is key to good reception
(as published on rec.radio.shortwave, February 1995)
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.
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
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.
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.
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!
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
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).
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.
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.
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.
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?
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
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 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.
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
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).
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