RF Grounding.
Rf grounding is considerably different than surge grounding. First thing is you are working with RF. Since it is an AC signal 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. While the DC resistance of a ground wire may be only a fraction of an ohm, the impedance (or the AC resistance at RF frequency) can easily be hundreds or thousands of ohms on the same wire. This can make it pretty difficult to get an effective RF ground. Remember an RF ground wire is just a short antenna! We want to make it as LOUSY an antenna as possible! We really don't need it radiating extra RF inside our shack. It is supposed to remove this stuff not cause it. An effective RF ground needs to be less than a quarter wave length at the highest frequency used. As you can see there is no such thing as an effective ground for VHF or UHF. We will concentrate our efforts to 10 meters and above. This means our ground wire from radio to ground must be about 9 feet or less! This is still pretty difficult. All radios, tuners, meters, etc in radio system should be grounded in a star ground configuration. The common point should be at the tuner if one is used, otherwise a ground bus bar can be purchased at an electrical house. All Connections to radios should be with either insulated or bare wire with as few strands as possible. RF likes smooth surfaces best. DO NOT USE braid for RF connections. This is an old wives tale! Your ground run should go directly to the ground where you should have a ground rod for the connection point, (which will be connected to all your other ground rods in the system as discussed above). This run must be less than nine feet to be effective. If you are on the second floor this will make this length impossible. Use of a shielded ground* wire can stop radiation of the ground wire but you will still have a lousy ground. Nothing can change this. Ground wire tuners only turn your ground wire into a counterpoise for your antenna, meaning it WILL radiate. This will only ensure that the low voltage point of your antenna will be at your radio. Next we need to form our RF counterpoise outside at our ground system. You will next need to add some bare copper wire at the RF feedpoint where your shack ground wire connects to. I prefer to use bare 8 gauge copper ground wire here. It is single conductor, bare copper and easily bent and run around house. Single strand is best but it should definitely be bare even if you have to strip insulation off wire. Run it around the house or anywhere it will stay out of the way fo lawn equipment but not buried deeper than 1/2 inches. This is CRITICAL. RF will not penetrate soil deeper than this at these frequencies. Those bonding wires you have between ground rods and ground rods do not exist to the RF! Burying this wire under wood chips or similar non conductive landscaping, etc is the way to go. This counterpoise should be as long as the wire antennas you have in the air. For most hams this will be about 130 feet. Longer is better. I run all the way around my house. I have found the eight gauge will push into the spacing used between driveway and foundation when persuaded with the proper tool, (READ HAMMER). You can connect the loop back on itself at the feed point. This can add several S units to the receive signal and dramatically reduce noise on the signal, though nothing will help all the noise on 80 or 160 meters. Years ago I installed a long wire antenna that was about 250 feet long and about 50 feet in the air. This should work fantastic you say. I had three ground rods outside window of shack with single ott solid copper ground wire direct to tuner. Ground wire length was only six feet. All three rods were spaced about eight feet apart with connecting bare wire interconnecting them....in other words, a really good surge ground. What I did not realize at that time was how lousy my RF ground was. We could not tune the antenna on most frequencies and we kept getting zapped from the radio or microphone when we transmitted. Also, our signal reports were lousy. SO, after consulting some experts, I added 250 feet of counterpoise around the building consisting of some bare 6 gauge copper wire I had. The radio was on while I rolled it out and a friend was listening to the broadcast on 40 meters, (OK it was night time---best time to do antenna work right!) Anyway he reported the broadcast was only about S 4-5 on meter. As I rolled out the counterpoise it rose to 40 over S9 and came in much clearer. We were able to tune everything easily now and SWR was rock stable. When we did a signal test, the station we had talked to before accused us of running a contest amplifier. We could not convince them it was only 100 watts, same as before and the same antenna! SUMMARY.
Don't underestimate the importance of a good ground system.
Include it into the planning of that ultimate shack you are working on. Don't scrimp on good copper wire and connectors. Aluminum can be used above ground but never in ground. Add one size to aluminum to achieve same current capability. Ground everything to the system. A ground run to ductwork in house can alleviate a lot of noise. A run to water pipes should go direct to ground....NEVER to radios, NEVER connect radios to ANYTHING inside the house for ground purposes. Always run all grounds from everything to ground directly. In other words, your furnace ducts will get one run, your water pipes will get one, etc. Don't daisy chain to save wire. If you have a chain link fence in back yard, run a bonding wire underground from ground system to it and bond well. A solid aluminum or copper wire run along bottom of fence as a bonding device will make it a great addition to the system. Weave it through the bottom fence fabric and bond every few feet with a split bolt connector. The power company does this with all their fences around their power stations.
* A shielded ground can be made using RG 8 or similar coax to replace the ground wire. Connect both inner and outer shields to the Ground rod and connect the center only to the radio. Add a .1uf 1000 volt cap between ground and shield at this end.
73 Bill - N8SA
RF Grounding
Posted - 10 Mar 2007 : 1:44:16 PM
