T4A08: Which type of conductor is best to use for RF grounding?
A. Round stranded wire
B. Round copper-clad steel wire
C. Twisted-pair cable
D. Flat strap
First, let’s define RF grounding: In the simplest view, radio frequency grounding provides a direct path to ground potential for electrical currents of radio frequency. So, any AC electrical currents with a frequency of alternation in the range of radio frequencies should run to ground potential via this path and not hang around on your station equipment to cause problems.
You’re probably wondering at this point, “From where do such stray and troublesome RF currents arise?” Any conductor attached to your station that is more than about 1/10 of a wavelength for your transmitting frequency will act as an antenna and pick up RF radiation, converting it into alternating electrical current of similar frequency. So, your antenna feedline, your equipment power cords, that computer interface cable, even the metal chassis or enclosures of your station components can pick up RF and create undesirable stray currents on your station. The safety ground on your electrical cable that connects to the safety ground of your shack’s power outlet will usually serve as an excellent undesired antenna for RF rather than a good RF ground. That safety ground will keep your 60 hertz power source grounded, but it was never designed to avoid picking up strong RF emissions from your transmitter! A separate and well-designed RF ground is needed in addition to the electrical safety ground to avoid stray currents on your gear.
Now you should ask, “What’s so bad about having a few stray currents running around?” Experimenting with this can become a self-critiquing exercise! With strong currents you may get a sharp shock from your microphone as some of those currents find a path to lower potential through your body. Currents flowing among the various pieces of equipment in your station are called ground loops, and they can interfere with your radio electronics causing unpredictable operation of some equipment (like computer interfaces!), inject noise into your receiver (usually a constant hum), potentially cause audio distortion or even result in erroneous SWR readings. Ground loops are evil and you should take action to prevent them with a good RF grounding arrangement.
So, you now question, “What constitutes a good RF grounding arrangement?” Several factors. You can learn about well-designed RF grounding arrangements in the HamRadioSchool.com Technician License Course book, Chapter 12, Avoiding Interference. The basic principles are as follows:
- Keep all wires and connections as short as possible to reduce RF effects
- Connect the chassis (metal enclosure) of each piece of equipment to a common ground panel or bus with solid bonds; this keeps all equipment grounds at a common potential, avoiding current flow between them
- Do not daisy chain separate grounds from equipment; each should be separately connected to the common panel or bus with a low impedance conductor
- Connect the common panel or bus to an earth grounded rod or pipe using a very low impedance conductor such as wide copper strap
- Keep the ground strap to earth ground as short as possible – if it approaches ¼ wavelength of your transmitting wavelength for any frequency it may resonate with the RF and create a hazard for RF burns if the conductor is touched (For the 10m band ¼ wavelength is only about 8 feet!)
- If a relatively long ground conductor is necessary for your situation, try using more than one length of conductor to avoid resonances and RF hot spots on the ground conductor
- In other challenging conditions try a piece of metal screen or wide foil/flashing placed underneath equipment and bonded to the common ground panel or bus. This can reduce RF hot spots with high RF voltages and minimize the chance of RF burn or shock from them
In many shacks it is difficult or impossible to comply perfectly with the ground connection length recommendation for all frequencies, especially the 10m band as noted above. Just do some simple calculations for wavelengths and be aware of the possibility of RF hot spots in your grounding system under some conditions.
Finally, notice the highlighted recommendation to use wide copper strap for the RF grounding conductor. RF currents tend to flow on the surface of conductors. This is called the skin effect. The impedance of the conductor is reduced as the surface area of the conductor is increased. Thus, a wide flat strap will have lower impedance for RF currents than a relatively small round wire. Low impedance means the currents will more readily flow to the ground potential to which the conductor is attached.
So, to very effectively rid your station of those pesky stray RF currents, provide them a short, low impedance, and direct route to ground potential with a wide, flat conductor, such as copper strap.
The answer to Technician pool question T4A08, “Which type of conductor is best to use for RF grounding?” is D: Flat strap.