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Grounding and RFI
Questions answered #1
Questions answered #2
Questions answered #3  

How to fix DSP-100 problems (NEW) in Questions answered #3

A SHORT PRIMER ON RF/RFI AND GROUNDING TECHNIQUES
by KA7GKN, Marty, Glendale Arizona

Many pages and many myths have been written about Amateur Radio Station set up and grounding systems. The days of two wire outlets are long gone and the need for excessive redundant ground systems too are a thing of the past. The fundamental need for proper grounding is personal electrical safety. I refer you to read the “National Electric Code book on proper grounding” and the ARRL publications “RF EXPOSURE AND YOU” and “THE ARRL RFI BOOK”. The reality is you’re not going to run out and buy any of these books. My goal here is to provide some basic information to help you determine the ground problems you’re having and some known solutions. 

With the introduction of professional audio equipment into your Ham Shack, you might experience RFI, hum, ground loops, RF in the shack etc. OK, so where do we start? 

1. In January 1998, the FCC rules of RF exposure went into effect. Simply, they are designed to protect YOU and your immediate area. For, example, if you’re on 20 meters and your RF output to the antenna exceeds 225 watts (PEP into the antenna) then you need to perform the appropriate calculations. I highly recommend you go through the exercise; it will help you determine what RF potential issues you really may have. An easy way to calculate the RF exposure levels is to go to:

http://n5xu.ae.utexas.edu/rfsafety/ 

Follow the instructions. I suggest you make hard copies of your results for future use and protection. If you run stealth or attic antennas even with less than the 225 watts I still stress the importance of you doing this exercise since you’re extremely close to the antenna. The radiation pattern of your antenna may envelope your shack making it almost impossible to eliminate RFI other than relocating the shack or the antenna or both. 

2. In a well-designed station all RF will be radiated by the antenna. In the Amateur Radio world, however, incidental radiation will occur. Here are some common problems that can result in excessive incidental RF radiation: 

• End fed antennas whose connections occurs directly in the shack
• Feed line radiation caused by antenna system imbalance
• Excessive feed line leakage caused by broken or missing shield connections or coax cables
• Excessive feed line leakage caused by inferior grade coaxial cables
• Improper grounding of station equipment
• Improper shielding of station equipment
• Other “RF in the shack” issues

Many of the station problems can be traced to defects in the installation or maintenance of the station! Properly grounding your station can minimize problems with RFI in the shack. 

3. Lets first visit your RF system. Ensure you use high quality coax. The stuff in a roll at the swap meet for 50 cents is to be questioned. There is a reason it’s 50 cents! Old coax does deteriorate. You also need to use the best RF connectors you can afford. Again, be cautious of the swap meet deals. Yes sometimes the stuff is just fine but think about why is that $2.50 RF connector being sold by this stranger for 50 cents? What’s wrong with it? OK, you get the idea.

Today you have many choices for the coax cables you can use. You should look at your station set up and goals to help you determine what cables to select. I use RG-214, this is a double shielded coax. Soldering the PL-259 connectors properly is critical. You’ll come across many methods of doing these. The wrong solder, the wrong heat, cold solder joints, internal shorts between the center conductor and shield are just a few potential problems you can have when making up the connectors. 

4. Your audio cable system should be made up of balanced line when possible. Use established audio standard color code, and keep the runs as short as reasonably possible. Here too the quality of the cable and connectors come into the picture. Solder technique is also paramount. 

5. Routing the cables is going to be a challenge. I suggest you as best as you can keep the RF, audio, and power cables away from each other. Keeping the RF and audio cables at least perpendicular to each other will reduce any common mode pick up due to mutual inductance. 

6. Use shielded power cords wherever possible. Radio Shack does carry one for about $5.00. Try to keep all your equipment connected to the same breaker. Use polarized plugs for the two wire devices, and avoid using the 3 to 2 cheater plugs! If you use outlet strips, use one for the audio, one for the radio equipment and plug one strip into the other. This keeps the grounds at the same potential. 

