April 2016 Archives

Grounding: Copper Strap vs Structural Steel

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In broadcast, multi-tenant, and microwave shelter design, it is desirable to prevent lightning from impacting operations. One way of doing this is by constructing a "halo" around the inside of the building at roof, wall, and/or floor-level or around the outside of the shelter of entire equipment installation.

The fundamental nature of lightning is that of an effective DC pulse channeled through a single point of contact. Because lightning is essentially a large DC pulse like a square wave with a trailing edge, low-frequency AC pulses are induced in nearby wires and radiated when lightning strikes. This is what produces the characteristic "slope" on a spectrum analyzer of a lightning strike.

Since lightning is a static charge equalizing through a single point, it is easy to see that equalization over an area is the desired result of the strike. A tree gets blown to smithereens, but the charge dissipates and equalizes over the area around the strike -- both above and below the point of contact. Using good conductors, the magnitude of voltage across a given area is minimized. This lowered difference in potential prevents large voltages from appearing across equipment.

By running ground strap across the inside walls of the shelter around the perimeter at the ceiling, middle of wall, and floor levels, the lightning is given a path around the equipment instead of through the equipment. This prevents damage as high-resistance points in the equipment won't result in arc flashover due to high voltage across those points. This is not a Faraday cage, but works in a similar fashion. By giving lightning an alternative path of low resistance and impedance, the vast majority of the current goes around rather than through.

NEC compliance confuses this point, because traditional RF grounding for lightning and power grounding are somewhat at odds. Military manuals and Motorola's guide to R25 installation help. In some cases, the halo ground system has been implemented with the power ground opposite the tower coax window / ground panel. This allows NEC compliance with respect to the power ground, while putting a ground rod at the tower base and connecting the two together via 0000 to 2 AWG copper conductors.

At this point, we will discuss the merits of a copper ground halo above equipment versus using building steel and steel bolts.

A 4" copper ground strap 0.022" thick has a total cross section of 0.088 square inches, equating to a little larger than the cross-section of 0 AWG copper wire. 0 AWG copper wire has a resistance of 0.1 ohms per 1,000 ft. Thus, 1,000 feet of 4" 0.022" thick copper strap has a resistance of nearly 0.1 ohms.

A carbon steel wire 0.5" in diameter has a resistance of 0.00034 ohms per foot, or roughly .34 ohms per 1,000 ft. Thus, three 1/2" diameter bolts are nearly equal to 4" 0.022" thick copper strap assuming no other losses.

This calculation does not factor for the resistance of zinc at steel-zinc-zinc-steel junctions. However, one can see that steel-on-steel contact under pressure, three or more (typically six) bolts are enough to allow the building steel to act as a halo, thus not requiring a copper halo be installed as long as the building steel is attached to the ground system.

1" diameter carbon steel has a resistance of 9.0 x 10^-5 ohms per foot or .09 ohms per 1kft, so one can see why big towers often are not connected together with ground straps from section to section but smaller towers are.

It would be preferable to have the building steel sections welded together, or jumpers cadwelded around each joint.

Bell Microwave Radio Reference

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This is an attempt at an exhaustive list of Bell System Technical Journal articles on Long Lines microwave technologies.

Published February 4, 1948
First introduction of the horn antenna and add/drop RF hybrid duplexer

BSTJ 30: 4. October 1951: The TD-2 Microwave Radio Relay System. (Roetken, A.A.; Smith, K.D.; Friis, R.W.)

BSTJ : The TJ Radio Relay System (Gammie, J.; Hathaway, S.D.)

Published July 4, 1960


BSTJ 39: 2. March 1960: Radio Frequency Interference Considerations in the TD-2 Radio Relay System. (Curtis, H.E.)

Published March 1960

BSTJ 62: 10. December 1983: The AR6A Single-Sideband Microwave Radio System: The Traveling-Wave-Tube Amplifier. (Balicki, J.F.; Cook, E.F.; Heidt, R.C.; Rutter, V.E.)


BSTJ 62: 1. January 1983: Maximum-Power and Amplitude-Equalizing Algorithms for Phase Control in Space Diversity Combining. (Karabinis, P.D.) Published January 1983

BSTJ 54: 1. January 1975: Space-Diversity Engineering. (Vigants, A.)

Published January 1975

BSTJ 44: 7. September 1965: The Triply-Folded Horn Reflector: A Compact Ground Station Antenna Design for Satellite Communiations. (Giger, A.J.; Turrin, R.H.)

Published September 1965

German microwave horn is based on this design. Musselhorn


BSTJ 42: 4. July 1963: The Autotrack System. (Cook, J.S.; Lowell, R.)

Published July 1963
Holmdel Horn tracker

BSTJ 42: 4. July 1963: The Servo System for Antenna Positioning. (Lozier, J.C.; Norton, J.A.; Iwama, M.)

Published July 1963

BSTJ 42: 4. July 1963: The Precision Tracker. (Anders, J.V.; Higgins, E.F. Jr.; Murray, J.L.; Schaefer, F.J. Jr.)

