Sooner or later, hams encounter some RF trickery above and beyond the usual connecting together of gadgets and antennas. This usually happens on the HF bands as a home station begins to grow or when a portable station starts to misbehave. It might be that RFI clobbers a PC or some kind of controller. You might experience a “hot spot” in the form of an RF burn—painful but not dangerous. There are lots of ways RF can get into or onto places and things it’s not expected to.
Why does this happen? Well, consider that most station equipment is very close to the antennas, electrically. A typical backyard 40 meter dipole is usually less than one wavelength (about 125 feet, more or less) from the station equipment. In the field, antennas can be as close as a few yards from the equipment and operator. (No closer, please!) In such cases, every wire and cable in the station is going to act as an antenna, picking up the transmitted RF. It is often said that everything is an antenna and the more you want it to NOT act as an antenna, the better antenna it will be!
Unless you operate from a shielded room, you will have to plan for this unexpected and possibly unwelcome RF. I call this RF management, and there are techniques for dealing with this RF in stations large and small. You might start by reading a copy of my book, Grounding and Bonding for the Radio Amateur, 2nd Edition (referred to as “G&B” from hereon). It deals with a number of related topics and how they interact: ac safety, lightning protection, and, of course, RF management. The topic of this article is a key component for dealing with RF in your station—the RF ground plane.
RF Ground Planes and RF Bonding
So what is an RF ground? There isn’t really such a thing as a point, enclosure, or wire that has zero ohms impedance and maintains zero voltage over the wide range of frequencies that hams use. The best you can do is keep the equipment or system at the same voltage. That’s the definition of bonding, so this article is really about RF bonding. If you use a metal surface and bond equipment to it, then that surface acts as the RF ground plane.
RF bonding may sound involved and expensive, but it’s really not. All it means is connecting equipment together with a low impedance conductor so that the different pieces of gear are kept at approximately the same voltage. By minimizing RF voltage differences, RF current flow is also minimized. Since RFI is usually caused by RF current getting to where it isn’t wanted, this also minimizes RFI.
What is a low impedance conductor? That means an electrically short conductor well under 1/20th of a wavelength so it doesn’t start acting like an antenna or feed line. The conductor should have low inductance to minimize inductive reactance. This means a short piece of strap, solid or tinned and braided, or a piece of heavy wire, #14 AWG or larger, solid or stranded.
Given the realities of assembling a collection of equipment into a budget-limited amateur station, it is common to use an RF bonding bus—such as a piece of metal tubing or bar at the rear of the equipment—and connect all the gear to that with short jumpers. Another common option is to use a metal desk or table or to put a metal sheet on the tabletop, creating an RF ground plane. This is not as direct as connecting the equipment directly together, but it is a manageable solution to the problem. Both the bonding bus and the ground plane approach are discussed in Chapter 5.3 of G&B.
What Connects to an RF Ground Plane?
To keep the discussion reasonably brief, let’s have the phrase “RF ground plane” include the RF bonding bus. In short, pretty much everything in the station that has a metal enclosure or is connected with a shielded cable or feed line should be bonded together—even gadgets like antenna switches that aren’t powered. (See Chapter 6 of G&B.)
What about dc power supplies? Only their metal enclosure should be connected. Check the manual for guidance on how to connect the negative output to the enclosure or any station ground. Don’t rely on the bonding connections to act as a dc return to the power supply. This invites RF into the power connections of the supply and the equipment it powers. Not good! Keep the return connection separate.
Each piece of gear with a metal enclosure probably has a ground terminal. If so, the bonding strap or wire should be connected there. Unpowered switches and similar gadgets may not have a ground terminal, but any enclosure screw can be used. Be sure to scrape off any paint or coating so a good electrical connection is made.
Today’s PCs and associated gadgets are a bit of a conundrum. If the PC has a metal case, use one of the case screws as your ground point. The threaded posts of a video or serial connector are usually connected to the case—check with a voltmeter. Laptops are usually floating and shouldn’t be connected to the ground plane. Computer gadgets like Ethernet switches and USB hubs usually have plastic cases and can’t be grounded. (Use shielded cables, though!)
Finally, the various cables and wires that connect gadgets and equipment are laid on the ground plane. This helps limit the amount of RF the cables pick up because the ground plane limits the field strength of an incoming signal. Use the shortest cables you can, but 3-foot or 1m is usually about the shortest length widely available. Coil the excess cable (either in a circle or figure-8) and place it on the ground plane. It’s not necessary to secure the cable to the ground plane except for neatness.
Lockwashers, Strap & Wire
As long as we’re on the subject of connections, make sure you have secure electrical contact! I recommend toothed lockwashers for all screw-down connections. A flat washer or screw head will eventually loosen enough for oxide or corrosion to degrade the contact. This is a good time to obtain an assortment of lockwashers for ground plane connections. As you use up the most popular types, you’ll know which to replace with a larger quantity. A split-ring lockwasher is intended to secure a nut mechanically, but it won’t help with electrical contact. Don’t use hose clamps since they aren’t intended for inflexible metal-to-metal connections and will loosen over time.
