Hi Glenn, Let me jump in here and comment. A Ufer ground is for 60 Hz and safety grounding only. It probably is not such a good RF ground, as that would depend on its wavelength relationships, ie how big is it in terms of wavelengths you use, what is it's capacitance to the dirt, what is the inductance of its embedded rebar, what is your local ground conductivity. How well are the rebar elements bonded to each other, and are they cad welded, so that rusting of tie wires holding rebars is not an issue re the resistance of the conductors?
Many hams probably confuse RF ground and earth/ safety ground, (or AC grounding). They are separate issues, although the RF ground should bond, for lightning safety, to the AC ground per the National Electrical Code. (All house grounds to be bonded). A single ground rod has poor chances of being low enough inductance and high enough capacitance to be adequate for RF coupling to the earth. Its area is too small. Grounds thru an intermediate medium, as the Ufer ground is, are also not the best candidate to be an RF ground, depending on local conditions. To have a good RF reference or counterpoise, we have to have a resonant or near resonant conductor. This is usually taken to be a quarter wave of insulated conductor leading out from the chassis of the transceiver. That will lessen RF on the rig problems. This can be used and then a simple bonding of the rig to the other AC safety grounds can be used. That keeps your rig from having a greatly different voltage upon its case than the AC wiring in case of lightning. If we have a connection to a rod, it does not have a good RF earthing characteristic, because of the small area of the rod, the usually round conductor used to bond the rod, and the length of the run of conductor to the rod. The grounding (RF) conductor must not be odd multiples of a quarter wave for the band in use or you will have a high impedance possible at the rig. The goal in safety grounding for lightning mitigation is to spread out the current from a strike or surge into the earth. One such method that satisfies the 10 ohm rule for AC earth grounds is 200 feet of bare no. 10 conductor put around the building perimeter. It does not have to be buried many feet deep, for it works by area in contact with earth. Such an extensive added earth conductor would augment your Ufer ground and give you added protection by bonding to it. Depending on the local state of earth at your QTH, this may or not, be adequate for your RF ground. The compounds of the earth around you may be poor RF conductors. In those cases, you generally have to add more low inductance area to your grounding scheme. Wide flat copper is one of the best conductors to use. Plates can be buried, and multiple rods are sometimes used. Hope that distinguishes the two types of grounding you need for safety and for low RF impedance to earth. Stuart K5KVH _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
