Impedances and transmission line effects...

[pwdrive]

Not unless your amp is a long ways away.

1/10 wave is sometimes used as the transition point for electrical transmission line modeling but for most folks like me it starts at 1/4 wave.

Lawrence.

[pwdrive]

Here's an interesting article (maybe) on transmission line reflections/ losses etc:

http://www.k6mhe.com/files/Reflect_rev1.pdf

So....Is reflected power dissipated in the transmitter or not?

Lawrence.

[ppppenguin]

Reflected power has to go somewhere. If the sending end is well matched it will indeed be disspated at the sending end. If both ends are badly matched it will bounce back and forth, gradually dissipating in the losses of the transmission line.

It gets more complicated when the terminations have significant reactance. Then as well as reflected real power you'll also have imaginary power. All those VARs still have to go somewhere. They may not be real power, as in making heat, but they are still real amps and volts that can heat up wires and cause flashovers.

[Hartley118]

Once upon a time, a pair of open telephone wires spaced 9 inches apart on ceramic insulators exhibited a characteristic impedance of 600 ohms. In other words, an infinite length of those telephone wires looked like a 600 ohm resistor. That's the net effect of the distributed parallel capacitance, series resistance and series inductance of the pair of wires. Interesting I think that those complexities can be reduced down to a resistance value.

So if a finite length of telephone wires is terminated in a 600 ohm resistor, it becomes indistinguishable from an infinite length. Therefore you get no reflection from the far end because the wires look infinite. This was an immensely important discovery in the early days of the telegraph and telephone.where engineers were plagued with echoes from the far end of lines many many miles long. However, it's a very long time since telephone wires were spaced apart like that in the open air. Nevertheless, that magic 600 ohm impedance figure remains in audio folklore for no particularly good reason.

Fortunately, audio cables, usually twisted pairs or coaxial, just look like capacitors, even up to several hundred metres long. That simplifies things, but of course the impedance of a capacitor falls as the frequency rises. That's why, as Mark makes clear, it's good to drive audio cables from a low output impedance because it avoids attenuation of high frequencies due to cable capacitance. Of course, in the real world, line drive amplifiers usually incorporate negative feedback and the capacitance of a cable slung directly on the output can cause instability, so it's common to buffer the output with a few ohms of resistance.

Interestingly, audio equipment reviewers normally ignore such interesting topics as different output impedances of preamps, CD players etc and their interaction with the capacitance of connecting cables. However, such factors can make audible differences in high frequency performance.

Martin

[Diabolical Artificer]

Thanks for the explanations and clarification. Its a bit confusing because AFAIK it's a cables capacitance not its resistance that matters with VHF signals. As AF has low ish reactance I guess it doesn't matter that my test cable collection all inherited from various places is a mix of 50, 75 and unknown. My test gear is also a mix of vintage 600 ohm and more modern high Z stuff. I made a 600 ohm terminator from two 1K2 resistors which I use rarely.

Andy.

[pwdrive]

I'm not sure about the effects of capacitance at VHF, maybe some one can explain, I always thought that the ratio of capacitance and inductance per unit length was constant as it's that which defines the cable's characteristic impedance, there's always the skin effect though.

Lawrence.

[Mike Watterson]

For coax, it's the ratio of inner diameter of braid  and the outer diameter of the core that mainly defines the characteristic impedance, though the dielectric constant of the insulation is a term too. The dielectric (and its RF losses) and absolute diameter and length affects the loss and maximum working voltage. Thus very low loss coax may be a tube for inner core and as fat as a D cell.  At RF the current flows mainly on the surfaces facing each other, the depth depending on frequency. Effectively skinny cable has higher capacitance and higher losses due to higher resistance too. One trick for Ham Radio on VHF/UHF transmitters is to put the power amp at the top of the mast as losing over half the power on input driving it doesn't mater compared with losing half the power on the output.

A RF mismatch can double the voltage on the output devices, open circuit or short, depending on length of cable to fault. So "tuning" (really matching) an HF aerial should always be done at minimum power, apart from it being annoying to listeners.

Resistive losses are significant, hence copper is better than gold and silver is good. At higher RF, the core can be hollow or copper plated steel as little or no current flows in it.

Aluminium phone wires were used for a while to save cost. They are rather poor for ADSL, which is using approximately 120 Ohm Cat 3 twisted pair designed for 3KHz max to over 1MHz, or much more for VDSL.  The 1Gbps ethernet uses Cat5e and must be fitted to patch panels and connectors carefully  and is "terminated" on the network card as it uses echo cancellation and transmits and receives simultaneously, like audio on phone lines, unlike 10Mbps that uses separate pairs. It uses all four pairs in parallel so to have 1/4 the clock rate.

[Kalee20]

At audio frequencies, it won't matter if you use 75Ω coax and 50Ω connectors, working into 600Ω load, 10kΩ load, or whatever - unless the cable is a significant fraction of a wavelength long. Which means, even for 20kHz, 2 miles-ish.

At high frequencies/long lengths, yes it is usual to match the receiving end to the cable so that no reflections occur. If this is done properly, the sending end won't matter - it could be a voltage source low source impedance) or a current source (high source impedance). Since the cable+load looks just like a pure resistor, there'll be no problems either way. It's only if the load is NOT properly matched to the cable, that a reflection will occur, in which case if the source impedance is not equal to the cable impedance, a second reflection will happen and the same signal will set off to the load, reaching it at two cable delays after the original.

Which really is just expanding on what Mark Hennesey and Gryphon have already said...

[Mark Hennessy]

The 1Gbps ethernet uses Cat5e

GB Ethernet (1000BASE-T) is fine on Cat 5. 5e not required...

1000BASE-TX uses only 2 pairs, but requires Cat 6 cables - it was a commercial flop as a result.

[Mike Watterson]

I've not seen plain Cat5 sold for maybe ten years?
Apparently a spec allowing 2Gbps to 5Gbps on Cat5e has been ratified, though people have been selling gear for a while. Good luck on getting anything individually to sustain more than 200Mbps on a home network.

I've used unbalanced audio over Cat5e, from a low impedance source though. I've used it also to extend VGA using off the shelf adaptors for a hall's Video projector, screened Cat5e for cables (installed, not loose) to mixing desk from boxes for balanced sources at stage, two x Cat5e of 15m spliced direct into a £2 HDMI 1m cable cut in half with no added hardware and to extend media centre "IR" remote for 30m.

I'm going to experiment next with using maybe 10 turns as a loop aerial, wiring the ends in series only for low frequency and parallel for high frequency.