Grounding Recommendations for Software Defined Radios
Grounding Recommendations for Software Defined Radios
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Content provided by: FlexRadio Systems Engineering
Grounding as it relates to the construction of an amatuer radio station has become a highly debated issue. There are many different opinions on the subject and this articles will not try to weigh the merits of these points of view, but will discuss its necessity as it relates to best practices for setting up a software defined radio.
The primary difference, and it is very significant, between a traditional radio interfaced to a computer and a software defined radio "system" is that the computer is an integral part of the radio itself and not a peripheral component. With a traditional radio/computer configuration, the radio is nothing more that a simple device that the radio is reading data from or sending to it in the same manner as if it were a modem or a TNC. Each component is not critical to the operation of another. Marconi didn't need a computer for his spark gap transmitter.
With a software defined radio, the computer is the radio. That is a very important and critical difference. Just as in a traditional radio, one IF stage sends signals to another IF stage, in a SDR, the radio hardware (aka "the black box") sends signals (I/Q data) to the DSP software running on the computer. In a traditional radio, the circuits are engineered to minimize loss and interference between IF stages. The same care and attention to detail must be taken with a software defined radio as well.
There are different types of grounds in an amateur radio station setup depending on the characteristics of the EMF they are designed to carry. They are usually A/C safety grounds, RF grounds and lightning grounds. All of these grounds perform the same basic function; that is to provide a proper and adequate electrical path to a point of lowest potential which is the Earth itself. A/C safety grounds are designed to handle voltages commonly found in household electrical systems and have a frequency of ~60 Hz (that is VERY low frequency RF energy) so impedance is negligible. RF grounds operate at much higher frequencies so impedance is a significant concern and can exhibit very high voltages and currents depending on the power, frequency and length of the conducting element. Lightning grounds are required to handle extremely high voltages and currents that have a mixture of alternating and direct current components along with very fast rise times.
For this discussion related to software defined radio systems, we will primarily consider RF grounds since it is assumed that you computer and radio's D/C power supply already have adequate A/C safety ground capabilities provided as required by national electrical code regulations. A comprehensive discussion lightning grounds are well beyond the scope of this article.
As noted above, we describe the computer and the SDR hardware together as a software defined radio system. They are two halves of the whole. Because of that, it is very important that both the computer and the radio hardware be as close to the the same electrical potential as possible. There are multiple ground paths between the SDR hardware and the computer. The following are just some of the ground paths that may exist
Since there are multiple ground paths between the SDR hardware and the computer which are usually not capable of providing a low impedance connection between them, it is imperative that an additional ground which has a much lower impedance than the other electrical connections be employed which will take precedence over the other higher impedance ground paths.
- Audio connections for I/Q data (Firewire, USB, Ethernet)
- Audio cables for modulated audio input and output (connections to computer sound cards)
- Audio cables connected to outboard audio equipment (equalizers, mic preamps, recording devices, Pactor modems,etc..)
- Cables for hardware control messages (parallel cables, Firewire, USB, Ethernet)
So the following recommendations are provided for creating a good low impedance connection between the radio and the computer.
Use as short of a ground cable as physically possible. RF is a alternating current signal therefore it has impedance. The length of the ground runs has much more to do with the fraction of a wavelength at the frequency involved than the DC resistance of the wire. The DC resistance (measured with an ohm meter) of your RF ground connection may be only a fraction of an ohm, the impedance can easily be thousands of ohms depending on its operating frequency. To keep the impedance low, it is best to keep your total ground connection less than 9 feet (3 meters) including connection to the external ground system (described below) if possible.
Use a low impedance cable for interconnecting devices in your SDR system. Alternating currents such as RF exist primarily on the surface of a conductor. This is know as the "skin effect". You want to use a cable that has the greatest amount of surface area. Solid conductor wires have the least surface area. Multi-strand cables are a bit better but still are significantly lacking in surface area. Braided ground straps that are at least 1" wide make a good low impedance conductor, but wide copper straps are the best. If possible, make sure that the conductor is insulated, because of the potential high voltages and currents evolved. This connection will also be an excellent path for direct current, which makes having insulated cabling even that more important.
Make sure all of your primary RF components are at the same ground potential. The computer and the radio are just two components that need to be considered in a ground system. If you are using a tuner and/or an amplifier, all of these devices need to be at the same ground potential. All radios, tuners, meters, etc in radio system should be grounded in a star ground configuration. The common ground point should be at the tuner if one is used, otherwise a ground bus bar or copper panel should be used. Daisy-chaining devices together is NEVER recommended.
Connect the star ground configuration to a good RF earth ground. With a star ground configuration, all the connected devices should be at the same potential, but not necessarily at the lowest potential. That requires that the center of the "star" be directly connected a good external earth ground "system".
NOTE: If your equipment is located too far away from a good earth ground such as when the shack is not on a ground floor, then omit the connection from the internal single point ground (star ground configuration) to the actual earth ground because the long connection will radiate RF energy potentially causing more RFI issues that it solves.
Construct a good external grounding systems that will handle the different types of ground requirement. Connecting your star ground configuration to a single ground rod is not sufficient for EMF at RF energies. You can think of this ground as a RF counterpoise where a lot of surface in contact between the conducting ground material and the earth itself is very important. In addition, the external ground system must be connected to the house A/C safety ground, telephone ground and TV cable ground (A NEC requirement in the USA), so the ground system must be designed as a good RF ground as well as a good safety and consequently a good lightning ground as well.
First construct the ground system to be a good A/C safety and lightning ground. Then modify the ground system to be a good RF ground as well. The best way to do this is again using a star configuration where there is a central point ground rod (8' in length, usually copper coated steel) connected by heavy gauge (#2 AWG or less) solid copper wire to additional ground rods laid out like the "spokes" of a wheel buried several inches under ground. The "spoke" wires should be connected to the ground rods using heavy duty bolt type connectors rated for under ground service that are well coated with a viscous insoluble non-corrosive conducting material to prevent the electrical connection from increasing in resistance over time. The number of spoke connections is dependent on your soil type and its conductivity. The rule of thumb is to have 4 spokes as a minimum and there is no maximum number (although there is a practical limit, of course). The length of the spokes should be as long as possible. If they are of substantial length, they will act as an RF ground (see below). If they are less than 10 meters (~33 ft), then they will not function as a RF counterpoise (ground). As with the RF ground connections inside your shack, you should never daisy-chain the ground connections together; use only a star configuration.
Now that you have a good very low frequency A/C safety ground, you will need to augment the systems to become a good RF ground as well. If you do not have perfectly balanced antenna system (which is just about impossible to achieve), there will be RF currents on the RF ground acting like a counterpoise depending on the operating frequency. Therefore you must connect a bare solid copper wire (8 AWG or less) to the center ground rod that is at least one half wavelength long at the lowest operating frequency of your antenna buried about 1" under the ground. If you have a 160m antenna, then you will need to use ~250 feet of wire. Longer is better and the exact length is not overly important. If possible, loop the wire around the dwelling and connect it back to the center ground rod (this will make the RF counterpoise "DC" grounded and can reduce the noise on your receiver by a S unit or two). You can also install short (2' or .5m) ground rods periodically along the path of the RF counterpoise to provide some addition anchoring of the counterpoise loop, but these are not necessary and do not increase the efficiency of the RF ground.
DISCLAIMER: No grounding system will provide full protection from a direct lightning strike; it has more energy than can be shunt to ground safely than most amatuer radio operators can afford to deploy (this is why it is good to have your equipment insured). An improperly installed grounding system can cause more damage from lightning strikes or electromagnetic pulses and this article does not go into any great depth about engineering a proper lightning ground system for the entire shack. This activity is best left to professionals in the field, of which we are not. There are several very good articles regarding complete ground systems contained on the PolyPhaser web site along with their professional consulting services. FlexRadio Systems is not responsible for any damages or consequences from using the information contained in this article.
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Last Modified:Saturday, March 14, 2009
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