7. Again many myths here for the ground system. I recommend you follow the ground system layout discussed in “the RADIO WORKS” catalog and website. You should purchase and install per their directions two T-4 isolators. The isolators will reduce or eliminate any RF leakage from your coax cable. Force yourself to remove the copper plate and pipes and myriad of 1” copper strapping from all your equipment. Think for a moment if your kitchen appliances required that same method to ensure a safe ground? Use the ground lug from your antenna tuner as the connect point for your grounds. Speaking of grounds, run a heavy gauge wire like #6 stranded. You can find this type of large flexible wire at a welding shop. Use mechanical crimp connectors. These to are available at Home Depot. Or a good hardware store! DO NOT SOLDER ANY GROUND CONNECTIONS! Connect a ground wire from your radio and from your linear and connect them at the ground lug of the antenna tuner. Run a #6 or #4 wire from the antenna tuner ground directly to a UL approved 5/8” 8 foot copper clad ground rod inserted into the ground with only 8” to 12” above ground exposed. Use the proper clamp and ant- seize compound, such as, NOLAX and secure it tightly. If you have a tower or mast antenna system attach a ground wire from the mast or tower leg to this ground rod too. Of course grounding towers and antennas properly is a different article and will not be discussed any further.

A short discussion about ground loops:

You hear the audio guys say “it’s gotta be a ground loop” Well what does that mean? A ground is supposed to be a zero-potential surface or point, but most float at some small voltage. In reality, our best hope is there is little circulating current [particularly RF]. Some households have multiple ground reference points: the ac power ground, the plumbing system, and sometimes separate grounds for the telephone and cable systems. Multiple grounds, however, are not a good practice; the National Electric code requires that all grounds be connected to a single point. Water pipes are buried, but their ground resistivity can be quite high, and there are no precautions to ensure electrical continuity [many homes use plastic pipe today]. As a result, the plumbing is usually tied to the ac ground return at some point in the house. This point is often at an outside faucet, where a wire is attached from the ac ground to the water pipe with a clamp. Improper grounding can introduce unwanted audio hum and may violate the National Electric Code of local regulations. An added ground for an audio system could create a big ground loop, which could act as antenna and worsen the RFI. (Taken from the ARRL RFI Book) The simple answer here is: TOO MANY GROUNDS CAN SPOIL AN OTHERWISE GOOD SYSTEM. 

A ground loop occurs when there is more than one ground connection path between two pieces of equipment. The duplicate ground paths form the equivalent of a loop antenna, which very efficiently picks up interference currents, which are transformed by lead resistance into voltage fluctuations. As a consequence, the reference in the system is no longer a stable potential, so signals ride on the interference. Ground loops are often difficult to isolate, even for those with experience. It could occur due to poorly designed equipment (which sometimes includes expensive equipment), or a poorly designed installation. The basic way to avoid ground loops is to utilize the “single point” grounding system; I again refer you back to “ the Radio Works” discussion, and methods they present.

8. Ferrite beads? What’s that all about? Without getting too technical, there are two types of conduction: COMMON-MODE, and DIFFERENTIAL-MODE. Common mode: all conductors except ground act in common: that is, as one conductor with reference to ground. Differential mode: where the signal arrives on a pair of conductors with a 180-degree phase difference between the pair. Each of these conduction modes requires a different RFI cure. Differential mode cures, typically a high or low pass filter, do not attenuate common-mode signals. On the other hand, a common-mode choke does not affect interference resulting from a differential-mode signal. 

The cure for differential mode interference is a filter that passes the desired signal but blocks the unwanted signal. For example using a “Low Pass” filter in your RF lines to reduce TVI. Common mode currents are in phase with each other and typically the return path is earth ground. Since all common mode currents flow in the same direction through all conductors in a cable, little field cancellation takes place. A cure for common-mode interference is placing a high impedance in the common mode without obstructing the differential-mode signals. This can be done by inserting a transformer with (with isolated windings) in series with the cable. A common mode choke is another solution.  A choke is formed by wrapping all conductors around a ferrite core or placing them through one or more ferrite bends. Their fluxes cancel within the core and no inductance results. 

