Brian Perfect. Well written.
Most hams don't know enough about common mode chokes. 73, mike va3mw On Oct 18, 2011, at 10:50 AM, Brian Lloyd <[email protected]> wrote: > 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. _______________________________________________ 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.
