Grounding is key to good reception
What's ground? If I connect the shield
of my coax (which is grounded outside) to the antenna input of my R8, I
hear lots of junk, indicating that there is an RF voltage difference between
the coax shield and the R8 chassis. Last night this measured about S5.5,
which is about -93 dBm (preamp off, 6KHz bandwidth). That's a lot of noise:
it was 18 dB above my antenna's "noise floor", and 26 dB above the receiver's
noise floor.
This sort of disagreement about ground
potential is characteristic of electrically noisy environments. The receiver
will, of course, respond to any voltage input that differs from its chassis
ground. The antenna, on the other hand, is in a very different environment,
and will have its own idea of what ground potential is. If you want to
avoid noise pickup, you need to deliver a signal, referenced at the antenna
to whatever its ground potential is, in such a way that when it arrives
at the receiver, the reference potential is now the receiver's chassis
potential.
Coaxial cable represents one way
to do this. Coax has two key properties:
1. The voltage between the inner
conductor and the shield depends only on the state of the electromagnetic
field within the shield.
2. The shield prevents the external
electromagnetic field from influencing the internal electromagnetic field
(but watch out at the ends of the cable!).
So, it's easy, right? Run coax from
the antenna to the receiver. Ground at the antenna end will be whatever
the antenna thinks it is, while ground at the receiver end will be whatever
the receiver thinks it is. The antenna will produce the appropriate voltage
difference at the input side, and the receiver will see that voltage difference
uncontaminated by external fields, according to the properties given above.
Unfortunately, it doesn't quite work
that way. It's all true as far as it goes, but it neglects the fact that
the coax can also guide noise from your house to your antenna, where it
can couple back into the cable and into your receiver. To see how this
works, let me first describe how this noise gets around.
The noise I'm talking about here
is more properly called "broadband electromagnetic interference" (EMI).
It's made by computers, lamp dimmers, televisions, motors and other modern
gadgets. I have all these things. In many cases, I can't get them turned
off, because it would provoke intrafamilal rebellion. However, even when
I turn them off, the noise in the house doesn't go down very much, because
my neighbors all have them too. In any case, one of the worst offenders
is my computer, which is such a handy radio companion I'm not about to
turn it off.
Some of this noise is radiated, but
the more troublesome component of this is conducted noise that follows
utility wires. Any sort of cable supports a "common mode" of electromagnetic
energy transport in which all of the conductors in the cable are at the
some potential, but that potential differs from the potential of other
nearby conductors ("ground"). The noise sources of concern generate common
mode waves on power, telephone, and CATV cables which then distribute these
waves around your neighborhood. They also generate "differential" mode
waves, but simple filters can block these so they aren't normally a problem.
So, let's say you have a longwire
antenna attached to a coaxial cable through an MLB ("Magnetic Longwire
Balun" [sic]). Suppose your next door neighbor turns on a dimmer switch.
The resulting RF interference travels out his power lines, in through yours,
through your receiver's power cord to its chassis, and out your coaxial
cable to your MLB. Now on coax, a common mode wave is associated with a
current on the shield only, while the mode we want the signal to be in,
the "differential" mode, has equal but opposite currents flowing on shield
and inner conductor. The MLB works by coupling energy from a current flowing
between the antenna wire and the coax shield into into the differential
mode. But wait a second: the current from the antenna flows on the coax
shield just like the common mode current does.
Does this mean that the antenna mode
is contaminated with the noise from your neighbor's dimmer?
The answer is a resounding, and
unpleasant, yes! The way wire receiving antennas work is by first moving
energy from free space into a common mode moving along the antenna wire,
and then picking some of that off and coupling it into a mode on the feedline.
In this case, the common mode current moving along the antenna wire flows
into the common mode of the coax, and vice versa. The coax is not just
feedline: it's an intimate part of the antenna! Furthermore, as we've seen,
it's connected back through your electrical wiring to your neighbor's dimmer
switch. You have a circuitous but electrically direct connection to this
infernal noise source. No wonder it's such a nuisance!
The solution is to somehow isolate
the antenna from the common mode currents on the feedline. One common way
to do this is with a balanced "dipole" antenna. Instead of connecting the
feedline to the wire at the end, connect it to the middle. Now the antenna
current can flow from one side of the antenna to the other, without having
to involve the coax shield. Unfortunately, removing the necessity of having
the coax be part of the antenna doesn't automatically isolate it: a coax-fed
dipole is often only slightly quieter than an end-fed longwire. A "balun",
a device which blocks common mode currents from the feedline, is often
employed. This can improve the situation considerably. Note that this is
not the same device as the miscalled "Magnetic Longwire Balun".