What’s the best conductor to use? The standard in commercial and military practice for fixed stations is solid 20-gauge copper strap at least one-inch wide. This can be hard to come by (and expensive) so a good backup is flat, tinned braid with securely attached terminals. Braid and copper ground straps are both available from providers like DX Engineering. Do not use braid removed from old coax—it will oxidize and the individual wires in the braid will start loosening as soon as the jacket is removed. (Using the braid still inside and protected by the coax jacket is okay.) Do not use braid where it will be exposed to water or chemicals. Auto supply stores sometimes have braid protected by a plastic jacket that can be used, but remember that automotive components are intended for dc, not RF.
Heavy wire, stranded or solid, will also work well. Because of skin effect and related behaviors, RF current in even wide braid and strap only flows along the edges. A large diameter wire, such as #14 or #12 AWG, will work about as well. Stranded wire can have crimp terminals attached. Solid wire, such as the kind removed from surplus pieces of Romex ac cable, has to have the terminals soldered on to avoid loosening. If I use an insulated wire, I try to use black or green insulation as a visual reminder of what the connection is for.
Whatever you use—braid, wire, or strap—keep the conductor short and avoid bends, turns, and loops. This adds inductance to the conductor and raises the impedance at RF, creating a voltage drop. I know that it may be hard to reach around behind equipment and connect that wire, but resist the urge to use a long conductor and then push the equipment back into place, creating a tangle. Some hams use snap-together Powerpole connectors for ground connections in awkward places.
What Is the RF Ground Plane Connected To?
That’s a good question, too! There are three basic functions of the RF ground plane: ac safety grounding (G&B Chapter 3), lightning protection grounding and bonding (G&B Chapter 4), and RF management bonding (G&B Chapter 5). Chapter 6 of G&B covers good practices for making the various connections.
Without question, in a fixed, ac-powered station, the RF ground plane and all exposed metallic surfaces shall (no wiggle room) be connected to the ac power safety ground according to the National Electrical Code. This may be a direct connection to the branch circuit’s “third wire” or through a station power supply. In generator-powered portable stations, this connection is not required for ground-independent generators.
Your RF ground plane also has to be connected to whatever lightning protection system you have in place. This is a very broad topic, so refer to Chapter 5 of G&B or to Ron Block, NR2B’s new book, Lightning Protection. Both books go into considerable detail about lightning protection techniques. The usual connection is either a direct connection to an external ground electrode (rod) or to an entry panel where feed lines are connected and also connected to an external ground system.
Second Floor Stations
This situation—a station well above ground in an upper floor room—is very common. In fact, the author’s home station is on a second floor. What to do? (See Chapter 6.5 of G&B for guidance.) The important thing is to focus on bonding. In the author’s home station shown in the photo, a sheet of aluminum flashing is secured to the shelf. All of the equipment is either connected directly to the flashing or to the ground bar, which is then connected to an external ground rod through the window panel. You can see the green wires that make the ground connections. While there is a ground rod outside below the window, the connection is too long to be effective at RF or for lightning protection. The equipment-to-equipment bond is what protects against sharp surges and transients.
Mobile and Portable Stations
A similar situation exists in these stations. In a permanent mobile station, a small ground plane panel can be constructed and mounted securely in the vehicle. For temporary portable station setups, a light metal panel or bonding bus can be effective with no ground electrode required. In fact, the author uses aluminum foil during Field Day as a disposable RF ground plane, with heavy clip leads connecting the equipment. This minimizes RF current flowing between pieces of equipment and the subsequent RFI.
Small Station Options
If you are just getting started with one or two radios and a modest collection of gadgetry, then an RF ground plane like the DX Engineering DXE-RGP-MA kit is a good way to get started and “see how it’s done.” Kits are available with either copper or aluminum plates. Either will work fine, although copper has a slight edge in conductivity and can be soldered to easily.
The plate in this kit is 16″ x 16″ and plates up to 20″ square are available. Note that external-tooth lockwashers are provided. This is a good habit to start. You can drill more holes for additional connections. Leave one hole for the connection to the station ground(s)!
Medium Stations
As your station grows (“Honest! I only need one more radio!”) it will probably expand to cover one or more tables or desks, probably with shelves. Purchasing a lot of ground kits would get pricey, and by this time you probably have a good idea of what’s involved. By happy coincidence, I just started building some new station tables for my contest and DX station. Each table has an RF ground plane covering most of the rear surface as you can see here.
The flashing is 10″-wide aluminum roof flashing. I used #8 “Lath” truss-head screws to hold the flashing down because they are flat and don’t obstruct the equipment. Having someone to hold the flashing flat against the tabletop helps keep the flashing from curling. If the flashing does curl, a strip of rug edging makes a nice-looking border.
A pair of ground bars intended for use in circuit breaker boxes are attached to the flashing at each end. Use a lockwasher under each mounting screw to ensure good contact between the ground bar and flashing over time. Ground bars are available from four to nearly thirty attachment points.
Here you can see the almost finished project. (Is any station truly done?) The radios, displays, and attached PCs are all connected to the flashing that forms the RF ground plane. A heavy wire (#6 AWG stranded) will connect each of the two stations to a central metal rack cabinet holding amplifiers, other antenna system gear, and ac surge protectors. The rack is then connected to the station’s external perimeter ground system which is just outside the basement wall.
Tying It All Together
Whether you use a pre-made kit or build a custom RF ground system, remember that you’re trying to minimize voltage between the equipment so that you don’t have high-voltage points or RF current flowing around. Bonding, bonding, bonding—that’s what ties it all together.