Published July 1963


BSTJ 42: 4. July 1963: Digital Equipment for the Antenna Pointing System. (Githens, J.A.; Peters, T.R.)

Published July 1963


BSTJ 38: 1. January 1959: Radio Transmission into Buildings at 35 and 150 mc. (Rice, L.P.)

Published January 1959


Published November 1952


BSTJ 38: 1. January 1959: Radio Attenuation at 11 kmc and Some Implications Affecting Relay System Engineering. (Hathaway, S.D.; Evans, H.W.)

Published January 1959

BSTJ 36: 3. May 1957: Interchannel Interference Due to Klystron Pulling. (Curtis, H.E.; Rice, S.O.)

Published May 1957

BSTJ 36: 2. March 1957: An Experimental Dual Polarization Antenna Feed for Three Radio Relay Bands. (Dawson, R.W.)

Published March 1957

BSTJ 47: 7. Septemember 1968: Microwave Radio Equipment and Building Considerations. (Skrabal, R.J.; Word, J.A.)

Published 1968-Septemember

BSTJ 38: 5. September 1959: A Network for Combining Radio Systems at 4, 6 and 11 kmc. (Harkless, Earl T.)

Published September 1959


Published November 1

BSTJ 50: 6. July-August 1971: Antenna Spacing Requirement for a Mobile Radio Base-Station Diversity. (Lee, W.C.Y. )

Published July 8, 1

BSTJ 60: 6. July-August 1981: Microwave Radio Obstruction Fading. (Vigants, A.)

Published July 8, 1

BSTJ 59: 8. Oct 1980: Horn-Reflector Antenna - Eliminating Weather-Cover Reflections. (Semplak, R.A.)

Published 1980-Oct

BSTJ 60: 8. October 1981: A New Approach to High-Capacity Digital Mobile Radio. (Henry, P.S.; Glance, B.S.)

Published October 1


BSTJ 45: 1. January 1966: The TM-1 / TL-2 Short Haul Microwave Systems. (Friis, R.W.; Jansen, J.J.; Jensen, R.M.; King, H.T.)

Published January 1966


BSTJ 50: 7. September 1971: TH-3 Microwave Radio System: Microwave Transmitter and Receiver . (Hamori, A.; Jensen, R.M.)

Published September 1


BSTJ 62: 10. December 1983: The AR6A Single-Sideband Microwave Radio System: Radio Transmitter-Receiver Units. (Heidt, R.C.; Cook, E.F.; Hecken, R.P.; Judkins, R.W.; Kiker, J.M. Jr.; Provenzano, F.J. Jr.; Wang, H.C.)

Published December 1983

BSTJ 37: 4. July 1958: Amplitude Modulation Suppression in FM Systems. (Ruthroff, C.L.)

Published July 1958

BSTJ 40: 1. January 1961: Mode-Conversion Filters. (Marcatili, E.A.)

Published January 1

BSTJ 50: 7. September 1971: TH-3 Microwave Radio System: Networks . (Drazy, E.J.; Sheehey, R.E.; Wang, H.C.)

Published September 1

BSTJ 47: 1. January 1968: An Improved Design of Waveguide Band-Rejection Filters. (Wang, H.C.)

Published January 1968

BSTJ 48: 2. February 1969: The Application of Delta Modulation to Analog-to-PCM Encoding. (Goodman, David J.)

Published February 1969


BSTJ 46: 1. January 1967: A High-Quality Waveguide Directional Filter. (Abele, T.A.)

Published January 1967

BSTJ 42: 5. September 1963: The TL Radio Relay System. (Hathaway, S.D.; Sagaser, D.D.; Word, J.A.)

Published September 1963


BSTJ 52: 5. May-June 1973: Computing Distortion in Analog FM Communication Systems. (Rainal, A.J.)

Published May 6, 1973


BSTJ 42: 4. July 1963: The Spacecraft Communications Repeater. (Davis, C.G.; Hutchison, P.T.; Witt, F.J.; Maunsell, H.I.)

Published July 1963


BSTJ 50: 7. September 1971: TH-3 Microwave Radio System: Modulators . (Giust, O.)

Published September 1

BSTJ 61: 8. October 1982: Error Probability of Partial-Response Continous-Phase Modulation withCoherent MSK-Type Receiver, Diversity, and Slow Rayleigh Fading in Gaussian Noise. (Sundberg, C.E.)

Published October 1982

BSTJ 40: 1. January 1961: Band-Splitting Filter. (Marcatili, E.A.; Bisbee, D.L.)

Published January 1

BSTJ 44: 4. April 1965: Index Reduction of FM Waves by Feedback and Power-Law Nonlinearities. (Benes, V.E.)

Published April 1965

BSTJ 50: 7. September 1971: TH-3 Microwave Radio System: Microwave Generator . (Bedell, H.R.; Judkins, R.W.; Lahlum, R.L.)

Published September 1

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