Ground Zero for your Receiver

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by Ted Benson (as published in Frendx 5/89)

Most of us who grew up in the “nuclear ages” think of ground zero as the impact point of a nuclear warhead. To radio enthusiasts, however, it can mean something completely different.

The ideal ground for radio reception represents zero ohms of resistance, and no impedance whatsoever. As with everything else in our hobby, however, there is the theoretical, end then there is the reality. In reality, perfect ground is impossible to attain. Commercial radio stations bury thousands of feet of copper wire to obtain a good ground, and even this is not perfect. But why is a ground connection important at all?

Electricity flows only when a difference in potential exists between two points. There are two power prongs on our wall sockets (the third is a ground – more on this later). At any one time, one prong has far more electrons than the other does. If you connect a circuit between the prongs (such as a lamp) the excess electrons will flow through the circuit to the other terminal, in an attempt to reach a balance. It is the flow of these electrons which performs work – in this case lighting the lamp. Unless current flow can occur, no work can be done. Ah, you say, what about the time as a kid when you stuck a key in the wall socket? Only one prong was connected, but you still got a nasty shock. Where did the current flow to as it coursed through your body? It flowed through your feet to ground. Because the ground wasn’t very good, only a little current was able to flow and you survived (literary folks – this is no joke). Many people are killed each year because the ground was good. Perhaps they were wet or standing in water, or barefooted on moist soil. The better the ground, the more work (or harm the electricity was able to do.

Your shortwave receiver is in essence a circuit, which is connected to a power source similar to your wall socket. In this case, though, the power source is the radio signal you wish to resolve. How can a potential exist when the entire signal flows through the air? The question provides its own answer! The radio energy is almost all in the air. Almost none of it is in the ground. The potential exists, therefore, between the air and the ground. But how do we plug our circuit – the receiver – into the air and ground? Well, the air connection is of course made by an antenna, which gathers as much signal as it can. The ground connection is made in any number of ways. Many people simply connect the ground terminal of their receiver to the little screw in the middle of the wall socket. Others drive a stake or rod into the ground and connect this to the radio’s ground terminal. Neither of these techniques is really adequate. Although the little screw on your wall socket is a moderately good ground for 60 Hz AC, it can literally appear as an open connection at radio frequencies. In other words, relying on this point as your ground is like making no ground connection at all! Driving a copper rod into the earth might provide a good ground connection, if the soil around it was very conductive. Most soil, though, is not. Some method must be used to increase the conductivity of the soil around the ground rod. Figure 1 suggests a technique for improving the conductivity of the soil.

Suggested ground rod installation.

Older reference books suggest using copper or nickel chloride instead of the table salt (sodium chloride) shown in figure 1. Although the copper and nickel chloride do work better than sodium chloride, they also contaminate the soil. In this age of ground water contamination, as is rampant here in the Silicon Valley, the last thing we need is to contribute more! Table salt, as shown, is also much cheaper than other salts. In fact, a huge bag of water softener salt works fine, and can cost as little as $5.00. In areas of very dry soil, it may also be necessary to “water” your ground occasionally to keep the soil moist in and around the pit.

Why all this bother about a ground connection? Well, as we saw earlier the ground is the other half of the power source. The better a connection you make to the power source, the more signal will flow through your receiver’s front end. And the more signal that flows, the more you will hear!

Connections

Now that we have a good ground, how do we connect it to our receiver? Many receivers, particularly the older ones, have a separate binding post for the ground connection – either as a ground terminal, or as one half of a balanced antenna input. Newer receivers often come with only a coaxial connector. The outer shell of this connector is ground. When using this type of receiver simply connect the ground wire to the shell of the connector. Be careful when connecting to any other metal part of a receiver – particularly tabletop models powered by the wall socket. In some of these receivers the chassis may be “hot”, that is, it may have a substantial amount of AC voltage present. This voltage will arc and perhaps trip a circuit breaker when grounded. Also, many receivers do not have all of the metal chassis parts connected together, so you may be connecting to an insulated bracket instead of the receivers’ front ends. Always use heavy gauge wire {#14 as is used for house wiring is good) when connecting to a ground, and keep the length as short as possible.

Exceptions to the rule

Many antennas are designed to be used without a ground. Instead of using the earth as the other half of the power source, these antennas contain two elements, and each element in effect becomes an opposite pole of the power source. Examples of these antennas are dipoles and Yagi beams. But even with these antennae, a ground can often improve reception, if for no other reason than it lowers the noise level. For the most common types of SWL antennas, like the random wire or vertical, a ground is absolutely essential.

One other aspect of the ground has not yet been discussed. Safety can be enhanced by the use of an outside ground. Most have never seen what lightning can do when it strike an antenna and enters a house while looking for ground. I have. As a Coroner, I once visited a home in which the amateur radio operator had grounded everything to the wall socket ground. Well, lightning can be very unpredictable. Before it eventually found the wall socket ground it travailed around his radio shack, and took a detour through him in the process! I implore you, if using a coaxial feedline use a lightning arrestor as well. If using a random wire or similar antenna, use a large switch as close to where your antenna lead enters the house as you can, and use it to short your antenna lead to ground when not in use. In either case, connect the arrest or switch to the outside ground. It’s a little inconvenience that can save a lot of inconvenience.

Grounding is key to good reception

by John Doty

(as published on rec.radio.shortwave, February 1995)

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).

Grounding System for DXers

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.

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