Another way is to ground the coaxial
shield, "short circuiting" the common mode. Antenna currents flow into
such a ground freely, in principle not interacting with noise currents.
The best ground for such a purpose will be a earth ground near the antenna
and far from utility lines.
Still another way is to block common
mode waves by burying the cable. Soil is a very effective absorber of RF
energy at close range.
Unfortunately, none of these methods
is generally adequate by itself in the toughest cases. Baluns are not perfectly
effective at blocking common mode currents. Even the best balun can be
partially defeated if there's any other unsymmetrical coupling between
the antenna and feedline. Such coupling can occur if the feedline doesn't
come away from the antenna at a right angle. Grounds are not perfect either.
Cable burial generally lets some energy leak through. A combination of
methods is usually required, both encouraging the common mode currents
to take harmless paths (grounding) and blocking them from the harmful paths
(baluns and/or burial).
The required isolation to reach the
true reception potential of the site can be large. According to the measurements
I quoted above, for my site the antenna noise floor is 18 dB below the
conducted noise level at 10 MHz. 18 dB of isolation would thus make the
levels equal, but we want to do better than that: we want the pickup of
common mode EMI to be insignificant, at least 5 dB down from the antenna's
floor. In my location the situation gets worse at higher frequencies as
the natural noise level drops and therefore I become more sensitive: even
30 dB of isolation isn't enough to completely silence the common mode noise
(but 36 dB is enough, except at my computer's CPU clock frequency of 25
MHz).
Getting rid of the conducted noise
can make a huge difference in the number and kinds of stations you can
pick up: the 18 dB difference between the conducted and natural noise levels
in the case above corresponds to the power difference between a 300 kW
major world broadcaster and a modest 5 kW regional station.
The method I use is to ground the
cable shield at two ground stakes and bury the cable in between. The scheme
of alternating blocking methods with grounds will generally be the most
effective. The ground stake near the house provides a place for the common
mode noise current to go, far from the antenna where it cannot couple significantly.
The ground stake at the base of my inverted-L antenna provides a place
for the antenna current to flow, at a true ground potential relative to
the antenna potential. The buried coax between these two points blocks
noise currents.
There has been some discussion of
grounding problems on this and related echos. I believe it has been mentioned
that electrical codes require that all grounds be tied together with heavy
guage wire.
I'm no expert on electrical codes,
and codes differ in different countries. However, I believe that any such
requirement must refer only to grounds used for safety in an electric power
distribution system: I do not believe this applies to RF grounds.
Remember that proper grounding practice
for electrical wiring has very little to do with RF grounding. The purpose
of an electrical ground is to be at a safe potential (a few volts) relative
to non-electrical grounded objects like plumbing. At an operating frequency
of 50/60 Hz, it needs to have a low enough impedance (a fraction of an
ohm) that in case of a short circuit a fuse or breaker will blow immediately.
At RF such low impedances are essentially
impossible: even a few centimeters of thick wire is likely to exhibit an
inductive impedance in the ohm range at 10 MHz (depends sensitively on
the locations and connections of nearby conductors). Actual ground connections
to real soil may exhibit resistive impedances in the tens of ohms. Despite
this, a quiet RF ground needs to be within a fraction of a microvolt of
the potential of the surrounding soil. This is difficult, and that's why
a single ground is often not enough.
A little experimentation with my
radio showed that the chassis was directly connected to the third (grounding)
prong of the wall plug. I am concerned that by connecting my receiver to
an outside ground I am creating a ground loop that involves my house wiring.
Can you comment on this?
Yes, you have a "ground loop". It's
harmless. In case of a nearby lightning strike it may actually save your
receiver. My R8 isn't grounded like that, so I had to take steps to prevent
the coax ground potential from getting wildly out of kilter with the line
potential and arcing through the power supply. I'm using a surge supressor
designed to protect video equipment: it has both AC outlets and feedthroughs
with varistor or gas tube clamps to keep the various relative voltages
in check. Of course the best lightning protection is to disconnect the
receiver, but I'm a bit absent minded so I need a backup.
This may seem like a trivial point
but I recently discovered that the main ground from the electrical service
panel in my house was attached to a water pipe which had been painted over.
