[Antennas] Grounding needs

Tue Apr 22 16:08:39 EDT 2008

I would like to add and comment a few points here :
1 )a 30' run of ground conductor  of any kind will most certainly be useless
for RF on any
frequency except the very low ones (160 possibly 75 ) unless you are
able to tune it to represent an electrical half-wavelength as seen from the
feedpoint
to the actual RF ground. A ground rod is of no or very little use as actual
RF ground in normal soil
The ground conductor would also radiate a lot, beeing an active part of your
system. Might not
please your family or nextdoor stereo :-) In other words: forget the ground
rod/downlead for RF
purposes. Go with radial/counterpoise system.

2 Lightning protection:
I have little knowledge of state/country regulations in your area. You must
check them well !
Heavy conductors are a must of course, But then I have some comments that
may be of value
to you all :
Do NOT look at a lightning strike as pure DC !!!
Sure, it is DC ...BUT the leading edge of the pulse (a lightning is a
DC-pulse) rises so fast from zero
(or in some cases a low so called pilot-stream current) that it must be
considered as RF !!
A simple test is to listen on 75m for QRN :-)   So the same calculations and
numbers must be considered
in designing a lightning protection as we would do in a feedline/RF-ground !
The first very steep part
of the pulse contains considerable energy far up into the HF range ! Then it
flattens out and decays and the rest
of the strike is more like "usual" DC .  The trick is to avoid that first
RF-part from taking shortcuts through
a building or the like.  It will behave like RF and go for the easiest
route,- which in this case is the route
with lowest impedance.  Remember that impedance is the resistance AND the
reactance in combination.
A very low resistance as in a very heavy, but "long and windy" conductor may
well be less tempting to a lightning strike than the
more direct route through a roof, into the buildings electrical wiring and
straight down through your living room !
Some people say that "ok-the first steep pulse may go that route, but all
the rest and really heavy current will
follow the heavy outside conductor"...  Experience has shown me and others
that this is not necessarily true...
As materials heat up and are heavily ionized as they will be along the first
"direct route" they tend to get very conductive
and a substantial current will flow. More than enough to leave you
homeless...
Conclusion : ?? I have no idea what your regulations say over there.... but
in some cases with an antenna of some size
placed on the middle of a wide roof... I would plan the lightning protection
into the buildings construction. Let it have
a direct and well sized and protected straight ground through the building
!!!   Crazy ? Yes...for a ham station it will
be for most people.  Why ?  Because you would need a lot of secondary
protection due to the intense magnetic fields
generated inside the building.  These will also be a problem with an outside
conductor, but to a lesser degree due to
"the square law factor" . I mentioned this last "inside conductor" to
visualize my thoughts. I have used this with great
success on a local FM-station that got blown to pieces 3 times in two years.
No problems...and no fires... no
damage to other wiring or electronics...after 7 years ...so far. But that
building was suitable for this installation. Do not attempt
this at home...but take it as a clue as to why impedance and not only
resistance is very important in lightning protection.
Excuse me for lecturing and going far beyond the topic... but if somebody
can learn a little from this it is worth it.
Plenty of sites can be found on the net covering lightning protection...but
find the pro's ... plenty of
strange and dangerous stuff out there !!
And to round it up :  Even if your local code tells you an 8' rod is
sufficient for safety ground it is most definitely not so
as a lightning ground !  On our station here (LA8W / LN8W )  we are using
numerous ( 32) rods and a mile or so
of heavy copper wire buried and interconnected. We also have a heavy copper
loop with 16 rods around the shack
building.  :-)  Going to the extremes ???   Well... we got a medium strike
here with absolutely no damage except
a burnt insulator in a "shorty fourty" beam...  But we DO use feedline and
mains protection as well...
Our neighbour 500 feet away lost his TV- his fridge and his water-pump
(farm-area) and blamed us afterwards
for attracting lightning  :-) We have now settled that matter by having the 
power company
installing a heavy duty protection unit on his line....
73 DE LA7SL
http://la8w.com

> Date: Mon, 21 Apr 2008 18:17:48 +0300
> From: "Alex" <alexeban at gmail.com>
> Subject: RE: [Antennas] Grounding needs
> To: "'Howard W3CQH'" <hsgorden at comcast.net>,
> <antennaware at contesting.com>, <antennas at mailman.qth.net>
> Message-ID: <480cb01e.0637560a.1bc3.5763 at mx.google.com>
> Content-Type: text/plain; charset="us-ascii"
>
> .....first of all, remember that it's going to be useless as far as RF is
> concerned!
>>From the point of view of the DC protection, copper strap or tubing are
>>also
> useless since the skin effect doesn't come into play. Go with some heavy
> gauge wire- the heaviest you can afford- for lightning protection.
> As for the RF install a counterpoise of 2 radials at least for every band
> you plan to use. Try to place each pair 180 degrees from each other so
> that
> their radiation will cancel. The other pairs can be spread quite close to
> the longest ones without undue interference occurring.
> Alex 4Z5KS
>
> -----Original Message-----
> From: antennas-bounces at mailman.qth.net
> [mailto:antennas-bounces at mailman.qth.net] On Behalf Of Howard W3CQH
> Sent: Monday, April 21, 2008 5:11 PM
> To: antennaware at contesting.com; antennas at mailman.qth.net
> Subject: [Antennas] Grounding needs
>
> I live in a 1 story home with a accessible attic.
>
> I have an Icom AH-4 tuner mounted in the attic, and need to run a real
> ground to the 8' ground rod we just pounded in.
>
> My question is - should the ground wire coming from the AH-4 be of a wire
> gauge of #12, or should I consider running 1/4" copper tubing?  The total
> length of the run is approximately 30'
>
> 73's
>
>
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