A square plate buried in the ground makes a poor ground for lightning. Lightning is dissipated in a sphere around a ground rod. The longer the rod the larger the sphere of dissipation. Length is what is important, not surface area.
On the other hand too long a rod does little good either. As length increases so does inductance. Going down 20 feet with a ground rod will do nothing more than an 8 foot rod will do unless the soil is very dry and non conductive. It is much better to use several 8 foot or so rods spaced by the sum of their lengths. (16 foot spacing for two 8 foot rods). This is because the sphere of dissipation around the rods will overlap and not be as effective if placed closer. 73 Gary K4FMX > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:amradio- > [EMAIL PROTECTED] On Behalf Of Jim candela > Sent: Saturday, August 05, 2006 5:15 PM > To: Mike Dorworth, K4XM; Discussion of AM Radio > Subject: RE: [AMRadio] Antenna Idea and lightning precautions > > > > > Mike, > > Thanks for the comments on this topic. At this point I am trying to get > an idea on what to do, and how to calm the XYL about lightning. I still > think erring on safety is the way to go, and mount my antenna elsewhere > away > from the house. That way I can disconnect the coax, and move the ends > apart > 20' or so. > Still, the tower on the roof idea has technical merit. Heck millions of > folks had TV antenna's on their roofs before cable or satellite TV was > popular. It was a rare installation that was properly grounded, and yet > even > rarer when there was a lightning strike to the antenna. > > I was 'googling' around on the topic, and I ran into this neat sight: > > http://www.lightningrod.com/ > > This is for DIY lightning protection systems for the home, and they > provide > the parts, and guidelines on what you need. I see some Ham radio uses for > these parts, like the 2 square foot copper ground plate instead of a > ground > rod. I notice that their 15/32" diameter copper wire is un-insulated. > > > Now If I cut down that tree in the back that provides late afternoon shade > for my neighbor (not for me), and also rains dead leaves into my pool, and > replace it with a tower and an antenna....my neighbor's wife will lynch > me!!! She had a fit last year when I trimmed that same tree (on my > property), and trimmed a few branches of her tree that hanged over my > property. Both trees ar 40' plus Live Oak's. That was after I told her > husband the day before what I was going to do. Apparently he isn't the one > wearing pants in that household! :-) > > Jim > JKO > > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] Behalf Of Mike Dorworth, K4XM > Sent: Saturday, August 05, 2006 2:59 PM > To: Discussion of AM Radio > Subject: Re: [AMRadio] Antenna Idea and lightning precautions > > > I can see this is the beginning of a long thread since everyone has their > own ideas. In commercial work a sharpened spike above the thing to be > protected is to DRAW the lightning to a well insulated and very well > grounded ground system. This is to protect the equipment below it. To > dissipate, the ball should be rounded like a car radio antenna to gently > discharge the corona. We put up a series of 150 foot towers at work with > a > 21 foot stainless sharpened lightning spike above the tower top to draw > the > lightning. All of our ( 92 each) microwave towers had a 3 or four inch > diameter sharpened brass rod 2 feet above the tip top of the tower. It's > ground cable was insulated from the tower all the way down. Of course the > tower and all the guys were also grounded to the common ground. A dipole > can > easily discharge static build up with a 100 k ohm resistor of at least 1 > fourth watt. This keeps the system equalized. Lightning usually hit the > HIGHEST ( though noy always) spot, so if there are taller trees they would > get it first. I like insulated wire instead of bare since the damp wind > will > not build up thousands of volts when it blows over..just before a storm. > For > fun take the antenna connector and put in a mason jar and place near > ground > and watch the 4 inch long blue firs just before a storm on a hilltop. A > Johnson Matchbox sounds like a fourth of July celebration if left > connected. > I guess, in the end a direct strike is bad news in every case. Most of us > are really talking about big static discharges I think. A real strike will > blow every receptacle in the house out and the wire on on side of every > power cord will vaporize and the fuse box will be blown off the wall. Let > the tall trees take that!.. 73 Mike > ----- Original Message ----- > From: "Jim candela" <[EMAIL PROTECTED]> > To: "Discussion of AM Radio" <amradio@mailman.qth.net> > Sent: Saturday, August 05, 2006 2:36 PM > Subject: [AMRadio] Antenna Idea and lightning precautions > > > > > > > > Hi All, > > > > I am contemplating putting up an inverted Vee antenna where the > center > > point is above my house suspended with a 30' Lowes push up mast attached > to > > my roof with a tripod mast base made for roof mounting. This would make > the > > apex at almost 50', and with the trees around my home, the ends at about > > 30'. Other locations that I might have the antenna apex at will be > densely > > surrounded by trees, and I am trying to avoid that. > > > > My question is about lighting concerns with this approach. I would > have > > multiple 12 awg ground straps from the mast base to earth ground via > copper > > ground stakes at least 5' long. This would act as a counterpoise for the > > antenna, and provide a DC ground reference for the 30' mast. My fear is > that > > the antenna would attract a lightning hit (direct) and that would cause > my > > home to burn up in a flaming fireball. > > > > Then I was thinking about how lightning rods work, and when done > > properly, don't lightning rods work by having a sharp point at the tip, > > where they bleed the static (a corona discharge) to prevent a lightning > > strike? If so, why can't I take a 1/8" stainless 8' whip with a point on > > top, mounted above the inverted Vee apex, and use that as a lightning > rod? > I > > guess I'd need to beef up my ground wiring scheme just in case of a > direct > > hit. Any suggestions? > > > > I am hoping for having more lightning protection with my antenna in > place > > over that of no antenna at all? Is this possible? > > > > Regards, > > Jim Candela > > WD5JKO > > -- > > No virus found in this outgoing message. > > Checked by AVG Free Edition. > > Version: 7.1.394 / Virus Database: 268.10.7/409 - Release Date: 8/4/2006 > > > > ______________________________________________________________ > > AMRadio mailing list > > Home: http://mailman.qth.net/mailman/listinfo/amradio > > Help: http://mailman.qth.net/mmfaq.html > > Post: mailto:AMRadio@mailman.qth.net > > AMfone Website: http://www.amfone.net > > AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb > > > > ______________________________________________________________ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:AMRadio@mailman.qth.net > AMfone Website: http://www.amfone.net > AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb > -- > No virus found in this incoming message. > Checked by AVG Free Edition. > Version: 7.1.394 / Virus Database: 268.10.7/409 - Release Date: 8/4/2006 > > -- > No virus found in this outgoing message. > Checked by AVG Free Edition. > Version: 7.1.394 / Virus Database: 268.10.7/409 - Release Date: 8/4/2006 > > ______________________________________________________________ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:AMRadio@mailman.qth.net > AMfone Website: http://www.amfone.net > AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb
