DX Engineering DXE-RGP-SC Hands On Radio #

Technical

Hands-On Radio

by Mark Spencer, WA8SME

H. Ward Silver, NØAX, [e-mail]

Notes on Bonding and Shielding

One of the challenges facing the average
ham trying to build a station is that the infor­mation needed is not collected in one place.
Electrical safety (ac safety grounding) is
discussed in one place, while lightning pro­tection is discussed in another, and control­ling RF is in yet other places. As we’ve
seen, however, the needs for all three have
much in common, including the physical
wiring. The ham needs to read up on all
three and approach station-building with a
comprehensive strategy.

Bonding Conductors

One of the more popular materials hams use
for “ground” connections is braided strap or
shield braid removed from old coaxial
cable, but that is not a good idea. Here is the
problem — the wires in a coaxial cable
shield are pressed together and protected
from moisture and contaminants by the
jacket. Once the jacket is removed, the
wires slowly begin to corrode and the
weave loosens. Pretty soon all those indi­vidual tiny wires start acting less like a
large, flat surface and more like, well, indi­vidual tiny wires with poor connections
between them. This is not very good for
conducting RF.

For bonding at RF, use heavy wire or strap,
such as copper or aluminum flashing. Solid
wire, such as #14 or #12 AWG from home
wiring cable, or stranded THHN wire
works fine. The important thing is to maxi­mize surface area because of the skin effect

1

For dc connections and for ac safety

at RF.
grounding, which are more concerned with
resistance, braided strap is acceptable.
Braid should also be used for bonding in
high-vibration environments, but that’s not
necessary for most ham shacks, even
mobile installations. For lightning protec­tion and bonding earth connections together,
use heavy wire or strap to minimize imped­ance and for mechanical strength.

Star Versus Bus Congurations

The answer to the question, “Should I use a

1

Notes appear on page 68.

Experiment #146

Equipment Enclosure

Long connections

Short

connections

Common

Point

Star Connection

(A)

Figure 1 — The star connection at (A) and the bus connection at (B) accomplish the same safety
functions for dc and ac power. As the frequency of interest increases, minimizing connection length
and loop area makes a bus configuration preferable if compact.

star or a bus connection?” (see Figure 1)
depends on what you are trying to accom­plish. At dc and low frequencies, where
resistance is the primary consideration, both
are equivalent and the star connection is
usually more convenient. At the higher fre­quencies involved for lightning protection,
inductance becomes the most important
characteristic. A star configuration with
reasonably short connections will still pro­vide adequate bonding.

At RF, however, the electrical length of the
connection is the dominant consideration.
The bus and star are trying to accomplish
the same thing — keeping all of the equip­ment at the same voltage — but the bus does
the job with shorter connections.
length of the bus must be kept to a mini­mum. Connecting the enclosures directly
together with individual wires is an alterna­tive to using a separate bus conductor.

While trying to address ac safety, lightning,
and RF control sounds complicated, mini­mizing the physical length of bonding con­nections addresses all of these concerns in
the typical strong RF fields of a ham station.

Bonding to

other systems

and Earth

connections.

2

The

Bonding enclosure to enclosure using short
conductors between interconnected equip­ment works for all three needs.

Minimizing Loop Area

The inductance of a one-turn circular loop
is directly proportional to the natural log
(ln) of the loop’s radius: the bigger the loop,
the greater the inductance. As inductance
increases, so will the voltage between any
two points in the loop whether the voltage is
induced by a magnetic field or a rapidly
changing current pulse.

What loop is this, you ask? The typical ham
station is full of loops created by the shields
of interconnecting cables, antenna system
and control cables, ground connections, etc.
Figure 2A shows the basic idea, and there
are many more loops than the ones indi­cated in the drawing. Every complete con­ductive path around enclosures and cables
counts as a loop.

Eliminating these loops is not realistic.
Minimizing their area, on the other hand, is
a productive strategy. First, use the mini­mum cable length. If you have two pieces of

Bus

Bus Connection

QS1503-HoR01

(B)

Reprinted with permission; copyright ARRL.

