As a result of several recent discussions and etc passed around on the
reflector, I thought I'd share a few thoughts on the subject. I profess to
be "no expert" on the subject but recalling my formal education and projects
professionally on "Lightening Protection for Remote Radio Systems" and
"Grounding for Broadcast Applications" lends a lot of experience and
knowledge.
Basically there are 3 types of ground systems to be considered as associated
with amateur stations:
(A) Safety ground - this is simply the AC neutral system, formed by a single
driven ground, and used for most all residential installations. Some
electrical codes still do allow and will accept connection to a cold water
pipe. This seems to be going out of style with the installation of PVC or
other "non-conductive" carriers of water. By all means, do not chop off or
isolate the 3rd pin ground on any power supply or other equipment. It is
there to keep you alive in case some component fails.
(B) Lightening ground - Associated with radio towers, buildings and other
"taller than local terrain" structures. Here again we find that several
ground rods are driven and spaced equally around the base of the tower. I
prefer the 1/2" or 5/8" x 8 ft. copper clad rods and if one has to drive
them in at an angle to avoid rock under the soil it is better at an angle
than not buried completely. These rods must be connected to each other as
well as the tower in the form of a ring or circle. Suggested spacing is
about 8 ft. to 10 ft. All conductors must be attached using the lowest
possible resistance connection system (crimp, clamp and cad-weld.....no soft
solder!) and conductors of massive surface area must be used. (I use a
length of 3/4" soft copper water line, flattened out so as to make a heavy,
easily workable copper strap.) The preferred method of attachment to ground
rods is a "CAD Weld" method.....basically a thermo weld....one time shot
system. Keep in mind that a single lightening stroke carries a lot of
energy and should be dispersed to as large of an area as possible.
(C) RF ground - The toughest of all. Any RF frequency can be defined by its
wavelength. Over this wavelength one will find that the voltage reaches
both maximum and minimum values. The distance between a minimum and a
maximum is 1/4 wavelength. Therefore as one moves away from a ground point
to a distance of 1/4 wavelength the potential goes from 0 volts
(theoretically) to maximum voltage. On 20 meters this is a distance of some
17 ft. On 10 meters this is a distance of about 9 ft or so. So where is
ground? Basic advice...... keep ground connecting systems, away from 1/4
wave multiples on bands you choose to operate. Keep conductors as large as
possible, the more surface area the better hence the lower inductance. (A
discussion on ground impedance will not be encountered here.)
Now in practice. In many and most cases a RF ground is not needed at your
station. In some cases it actually may be causing you more harm than good.
Unless you are running an antenna system that is "unbalanced against ground"
i.e. long wire or off-center fed antenna to reference a couple of types.
Due to the nature of coax fed dipoles and beams as well as balanced fed
dipoles, a ground is not needed FOR RF PURPOSES. Do pay heed to lightening
protection however. (Balanced fed antennas and the use of baluns is another
topic of discussion. FYI - I believe in both.)
So how does RF get into my shack? Most likely, RF is being picked up by the
house wiring from a nearby, low antenna, (one that is located over the
house, or attached to one end of the house) fed back via the 3rd pin AC
ground and to the rig. SWR on feedlines is also a contributing factor but
not as much as one is often led to believe. No amount of filtering,
shielding or added grounding will correct a RF radiation problem of this
type. Move the antenna or move the radio to gain more distance. (I
frequently operate a 75M coax fed dipole on 10M and 20M (using a tuner to
keep the transmitter happy) and have no RF in the shack. The SWR on the
coax feed line is often 10:1. There is no RF ground in use either. How
does this work....... well the antenna is some 100 ft from the shack, 70 ft
in the air and does not cross over the house.
Now, if possible, one can take advantage of a good RF ground (staying away
from the 1/4 wave numbers) and improve the situation. A simple,single,
short conductor from the operating position to a driven ground may help. A
word of caution about adding driven grounds. It is little understood, and
most often overlooked, but most important, ALL DRIVEN GROUNDS must be bonded
together. Yes, the tower ground, the radio ground the residential AC
ground, the telephone and cable grounds must be bonded or connected
together. For details on this subject, locate the National Lightening
Safety Institute web page and read the real truth and facts for yourself.
What they have is current practice and should be taken seriously.
Regarding bonding of all grounds, I've been told that connecting the tower
lightening ground system to the house ground system will cause the house to
"burn up" because of a lightening strike. Poo poo on this. Keep in mind
that during a nearby lightening strike the energy is dispersed over a wide
area. ( Several 1000 ft in all directions.) Should there be 2 driven
grounds in the area a "step voltage" is produced and these grounds are not
at the same potential. Look at your system. Is your radio equipment
connected to 2 different grounds? If so the difference in potential will
find the path of least resistance.......typically through the radio
equipment. Please bond all grounds. (Comment: should your system, tower or
house take a DIRECT lightening strike......nothing short of the fire
department can help. Maybe they can arrive in time to save the lot.)
I've used enough bandwidth on this subject for general information. If you
have questions, agreement or disagreements, please respond to me direct.
Let's not take up more space on the reflector. Otherwise, if you have
questions ask.
73
Bob K4TAX
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