AE6PM 11/06/2007 1

Heard at Pacificon2001-2007

Technical tidbits from selected presentations

AE6PM 11/06/2007 2

Steve Stearns, K6OIKPacificon 2001

Coaxial cable characteristic impedance

• Zo = 50Ω has no special significance

• For minimum loss Zo = 77Ω

• For maximum breakdown voltage Zo = 30Ω

• For minimum temperature rise Zo = 60Ω

AE6PM 11/06/2007 3

Steve Stearns, K6OIKPacificon 2001

Round Open-Wire Transmission Line Impedance Formulas:

• Approximate, widely published, but accurate only for large spacings: s/d>3.

• Z0 = 120 ln(2s/d) = 276 log10(2s/d)

• Exact, accurate for all spacings & impedances• Z0 = 119.917 cosh-1(s/d)

AE6PM 11/06/2007 4

Comment

Yet another equation:

Z0 = 276 log10(s/d+ sqrt((s/d)2 – 1)

d = conductor diameter

S = center-to-center spacing

AE6PM 11/06/2007 5

Steve Stearns, K6OIKPacificon 2006

• 1/2λ transmission line repeats the impedance of the load• Zi = ZL

• 1/4λ transmission line repeats the reciprocal of the load Z• Zi = Zo

2/ZL

AE6PM 11/06/2007 6

Steve Stearns, K6OIKPacificon 2006

• 1/8λ transmission line always shows the impedance of the transmission line• XL = Zo tan l for a short-circuited line less than

1/8λ in length (l)• 1/8λ = 45o and tan 45o = 1

The impedance at the antenna is almost never what is seen at the input end of the transmission line.

- AE6PM -

AE6PM 11/06/2007 7

Steve Stearns, K6OIKPacificon 2004/2005

• For an exact half-wave dipole, l = λ/2:• ZA = 73.08 + j41.52• Independent of wire diameter

• For a resonant dipole, l < λ/2:• ZA = 73.08 + j0 • Depends on wire diameter

• Resonance is just an impedance property• Maximum gain for a dipole is 5.18 dbi at a

length of 1.269λ

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Steve Stearns, K6OIKPacificon 2006

• QEX Sep/Oct 2001 is not the way to match.• Forget conjugate match (which assures

maximum power transfer).• The important thing is to prevent

reflections (which cause waveform distortion and degrade information transfer).

AE6PM 11/06/2007 9

Comment

• The QEX article in question is evidently:

“A Flat Impedance Bandwidth for any Antenna”

by Grant Bingeman, KM5KG

AE6PM 11/06/2007 10

Comment

• So which is the best?– Conjugate match (Maxwell)

• Maximum power transfer

– Maximum SWR bandwidth (Bingeman)• Maximum operating convenience

– Minimum waveform distortion (Stearns)• Maximum fidelity for digital modes

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Steve Stearns, K6OIKPacificon 2006

• A Dipole antenna differentiates the signal once

• A Loop antenna differentiates the signal twice

• Digital TV will require a discone antenna or a bow tie antenna or a ?

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Comment

• Conversation with Steve Stearn 10/19/07:– Yagi and log periodic antennas

introduce phase distortion.– Bow tie and discone antennas do not

introduce phase distortion.– Most digital modes are intolerant of

signal distortion.

AE6PM 11/06/2007 13

Tom Schiller, N6BTPacificon 2006

• Round elements are more efficient than square or rectangular.

• Solid wire for radials is more efficient than stranded.

• Stainless steel is a very poor conductor (high resistivity).

• Galvanized wire is a very poor conductor (coating too thin: skin effect).

AE6PM 11/06/2007 14

Tom Schiller, N6BTPacificon 2006

• Maybe 12 dB improvement by silver plating stainless steel.

• Copper clad, not copper coated or copper plated (again, due to skin effect).

AE6PM 11/06/2007 15

Comment

= (2/)Where: = skin depth = resistivity of conductor = angular frequency of current = absolute magnetic permeability of conductor

Copper @ 1 MHz = 2.84 milsCopper @ 10 MHz = 0.897 milsCopper @ 30 MHz = 0.518 milsZinc = ? Steel = ? Aluminum = ?

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Steve Stearns, K6OIKPacificon 2007

RLdb = -20 log ||

Errors correcting errors …. stuff happens:• Return Loss in decibels is 20 times the logarithm

of the Reflection Coefficient– WorldRadio, January 2007 p52 (wrong)

• Return Loss in decibels is 20 times the logarithm of the Voltage Reflection Coefficient– Correction in WorldRadio, March 2007 p52 (wrong)

• Return Loss is 20 times the reciprocal of the Reflection Coefficient– Correction in WorldRadio, June 2007 p52 (wrong)

AE6PM 11/06/2007 17

Comment

Errors correcting errors ….• The correct answer is:

RLdb = -20 log ||

Where = reflection coefficient

= Er/Ef = Ir/If

Ref: The ARRL Antenna Book, 21st Edition, p24-8

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Articles by Steve Stearns

• “Mysteries of the Smith Chart: Transmission Lines, Impedance Matching, and Little Known Facts”

• “Antenna Impedance Models – Old and New”• “New Results on Antenna Impedance Models

and Matching”

http://www.fars.k6ya.org/docs/k6oik

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Articles by Jim Brown, K9YC

• “Understanding and Eliminating RF Interference”

• “Transmitting Chokes”• “RFI, Ferrites, and Common Mode Chokes for

Hams”• “Measured Data For HF Ferrite Chokes”• ….. and many more

http://www.audiosystemsgroup.com/publish.htm

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K6OIK’s Broadband Equivalent Circuit

98.4’ 1/2 @ 5 MHz, thin wire dipole resonant at 4.8 MHz.

See Pacificon presentation chart for component values.

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Accuracy of K6OIK’s Broadband Equivalent Circuit