On Tue, Oct 18, 2011 at 5:19 AM, Bob McGwier <[email protected]> wrote:
> The most import point needs to be reinforced. DC ground IS NOT RF > ground. A dead short at DC can be like an open circuit at RF. Remember > those chokes keeping RF out of your DC supply in your final amplifiers? DC > ground is great for elimination of static build up, lightning protection and > then you have to analyze the impedance to ground at RF. > Actually, DC ground does not help with lightning protection nearly as much as people think because lightning is a dynamic (AC/RF) event. Ever wonder why a lighting strike does not always follow what appears to be the direct (DC) path to ground? It is following the lowest impedance path to ground. When lightning strikes you have very rapid voltage and current rise times. Fast rise times = RF! That is why they call it an electromagnetic pulse. So it is entirely possible that a good DC ground is not necessarily a good lightning ground. (But there should still be DC continuity on the ground system to drain away the static charge that builds up on any conductor that floats above ground.) So even lightning provides a similar problem to RF in the shack. Turns out you can treat both problems the same way and the first thing to realize is what Bob just said: DC ground does not equal RF ground. Insofar as lightning protection goes, if you could impose a high impedance to the fast-risetime current pulse from the EMP of the lightning strike, the pulse will take another path to ground and not through your radios. It is useful to understand this and then look at how others have built lightning-tolerant systems. The guys who build cell towers and mountaintop communications sites have figured this out. The formula is actually pretty easy to duplicate if you are willing to go through the time and effort to do so. Here are the rules-of-thumb: 1. Take the coax from the antenna and run it directly to the nearest good ground at the ground. If the antenna is on a tower, that would be at the base of the tower. A large concrete tower base build with rebar that has been bonded together and to which the tower is bolted is going to be the best RF and DC ground you are going to find. You don't even need extra ground rods. 2. The shields of all the coax cables from all the antennas are bonded to this ground using lightning arrestors. If you have an antenna rotator, it's cable should be bonded to ground through a lightning arrester at this point also. 3. If the tower is next to your house, this is the entry point where your cables are going to go to your shack. 4. If the tower is separated from your house/shack you need to run the coax underground in conduit to the desired entry point. At this point you will build another good ground plate with lightning arrestors. Remember, when you have long runs of any kind of wire, they will have high currents and voltages induced in them due to capacitive and inductive coupling. You need to shunt these currents to ground too even if your antenna system has not sustained a direct lightning strike. In most cases these induced currents are what damage your equipment, not the lightning itself. 5. When your coax reaches your station, the shields should all be bonded together at a single point. This is an ideal reason and place to put that antenna patch panel you have always wanted. 6. Every piece of equipment should be bonded together with very short pieces of low-impedance strap. 7. If you want to do it 100% right, make a metal ground-plane under all your equipment and run the straps directly to that. That is of course also bonded to our single-point coax shield bonding point. 8. Here comes the controversial part of this: don't bother with a separate ground wire at the shack. The DC ground through the household protective ground (green wire) is sufficient to provide a protective ground from a safety point-of-view. That extra ground wire is most likely NOT a good RF and EMP ground and may actually serve to carry the induced RF and EMP currents to your station, undoing all the work you have gone through! The DC ground through the household protective ground and through the coax shields is ample DC ground. Remember that we want to increase the impedance of the path to both RF and EMP so that it takes a different path to ground rather than through your equipment. Here is how we do that: 1. Place a common-mode choke on each piece of coax at each antenna. 2. Place a common-mode choke on each piece of coax where it reaches the first ground plate with the lightning arresters on it. 3. Place a common-mode choke on each piece of coax where it leaves the ground plate heading for your shack. 4. If you have a remote antenna tower and a second ground plate at the entrance to your house/shack, for each piece of coax place common-mode chokes on either side of the lightning arresters there. 5. Place common-mode chokes on each piece of coax as it reaches your shack, just before the patch-panel/bonding-plate. The purpose of chokes is two-fold: you want to preset a high impedance to any RF/EMP currents that have been conducted to and induced on the outside of the coax, making the separate path to ground the preferred, low-impedance path. Remember Kirchhoff's laws: the current leaving a node equals the current arriving at a node. We want to make the current leaving the node take a different path from the one going to our shack. At each of these points we are giving the RF and the EMP two different paths: a low impedance path to ground and a high impedance path toward our equipment. Which direction do you think it is that the power is going to take? And you may be saying, "Aren't all these chokes overkill?" The answer is a resounding, "NO!" Remember the induced currents? Think of all that coax as an antenna. The chokes substantially reduce induced current along the way. And this works both ways as well. This also works to reduce EMI picked up inside your shack/house from things like switching supplies and light dimmers from being carried on the outside of the coax, up to the antenna, where it will be picked up by the antenna and sent to your receiver. So this wiring technique will also reduce the noise floor at your receivers! So, yes, doing a proper "grounding" job is hard work and can be expensive. OTOH, it provides much better protection for your equipment AND it will reduce the receiver noise floor from local noise sources. Equipment damaged or destroyed by the EMP from a nearby lighting strike is expensive too. -- Brian Lloyd, WB6RQN/J79BPL 3191 Western Dr. Cameron Park, CA 95682 [email protected] +1.767.617.1365 (Dominica) +1.916.877.5067 (USA) _______________________________________________ Flexedge mailing list [email protected] http://mail.flex-radio.biz/mailman/listinfo/flexedge_flex-radio.biz This is the FlexRadio Systems e-mail Reflector called FlexEdge. It is used for posting topics related to SDR software development and experimentalist who are using beta versions of the software.