I stripped the paint from the pipe and re-attached the grounding clamp
and I noticed a reduction in noise from my receiver.
Not trivial. Not only did you improve
reception, but your wiring is safer for having a good ground.
I suspect part of the reason I see
so much noise from neighbors' appliances on my electric lines may be that
my house's main ground wire is quite long. The electrical service comes
in at the south corner of the house (which is where the breaker box is),
while the water (to which the ground wire is clamped) enters at the east
corner. All perfectly up to code and okay at 60 Hz, but lousy at RF: if
it was shorter, presumably more of the noise current would want to go that
way, and stay away from my receiver.
I am also a little confused by what
constitues an adequate ground. I have read that a conducting stake driven
into the ground will divert lightning and provides for electrical safety
but that RF grounding systems have to be a lot more complex with multiple
radials with lengths related to the frequencies of interest. Is this true?
Depends on what you're doing. If
you're trying to get maximum signal transfer with a short loaded (resonant)
vertical antenna with a radiation resistance of, say, 10 ohms, 20 ohms
of ground resistance is going to be a big deal. If you're transmitting
50 kW, your ground resistance had better be *really* tiny or things are
going to smoke, melt or arc.
On the other hand, a ground with
a resistance of 20 ohms is going to be fairly effective at grounding a
cable with a common mode characteristic impedance of a few hundred ohms
(the characteristic impedance printed on the cable is for the differential
mode; the common mode characteristic impedance depends somewhat on the
distance of the cable from other conductors, but is usually in the range
of hundreds of ohms). Of course, if it was lower a single ground might
do the whole job (but watch out for mutual inductance coupling separate
conductors as they approach your single ground).
In addition, a ground with a resistance
of 20 ohms is fine for an unbalanced antenna fed with a high impedance
transformer to supress resonance. Such a nonresonant antenna isn't particularly
efficient, but high efficiency is not required for good reception at HF
and below (not true for VHF and especially microwave frequencies).
Much antenna lore comes from folks
with transmitters who, armed with the "reciprocity" principle, assume that
reception is the same problem. The reciprocity principle says that an antenna's
transmission and reception properties are closely related: it's good physics,
but it ignores the fact that the virtues required of a transmitting and
receiving antenna are somewhat different. Inefficiency in a transmitting
antenna has a direct, proportional effect on the received signal to noise
ratio. On the other hand, moderate inefficiency in an HF receiving antenna
usually has a negligible effect on the final result. A few picowatts of
excess noise on a transmitting antenna has no effect on its function, but
is a big deal if you're receiving (of course, one might not want to have
transmitter power going out via unintended paths like utility lines: this
is indeed the "reciprocal" of the conducted noise problem, and has similar
solutions).
by John Doty
(as published on rec.radio.shortwave,
February 1995)
Grounding System
Perhaps one of the most overlooked
aspects of setting up a listening post is a ground system. Any listener
with a table top receiver will need a good ground system to operate their
unit at its optimum level. This piece will deal with setting up a simple,
yet effective, ground system that can be installed in a short period of
time with a minimum number of tools.
The first thing you will need three
parts:
The first is of course a good ground
rod.
You will also need a buss bar if
you have more than one unit to ground.
And finally some ground wire to
tie the whole thing together.
Let us look at the ground rod.
Not all ground rods are created
equally. When I went to put in my present system, I talked with several
people from our local hydro electric company to see what they were using.
They all agreed on one thing: you need a full sized 10 foot ( 3 metre)
rod to be effective.
This length will almost guarantee
that the rod will stay in contact with moisture in all but the driest years.
The ground can dry out to quite a depth during long hot dry periods, leaving
shorter four to five foot rods useless. The rods must be kept moist to
give a good ground, but more on this later.
I also purchased a rod that had
a built in connector so I did not have to purchase one to keep the ground
wire attached. These work best and are easily found.
Next was the selection of the wire
I was to use. I ended up selecting 10 gauge copper wire that was covered
in a heavy vinyl jacketing. What kind of wire to use is open to all sorts
of opinions. I picked the 10 gauge as it was readily available and, although
stiff, you could work with it fairly easily.
A coated or insulated wire was chosen
to make life easier for me. By using a coated wire it meant I could run
the wire easier as I did not have to worry about it touching objects that
are conductive in nature. Your ground wire must never touch any thing conductive
as it will ruin the ground. A clear and unrestricted path from the radio(s)
to the ground rod is a must and coated wire gives you more options of how
and where to run the wire.
