coax for interconnects

[Lawrence001]

Just out of interest, ohms measure resistance (or impedance), what 2 points on a 75ohm cable, or connector, is the value measured between?

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[The Masked Avenger]

It’s not a balanced cable, it interested me that so many people on the diy audio forum advocated the use of coax rather than mic or twisted pair types of cable for interconnects.

There is, of course, the pairing in a shield commonly known as a "balanced co-axial".

[Firebottle]

Just out of interest, ohms measure resistance (or impedance), what 2 points on a 75ohm cable, or connector, is the value measured between?

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The impedance of a 75 or 50 ohm cable can only be measured at radio frequencies, it does not equate to a resistance that can be measured with a multimeter or at audio frequencies.
It is also a 'distributed' impedance, between the centre conductor and the outer shield. The impedance will measure the same wherever it is measured along the cable, this is the whole point of a constant impedance cable so that RF signals don't meet any discontinuity in impedance.

Discontinuities will cause mismatch and reflections resulting in a high Voltage Standing Wave Ratio (VSWR). In extreme cases this high VSWR can cause damage to RF equipment.

[goraman]

The impedance of a 75 or 50 ohm cable can only be measured at radio frequencies, it does not equate to a resistance that can be measured with a multimeter or at audio frequencies.
It is also a 'distributed' impedance, between the centre conductor and the outer shield. The impedance will measure the same wherever it is measured along the cable, this is the whole point of a constant impedance cable so that RF signals don't meet any discontinuity in impedance.

Discontinuities will cause mismatch and reflections resulting in a high Voltage Standing Wave Ratio (VSWR). In extreme cases this high VSWR can cause damage to RF equipment.

Absolutely correct and it's very important in microwave Communications. Transmitting in the gigahertz is a whole different ballgame than HF / MHZ.

[Firebottle]

Ever played with 10GHz stuff Jeff?
I used to do amateur TV up to 10.5GHz, built a repeater station, converted LNBs to transmitters etc.

[Light Dependant Resistor]

The impedance of a 75 or 50 ohm cable can only be measured at radio frequencies, it does not equate to a resistance that can be measured with a multimeter or at audio frequencies.
It is also a 'distributed' impedance, between the centre conductor and the outer shield. The impedance will measure the same wherever it is measured along the cable, this is the whole point of a constant impedance cable so that RF signals don't meet any discontinuity in impedance.

Discontinuities will cause mismatch and reflections resulting in a high Voltage Standing Wave Ratio (VSWR). In extreme cases this high VSWR can cause damage to RF equipment.

However , https://theartofsound.net/forum/show...speaker-hookup
Cyril Batemans article indicates speaker cables do comply with established transmission line behavior.

Regarding measuring - rather than cable impedance that is indeed difficult with a multimeter, I recall measuring what appeared as actual antenna impedance with a multimeter, I recall measuring close to 30 ohms of a monopole antenna atop a roof, using nothing more than a multimeter.

[Barry]

The concept of characteristic impedance and the treatment of cables as transmission lines only applies when the length of the cable is much greater than the wavelength of the signal/(2*pi). For most insulating materials the signal propagation velocity in the cable will be ~ 2 10⁸ms^-1. Thus at 20kHz, the cables would have to be longer than 10km for transmission line theory to apply.

It is impossible to measure the characteristic impedance of a cable with a simple multimeter.

Anyway we are not interested in matching amplifier to speaker, in the sense of desiring maximum power transfer as one does at VHF and microwave frequencies. Amplifiers are effectively voltage sources and have a very low source impedance. It is the ratio of source impedance to speaker load impedance that allows the amplifier to dampen the back emf generated by the speaker. (i.e. to prevent the 'tail wagging the dog'). The loop resistance of the speaker cables appears in series with the amplifier's output impedance, so need to be kept as low as possible so as not to degrade the damping factor.

[Light Dependant Resistor]

Hi Barry
Did you take a look at the Cyril Bateman article ?
http://web.archive.org/web/200910270...wnloads_5.html

"Many writers claim that at audible frequencies, our loudspeaker cable cannot act as a transmission line
because it is much too “short”. This view became established some years ago following the widely
published and much copied paper by R. A. Greiner, which simply stated some facts about cable lengths
etc., without going into any form of proof. Sadly his basic calculations/assumptions were some 600% in
error, mostly due to the slowing down affect of the cable’s insulation. He also assumed reflections would
only occur when a cable was one wavelength long, when in fact the most troublesome reflections occur
at odd one quarter wavelengths and with cables very much shorter than one quarter wavelength long."

And we are all agreeing repeated a few times that a multimeter cannot measure cable impedance,

however as I found when loaded by some types of antenna, a resistance measurement approximating
the expected characteristic impedance of that antenna ( vs cable) appears, using nothing more than
a multimeter. Multimeter was a Fluke 12. As I say I was atop a roof, but noted it at the time some 30 years ago.
antenna was a monopole, where just over 30 ohms was expected.

[Alan D]

Ever played with 10GHz stuff Jeff?
I used to do amateur TV up to 10.5GHz, built a repeater station, converted LNBs to transmitters etc.

I built the Practical Wireless 10GHz Wideband transceiver around 1986

http://www.microwave-museum.org/exhibits/mwm-003.htm

73 de G4PSU

[Barry]

I have read many of Cyril Bateman's articles in Wireless World magazine (well actually Wireless World and Electronics magazine), when it was editorial policy to allow more speculative articles that questioned established thinking. Some of these were thought-provoking, such as Ivor Catt's maverick questioning of the need for displacement current to complete Maxwell's equations, and the motion of charge on the electrodes of a capacitor as it is charged. So this lateral and open thinking was indeed refreshing, but it also permitted some 'voodoo science' to be promulgated. Sadly a few articles on the propagation of audio signals in cables fell into this category.

Much of Bateman's work is well done (the measurements especially), but his theoretical thinking is muddled, and his maths incorrect; as pointed out in subsequent letters to the magazine. His work on the possibility that speaker cables can act as very inefficient antennae to RF is interesting, and the idea that the use of screened cable to counter this effect may have influenced his thinking that speaker cables should be thought of as transmission lines at audio frequencies.

But to show that transmission line theory can be applied at audio frequencies, it was noted in the '30s that some mains distribution networks were being overloaded but with no obvious cause. It was eventually found that some unterminated branches of the network were approaching a quarter wavelength long (which at 50Hz, is approximately 75km long), and the open circuit at the end was being transformed to a very low load impedance at the junction.