For stations with freestanding cabinets or racks in addition to an operating desk, the timing issues become more significant due to distance. This size station necessitates separate cabinet/rack direct ground connections to the SPGP.  In addition, stations of this size have other special considerations, such as concrete floor conductivity, that are not covered here.

Don’t forget to allow for future growth of your station in the SPGP layout.  Typically this means leaving room for an additional coax protector or two or maybe a rotor control protector.  It is easier to plan for these expansion items now rather than have to re-arrange the protectors on the panel later.

If the form of SPGP you have chosen is a metal plate mounted in a window or it is a full fledge through-the-wall entrance panel, you can ignore the remainder of this section.  The next major consideration is the placement of the SPGP with relation to the radio equipment.  The SPGP is ideally mounted on an interior outside wall with access to an earth ground and within a few feet of the radio equipment.  Sounds easy, depending on your radio room, it may be next to impossible.  Let’s work it through. 

These real-world constraints sometimes present some real challenges.  One of the biggest challenges is how do I ground the SPGP.  A number 6 AWG wire to the radiator or water pipe is usually NOT acceptable!  I say “usually” because if your radio room is on the top of a high-rise building, that may be all that you have.  Let’s cover the real requirements and the come back to this type of problem.

The purpose of the ground connection is to take the energy arriving on the antenna coax cables and control lines (and to a lesser extent on the power and telephone lines) and give it a path back to the earth, our energy sink.  The impedance of the ground connection should be low so the energy prefers to follow this path and is dispersed harmlessly.  To achieve a low impedance the ground connection needs to be short (distance), straight, and wide.  Let’s examine each of these attributes.

Short

We all know that a conductor, no matter what size or shape, has inductance.  In connecting the SPGP to the external ground system, it should be done with the shortest possible wire.  Did I say wire?  Be sure to read about Wide.

Straight

Rarely is it possible, in the context of an amateur radio station, (unless the structure was designed around the radio station) to go directly in a short straight line from the SPGP to the external ground system.  Most of the time we are encumbered with an existing structure that is less than ideal and further encumbered with self (or spouse) imposed constraints regarding just how much of a mess we can make.  So, we do the best we can.  Straight then becomes a relative concept.  You run the wire (there's that word again) as straight as possible.  Keep in mind that every time the wire makes a turn, the inductance of the path is increased.  Not a lot (~0.15uH for a 90 degree turn in less than one-inch), but the cumulative affect could be meaningful.  By the nature of its current (magnetic) fields, a wide wire (strap) has lower inductance per length, compared to round conductors, and has minimal inductance for turns. 

Also keep in mind, speeding electrons don’t change direction.  The inductance in each bend or turn represents a speed bump, causing the electrons to bunch-up.  If enough bunching occurs, some of the electrons are likely to leave the wire and find another path to ground.  This is not desirable; we have lost control.

Wide

We all know that wire no matter what size has inductance.  Yes, the larger wire sizes have less inductance than the smaller sizes.  We also know, RF energy travels near the surface of a wire as opposed to within the central core of the wire (skin effect).  If we put these together and extend the hypothesis a little, it would seem reasonable to use a railroad-track size bus bar as an excellent connector between the SPGP and the earth ground.  While this would work well, it has lots of surface area and a massive core; the cost would be prohibitively expensive and it would be extremely cumbersome to work.  However we can have the benefits of the large bus at a very reasonable cost if we use multi-inch-wide copper strap instead.

One and a half inch wide, 26 AWG (0.0159 inch), copper strap has less inductance than 4/0 AWG wire, not to mention that it is less expensive and much easier to work.  We can use thin copper strap to safely conduct lightning surge energy because the energy pulse is of very short duration and the cross sectional area of this strap is larger than a #6 AWG.  Also keep in mind that the strap has a large surface area that makes it ideal for conducting the strike’s RF energy.

Remember the goal is to have the path leading away from the SPGP that is more desirable than any other path.  In order to achieve this we need to find the total amount of coax surface area coming to the SPGP from the antennas.  A single run of 9913 coax represents about 1.27 inches of incoming conductor surface.  To make our ground path appealing to the surge energy, we ideally need more than 1.27 inches of conductor surface leaving the SPGP.  So, the use of a single one and a half inch wide conductor leaving the panel is reasonable, three or more inches would be better.  Remember, inductance is calculated on the length of the connection between the SPGP and the ground, as well as the number and sharpness of the turns.  Think about what you would need if you had three 7/8-inch hardlines – a minimum of nine-inches; twelve would be better.