With coaxial cables a common mode signal is typically on the outside of the cable shield. When the cable is wrapped around a ferrite torroid core the choke appears to be a reactance in series with the outside shield, but has no effect on signals inside because their field is confined inside the shield. These differential fluxes cancel each other and there is no net reactance for the differential signal. Again, to common-mode signals the choke appears as a high reactance in series with the line. This is also a very simplistic explanation as to why the T-4 isolators work too. With your audio cables you use split beads and in essence choke off the interference. There are many types of ferrite materials out there with many specifications. The most common ferrite chokes, bead, split beads, etc you’ll hear discussed are: mixes 31,43,73,75,77 …. I suggest trying the newer mix “31” offered by he Radio Works. I have had good reports about them. If you do not know the mix material your ferrite is, you really won’t know if it’s working to eliminate the common-mode interference. Also, it may require more than one bead or choke to eliminate the problem. This is where a RF sniffer can come in handy. 

Summary:

RFI elimination and station grounding is a complex topic not just related to audio. My goal here has not been to mislead you or give you a false sense of security, rather to increase you desire for more specific information on the topic. Good grounding is essential for your protection, electrical safety, reducing the possibility of lightning strikes, and more! Eliminating ground loops will improve your overall Ham Radio signal, making the experience enjoyable. Controlling and eliminating RFI also addresses the topics of TVI, phone RFI, computer RFI, computer RFI etc. Understanding and being able to identify the type interference you have will without a doubt increase your success in eliminating the problem. I encourage you purchase and read the ARRL RFI Book. It will provide you with a good foundation to help you maintain a safe good quality Ham Radio station. I wish you good luck in your venture and in identifying facts from myths!

GROUND LOOP RFI ELIMINATION PROCESS CHECKLIST: 

1.Verify all RF coax cables, connectors, and adaptors are OK. This means visual inspection to ensure all soldering is of high quality and there is no obvious corrosion. Measure with a dc ohmmeter continuity to ensure there are no hidden opens and or shorts. Properly re-tighten all connections ensuring no cross-threading or over torquing. 

2. Use an antenna analyzer such as the MFJ-259B to verify your antenna system is functioning properly. Do the RFI exposure level computations  http://n5xu.ae.utexas.edu/rfsafety/  to determine if your radio operating position is within the specified RFI exposure levels. 

3. Verify your AC power distribution is such so not to introduce ground loops, polarity reversals, and phase changes. Basically ensure all equipment is connected to the same circuit breaker with ground and any 2 wire cords be of the polarity design. 

4. All audio connections should be XLR and balanced types if possible, Use double shielded cables for all audio connections. 

5. The station ground system should be as The Radio Works single point system. Refer to the NEC for specific details. Use the appropriate UL approved ground rods minimum 8-foot long and proper connectors. Do not solder anything in the ground system. 

6. Install two T-4 isolators as described in The Radio Works catalog. 

7. Install mix 31 ferrite split beads on all audio and control cables, one at each end. 

8. Install a power conditioner similar to the FURMAN PM-8. This type is used by professionals to eliminate ac power system loops and noise. The units have a better common mode and RFI rejection than the ones you find at Radio Shack and the computer store. 

9. Revisit all of your cables to ensure they are soldered properly, all are wired correctly. Be sure to establish a standardized wire color code, such as, red = + audio, White = - audio, Shield = ground, black = chassis ground and so forth. Having all cables wired with the same color code will simplify troubleshooting. 

10. Try to re-route your cables isolating the audio RF and power cables from each other as best as possible. When cables must be near each other try to place them perpendicular to reduce common mode pick up. 

11. If you still are having problems, you then need to start to isolate your equipment to help find the source. Remove all audio devices and determine if the problem has been eliminated. Be aware often the problem is still there but you don’t hear it. If it seems that the problem is gone reinstall your audio system one box at a time. You might have an equipment conflict, causing the ground loop. For example. The HY-GAIN Tailtwister rotator control would hum when the brake release is pressed. I’ve had this for 8+ years. Finally after re-visiting the schematic and looking for ground loops I found the culprit! The AC ground is tied to the brake release secondary ground. A millivolt difference between the two grounds. I lifted the AC power cord ground, ensured the control box is still connected to a viable ground and magically the hum has gone away! 

12. Finally….ALWAYS BE ELECTRICALLY SAFE! IF YOU FEEL BEYOND YOUR ABILITY STOP! AND GET QUALIFIED HELP. THIS IS ONLY A HOBBY AND YOUR LIFE IS MORE IMPORTANT!

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 Copyright © 2001- 2006 KA7GKN Martin Shapiro All rights reserved