QST® – Devoted entirely to Amateur Radio www.arrl.org March 2015 67

Cables

(A)

(B)

Loop

Keep Cables

Together

(A)

Figure 2 — Each conductive path through enclosures and cables creates a loop (A) that can pick
up and radiate signals. Minimizing cable length and loop area by keeping cables together as at (B)
can reduce pickup and radiation of RF.

equipment 6 inches apart connected with a
6-foot cable, replace that cable with a
shorter one and coil up any extra length.

Next, minimize the area of each loop.
Where several cables run in the same gen­eral area, use wire ties or a cable tray to hold
them close together. Separate wires for
power and ground should be twisted
together, as should audio connections to
speakers. If you use a bus for bonding
equipment enclosures together, consider
running your interconnecting cables along
the bus as in Figure 2B, further reducing
loop area.

Connecting Shields

Most hams understand the need to connect
the shields of RF-carrying coaxial cables to
metal enclosures, as in Figure 3A. One
analogy is that of creating a “water-tight”
connection as if the differential-mode RF
signal was a fluid to be kept within enclo­sures and cable shield. This keeps any con-

taminating common-mode RF currents on
the outside of the cable or enclosure. It also
eliminates unwanted radiation of the RF
signal from the internal electronics.

The same concerns apply to unbalanced
data and audio connections. Figure 3B
shows a two-wire connection with a shield,
such as for one circuit of an unbalanced
RS-232 interface. From the perspective of
both susceptibility to interference and inter­ference-causing, it is important that the
external shield be connected to the enclo­sure and that any separate signal ground
connections not connect to the enclosure.
Keep internal signals and RF inside the
cable and external RF out of the cable.
Using twisted-pair cabling also helps pre­vent RF pickup and radiation.

Unfortunately, it is common practice in
amateur equipment for multi-circuit con­nectors such as mono and stereo phone
plugs to have their shield connections (usu-

QS1503-HoR02

(B)

ally referred to as the sleeve) isolated from
the metal enclosure. Instead, the shields are
often connected to an internal common
point on a circuit board which is then con­nected back to the enclosure. Known in pro
audio circles as the “Pin 1 Problem”
because shields of audio cables are con­nected to Pin 1 of the standard XLR con­nector, this routes RF current on the outside
of the cable shield to the internal electronics
where it can do the most damage. At the
same time, RF noise and harmonics from
the electronics are routed to the outside
world.

Worse, shields are sometimes left com­pletely unconnected. This allows the RF
current to enter the cable and flow into the
electronics of whatever is connected. For a
demonstration, turn on your HF transceiver
and connect it to a dummy load. Even a
short cable will do. Turn off any preamp or

3

front-end filtering.

Note the level of back­ground noise on, say, 20 meters. Now
unscrew the shell of the PL-259 at the
transceiver, leaving the center pin con­nected. The dramatic increase in noise is
from signals being picked up by the cable
shield and entering the cable to become
differential-mode signals at the open end of
the shield.

The ARRL’s Safety page (www.arrl.org/

safety) and RFI page (www.arrl.org/
radio-frequency-interference-rfi) list

many references and detailed articles to
teach more about these topics.

Notes

1

The ARRL Handbook, 92nd edition, (Newing-

t on: 2014), “Chapter 5: RF Techniques,” p 5.1.

2

See the Hands-On Radio web page (www.

arrl.org/hands-on-radio) for Experiments
#145 and #146.

3

If the receiver has a high-impedance input,

this demonstration may require a 6 to 10 dB
50 Ω attenuator at the inputs.

Enclosure Enclosure

Electronic

Circuit

Bonding connections
to other equipment.

Figure 3 — It is important to keep common-mode RF currents flowing on the outside of cable shields from entering electronic equipment. Similarly,
proper shield connections prevent RF signals generated by the equipment from being radiated by the cable.

68 March 2015 ARRL, the national association for Amateur Radio

Shield

Differential-mode

Common-mode current

on outside of shield

Coaxial Cable

Electronic

Circuit

Bonding connections
to other equipment.

®

www.arrl.org

Shield

Differential-mode

Common-mode current

on outside of shield

Shielded

Two-Wire Pair

QS1503-HoR03

Reprinted with permission; copyright ARRL.