be getting interference from a number of different sources. It may not disappear all at once. Look for small improvements. Once identified, use one of the procedures discussed above to mitigate or eliminate the problem.

Another method to use to find issues both inside and outside your home is the use of some simple DSCN0675-smalldirection finding equipment. I have used the MFJ-852 noise level receiver along with a small homebrew 3 element beam to track down numerous problems. These include bad switching power supplies, faulty street lights, and faulty power factor correction capacitors and insulators on the local power distribution systems. Myself and some other local hams recently used this system to track down a faulty home security system that was wiping out the receiver of a fellow club member. He is now able to get back on the air. The MFJ 852 is a 135 mhz receiver and with the three element beam is very directional. Most noise is pretty broad banded. The interference may not sound the same on 135 Mhz as it did on a lower frequency, but in most all cases I was still able to hear it. MFJ now sells a version of the MFJ- 852 that comes with a direction antenna.  I built one out of PVC pipe and some stiff alloy rod (similar to coat hanger wire). It is a simple center feed dipole cut to 135 mhz with a reflector and director that was optimized for maximum front to back gain using the free MManna antenna design program. I must give credit to Brian (KG8CO). He is the actual owner of the MFJ-852. Me, I just designed and added the antenna. I keep it at my house due to my central location with any area hams that may need to use it.

Resolving the Problem: If the problem is found to be in your home, it is normally pretty easy to resolve. You can add ferrite chokes or replace the device. Whatever it takes to get the job done!  Sometime you can’t, or it’s not worth fixing. Our clothes washing machine is a good example. It has an electrically commutated motor that kicks up some pretty good modulated noise spikes every 10 Kc’s or so. I live with it in the interest of clean clothes. If there is some rare DX that I need to work and the noise is an issue, I just shut it off. No problem as long as I restart it!

When it comes to dealing with the power company I have never had a problem. When you have done all the investigation and know where the noise is coming from, my experience is that they will fix it. Now if you call them up and just report that you have a noise. Good luck!

When dealing with neighbors, some interpersonal skills are required. I have had cases that have involved a touch lamp and a switching power supply. Both were resolved by buying them new stuff. This is a small price to pay.  More recently there was the previously mentioned case of the home alarm system that was tearing up the bands in a club member’s neighborhood. In that case a letter explaining the situation coupled with a friendly and cooperative neighbor resolved the situation. He changed the time the alarm system timer came on and everybody was happy.  Informing a neighbor that they have a device that is emitting EMI in excess of the allowable FCC class B requirements and that they are required to fix it is a worthless exercise and will only make things worse.

Be patient, Identifying, analyzing, locating and resolving these issues takes time!  

Good Luck

Mark NU8Z

Radio Interference Links