The buss bar can be installed if
you have more than one radio to ground, or if you plan to add to your listening
post with other equipment that may require grounding.
The bar is usually made from copper
because of its conductivity. The bar need not be large. Mine is 3/4 of
an inch (2 cm) wide and 10 inches (25cm) long. I can ground five to six
pieces of equipment on it with no problems at all.
Now that all of the parts have been
purchased we can start on a simple but effective ground system that will
last for years.
You must first of all choose a site
for the rod to be put in. One very important thing to consider is to keep
the run of ground wire as short and as straight as possible. This will
insure a better system.
Keep the rod as close to the side
of the house that your listening post is located. If your home is like
mine, you may have underground hydro, telephone, and gas lines as well
as water and sewer lines, so please call your local utilities to have them
located before you start putting in a ground rod. You do not want to drive
your ground rod into any of these lines. Putting a ten foot metal rod into
a hydro or gas line can ruin your day!
Once you have selected your spot
you will have two options:
1) You can pound the rod into the
ground leaving about 8-10 inches (20 cm) of it above ground;
2) Or you can for the deluxe option.
I have gone for this latter route
as it will over time help you keep the ground rod damp during dry times.
This involves more work but if you live in climate like mine where the
weather varies over a large spectrum or has long dry spells it is worth
the extra effort. Also if you have heavy clay soils during rains the water
will have an easier time to soak into the rock pit instead of running off.
You can mark the ground where you
wish to put the bar and measure one foot (30 cm) in all directions from
this point. Mark the area off and then dig a hole in the area.
This will result in a two foot (60
cm) square or diameter hole depending on how you dig it out. Either is
acceptable. You should dig a hole that is about 2 feet (60 cm) deep, more
if you wish.
Once the hole is completed place
the tip of your rod in the centre of the hole. You can now pound the rod
into the ground leaving it the 8 inches (20 cm) above ground level (not
the bottom of the hole). Have a friend help hold the rod as it will move
around as you pound it in. Be careful not to hit your friend, as this may
hurt the relationship as well...
Once the rod is in place test it
to insure it is in in tight. Try pulling and wiggling it to see if it moves.
If it is in tight you have been
successful.
If it is close to a foundation or
is in loose or sandy soil it will move around. This will not produce a
good ground, so check it out.
If you went the deluxe route you
must now fill the hole with rock. Insure it is hard rock that will stay
loose. Rock such as limestone is of no use as it will break up and form
a hard packed area. You need loose rock fill that will not pack over time.
You may also want to put is in a
bag of rock salt before the rock. This salt once wet will start working
on the rod to give better conductivity. This rock pit is put into place
for one important reason: moisture.
During dry periods I water the rock
pit to insure moisture is getting down to the lower levels of the rod.
The neighbours do kid me about it so if you embarrass easily do it at night.
The next step is to install your
buss bar is in your listening post. If you are going to use one it is easy
to install. You can make one or buy one ready made.
To build one just take your flat
piece of copper and drill two holes is in it. One at either end that will
act as anchor points to mount it on the wall near as possible to you equipment.
You can now drill as many holes as you have pieces of equipment plus one
more for the common lead into the bar.
This will mean if you have four pieces
of equipment to ground you will need:
Two holes to mount the bar, one
at either end.
Four holes for the equipment between
the two anchor holes.
And one hole for the common lead,
also between the anchor holes.
Each of the holes, excepting the
anchor holes at the top and bottom, will be drilled to put in a bolt and
washer to attach the radios etc to. Use what ever you have at hand.
Put in the bolts and washers into
the pre-drilled holes. Using the two mounting holes screw the buss bar
to the wall near to your equipment. Try to keep it centrally located to
keep leads to the equipment as short as possible.
Now that the bar is mounted run short
straight pieces of heavy wire from each piece of equipment to the bass
bar.
You should use coated wire here
to insure no wires touch each other or anything else. This is very important.
Attach the other end of the wire to the lowest bolt and work your way up
to the top. Insure the wire is under the washer so it presses the wire
onto the buss bar insuring a tight and solid contact fit. This is a must.
You can now attach a run of wire
to the common at the top of the bar and run it to the ground rod outside.
Once again insuring a solid contact . If your rod had no built in clap
you can use metal strapping to get a solid tight fit to the rod.
