[TowerTalk] TowerTalk Digest, Vol 35, Issue 79

[Gary Schafer]

> -----Original Message-----
> From: towertalk-bounces at contesting.com [mailto:towertalk-
> bounces at contesting.com] On Behalf Of Dudley Chapman
> Sent: Monday, November 21, 2005 10:46 AM
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] TowerTalk Digest, Vol 35, Issue 79
> 
> >Message: 5
> >Date: Sun, 20 Nov 2005 23:13:34 -0600
> >From: "Jim Miller" <JimMiller at STL-OnLine.Net>
> >Subject: [TowerTalk] Radials
> >To: "TOWERTALK" <TOWERTALK at contesting.com>
> >Message-ID: <032201c5ee5a$52c7a490$6600010a at hmjm500>
> >Content-Type: text/plain;	charset="iso-8859-1"
> >
> >Would the lightning protection provided with numerous bare radials fanned
> >out on the surface of the ground from a ground mounted vertical antenna
> be
> >greatly reduced or eliminated by using insulated radials instead?
> >
> >TIA, 73, de Jim KG0KP
> >
> 
> Jim,
>     Radials for lightning protection have different criteria than radials
> for rf.  So it's best to consider them separately.  The reason why AM BC
> stations can combine the functions is because they have enough money to
> make
> the RF radials robust enough to perform the lightning protection as well.
> 
> You could do this in a ham radio installation but it would be massive
> overkill in cost, materials, and labor.
> 
> Lightning radials are usually made of solid or stranded wire of #6 or
> larger.  There are usually 5 radials of up to 50 feet welded to ground
> rods
> placed at intervals along the radials.  All of this is shallowly buried to
> keep people from tripping over the thick wire and ground rods.  The wire,
> rods, and welding jigs can add up to a lot of money.  The labor adds up as
> you dig the shallow trench, drive each ground rod and weld the rods to the
> wire.
> 
> RF radials are a different story.  You need somewhere between 20 to 60 of
> them but fortunately, you can use really cheap insulated wire laid very
> casually on top of the ground.  You do not put ground rods along their
> length.  The biggest challenge for rf radials is just figuring out a good
> way to lay them down cheaply and easily but not have people tripping over
> them or sucking them up in your lawn mower.
> 
> By keeping these two as separate problems, you can put down a lot of cheap
> rf radials, where you need lots of them.  Then you can save your money for
> the expensive materials and labor for the lightning radials.
> 
> Dudley - WA1X
> 

A quote from Polyphaser: "A good lightning ground is also a good RF ground.
But a good RF ground is not necessarily a good lightning ground."

The RF ground and lightning ground need not be treated separately. 

Heavy radials are not necessary if there are more than a few. The lightning
energy will be divided among all so no single radial will carry high
current.

Insulated wire radials will dissipate the higher frequencies involved in
lightning but bare wires will be required for the low / DC component of
lightning. The insulated wire will arc to ground as voltage rises but bare
wire will help keep that arc point lower. 
Arcing to ground with bare wire still happens at times in some soil types.

Some broadcast stations employ ground rods along the radial lengths when
difficult soil conditions are present for lightning dissipation.

A properly installed radial system can be both an excellent RF ground and
lightning ground as a large portion of lightning energy is in the RF
spectrum. 

As a mater of interest, you do not have a good lightning ground unless it is
a good RF ground in addition to a good DC ground.

73
Gary  K4FMX