64 votes

What is this object found on transmission lines?

The are called aeolean vibration dampers, or Stockbridge dampers. They aren't rigid weights: the weights vibrate on the ends of the centre-clamped bars. You can see the wavelength that they are ...
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42 votes
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Why must some HDMI repeaters know the cable length?

Some repeaters have a model for the frequency-dependant characteristics of the cable (which varies with length), and pre-emphasize the signal in the output driver to compensate for those ...
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36 votes
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How am I getting 1 Gbps through a CAT3 cable?

100 feet is not extremely long; so, as Dave says, you've got lucky, and your Gigabit Ethernet hardware is just exceedingly robust. You have to realize that "Cat3" doesn't put an upper limit ...
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32 votes
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What is this device attached to a high-voltage power line?

These seem to be optical fibres. Figure 1. Close-up of fibres branching onto each of the lightning protection wires. Thanks to your high-res photo we can see the fibres at (1) and (2). Note that ...
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31 votes
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Why weights on cables between utility poles?

There are several modes of vibration on conductors between poles. Different devices damp different vibrations. These weights are intended to primarily dampen torsional vibration. Torsional vibration ...
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28 votes
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Physics behind signal reflections and series termination

With electrical transmission lines, it all has to do with the speed of light being finite, thus so is the speed of EM propagation in a wire. You can think of a wire as a long series of infinitesimal ...
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26 votes

Purpose of cylindrical attachments on Power Transmission towers

I'm assuming you mean these two: The official name for these is "Distribution transformer". They are the "local transformers", here's a schematic of how they could be connected: Which is from this ...
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24 votes

Why not implement 1Gbps, when all I need is 20Mbps?

TTL (single-ended, unterminated) signals can easily handle 20 Mbps or more — look at SPI, for example. If you're only going a few inches, ribbon cable and IDC connectors (or a backplane of some ...
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24 votes
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Why not implement 1Gbps, when all I need is 20Mbps?

A few reasons: Power Faster speed means more power. Not only do you need faster analog circuits, which will consumer more power, all your electronics surrounding them need to be faster. Your digital ...
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24 votes
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Why does the cable resistance jump from a low value to high value at a particular frequency?

Your tooling seems to be the cause there, not the cable. From https://www.keysight.com/main/editorial.jspx?cc=US&lc=eng&ckey=1428419&nid=-32775.536879654&id=1428419 The 4294A ...
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23 votes

Why weights on cables between utility poles?

I think it's unfinished business. I'll get to that lower down. When you have a pole-to-pole run of delicate communications cable it needs to be supported in several places from a "structural" steel ...
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21 votes

Physics behind signal reflections and series termination

The impedance of a transmission line, in ohms, is the ratio of voltage wave and current wave that travels down the line. For a 100 ohm line for instance, a 1 volt wave will always be accompanied by a ...
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20 votes

Why does the cable resistance jump from a low value to high value at a particular frequency?

Something as simple as a cable does not have discontinuities like that. There may be a clue in the fact the problem occurs at a nice round number, 5MHz. Is this a place where your test set changes ...
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17 votes
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Does current flow on an open transmission line?

Yes, there will be a brief pulse of current through the bulb as the portion of the transmission line (i.e., its capacitance) to the right of the bulb charges to the supply voltage.
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17 votes
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What are these teardrop-shaped loops in power lines for?

I always assumed it was a way to store slack in a cable like coax or fiber that can't be bent sharply without damaging it. I followed the "Snowshoe" link Michael posted, and it appears that ...
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16 votes

Does current flow on an open transmission line?

There will be a slight current pulse at switch on even if you consider the circuit a lumped element circuit, i.e. without resorting to transmission line theory. Just keep in mind that in a real ...
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16 votes
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Why does reflection only applies to transmission lines?

Reflections happen everywhere, not just in transmission lines. Transmission line is a model of the physical situation, which is easy to apply to a pair of conductors whose length is comparable to or ...
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16 votes
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Why do we need to use transmission line theory?

Why do reflections happen only when the wire is at that length? Reflections always happen, it's just that when the line is really short compared to the frequencies involved the effect of the ...
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15 votes

Why weights on cables between utility poles?

I'm pretty sure they serve a specific mechanic purpose, that nowadays is usually solved with Stockbridge dampers: They absorb the energy of mechanical oscillations in the line.
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15 votes

Purpose of cylindrical attachments on Power Transmission towers

The large cylindrical devices as indicated below are power line transformers. Their function is to step down the higher distribution AC voltage down to the common mains line voltages that feed into ...
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15 votes
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Can we reduce power consumption of digital interfaces by using high impedance transmission lines?

While, in theory, high impedances would reduce power dissipation for the same voltage swing, there are several important issues in practice. 1) It's the power, not the voltage, of a signal that ...
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14 votes

Why not implement 1Gbps, when all I need is 20Mbps?

The obvious question is, "Does 1 Gbps mean 1000BASET Ethernet?" If that's what the customer is thinking, your requirement that, "we don't have room for things like magnetics" rules that out right away....
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14 votes

Understanding data transmission rates over copper wire

Shannon's Theorem sets the ultimate limit of information bandwidth on a cable. Here's some more info about that: https://www.gaussianwaves.com/2008/04/channel-capacity/ tl; dr version: the Shannon-...
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14 votes

How am I getting 1 Gbps through a CAT3 cable?

The Ethernet speed negotiation process takes into account the capabilities of the devices at each end of the connection, but not the cable between them. Essentially, this means that if you connect two ...
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13 votes
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What is wrong with my 50 Ω grounded coplanar waveguide?

You should not use 'thermals' when grounding the SMAs. Those ground tabs should go straight onto the big unbroken ground plane. It won't even be harder to solder, the bulk of the SMA has to be heated ...
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13 votes
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Signal velocity in PCB traces

The rules of thumb given in the previous answer are good enough for many designs. But I want to add one additional thought. The velocity factor is basically going to be the inverse square root of ...
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  • 121k
13 votes
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What is the meaning of "longer electrical length = more wavelengths"?

While the document talks about more wavelengths, usually the threshold where large impedance mismatches start to matter is regarded as 1/10th of a wavelength for digital systems. Below that the ...
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12 votes

Why does the USA use 110V and UK use 230-240V?

You should not be surprised that they use different voltages and frequencies, you should be surprised that there are only two big voltage/frequency standards. When electricity was first introduced ...
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12 votes

What is this device attached to a high-voltage power line?

It may be a splicebox for optical fiber. Many power lines carry fiber inside one of the wires for telemetry and control.
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