When you connect any end of the
wire to any piece of equipment or the buss bar or ground rod, insure you
strip the wire and then using sanding or emery cloth clean the bare wire
to insure there is a clean contact.
You should use washers on binding
posts to wire up the equipment. This will insure a solid contact. Loose
contacts are of no use so make sure all contacts are good ones.
Your ground system is now completed.
Maintenance is little if any. You should from time to time check the connections
to insure they are tight and in the case of the ground rod connection there
is no corrosion. It may need to be cleaned once a year.
When it is dry water your rock pit
to insure a good ground year round. I flood mine until I can see the water
sitting on top.
That is it, you now have a good ground
system that will last years.
Great ground with kitten litter
Bentonite is great for getting an
excellent conductive ground.
Bentonite?
That's the stuff used in clumping
kitty litters.
I had forgotten that the "clumping"
litters are Bentonite. The brand we buy for our Siamese cat says that it's
"a natural clay product...".
That's the Bentonite!
If you can locate a supply of the
mineral called Bentonite, it makes an excellent ion-rich (and non-corrosive)
backfill for ground rods and grounding systems. Professional antenna installations
and electrical substations use Bentonite for lowering the resistance to
earth.
I have read that Bentonite is sold
at animal feed stores; it is used as an additive to cattle & pig feed.
Also, ceramic supply stores sometimes carry Bentonite, as it is a special
clay used by potters.
During a vacation one year, I got
my initial supply of Bentonite directly from a refinery in Wyoming where
they mine the stuff... they gave me some bags of Bentonite free because
the bags were slightly ripped and they couldn't sell them.
To use the Bentonite, you dig a hole
eight inches or more in diameter, perhaps three feet deep (post-hole digger
helps), and suspend the ground rod in the middle. Put in a few inches of
the powdered or crushed Bentonite, water it thoroughly (it swells up tremendously
with water), and then add another layer. Water that layer, and continue
with water-soaked layers of Bentonite until the hole is filled up.
The Bentonite absorbs the water,
expands, and holds the ground rod very tightly in the center. Because of
the expansion and Bentonite's conductive qualities, a lower resistance
path to ground is achieved. It's sort of like using an eight-inch diameter
ground rod at your site. The Bentonite never drys out, as it is "hydrophilic"
and absorbs moisture from the soil to remain hydrated.
If you do a web search on the term
"bentonite" you'll come up with a lot more information about this mineral.
Besides improving ground systems, it is used as a colloidal (suspension)
product for everything from vinyl plastics to chocolate(!) to cosmetics.
Also, Bentonite, in a highly refined form, is the key active ingredient
in disposable baby diapers (I'm not kidding!). I know this trivia because
Bentonite mining is one of the industries in the region of Wyoming where
my wife's family resides.
[It is also to be used as protective
shock absorber for nuclear waste, dumped deep inside Sweden's rocky ground.
-- hcdx editor]
Here in Bonney Lake, WA, our soil
is very rocky, ancient glacial debris from Mt. Rainier. Even though we
live on a small island and are surrounded by water, the soil is still very
dry and rocky. Bentonite surrounding my ground rods has improved the directionality
of my impedance-matched, terminated 175 ft. longwire. In my opinion there
was an improvement in directivity to Papua New Guinea and Irian Jaya after
improving the ground with Bentonite.
I also use the same ground for K9AY
electronics (the head unit), although I'm not positive this is really helping.
It's worth a try, though, especially for poor soil.
There was a IEEE paper written some
years ago about Bentonite grounds. The careful measurements and comparisons
they did showed clearly that resistance-to-ground was lowered when Bentonite
was used. This method is clearly preferred over adding various salts to
the soil... Bentonite won't corrode the ground rod nor harm the soil or
surrounding vegetation.
By Guy Atkins
hcdx list, August 10, 2000
Better ground with conductive
cement
The resistance of an ordinary ground
rod can permanently be cut in half by embedding it in a 3 to 6-inch radius
of conductive cement such as Earthlink-101.
The cement is poured in the 6-in.
or 12-in. diameter hole, around the rod.
It sets up as it absorbs moisture
and stays moist. It's conductivity is 25 ohm-cm versus 5000-6000 ohm-cm
for ordinary concrete.
We tested this at Lucent Bell Laboratories
in Chester, NJ.
By Clayton Hallmark
April 2000
