# Tag Info

36

Coax was used for its controlled impedance, its bandwidth and its self-shielding properties. Sure, twisted-pair wiring has existed for a very long time, mostly used to carry audio frequencies in telephone wiring. That isn't where the technical advancement was required. In order to compensate for twisted-pair's lossiness and impedance issues, major ...

31

A "better" way is to send 12 V (or more) over the coax and have local regulation to 5V at each RaPi outlet. You can use cheap buck regulators available on Ebay (a few GBP or dollars) to take the 12 V DC and efficiently convert to 5 V locally. With 12 V being sent down the wire and with local switching buck regulators, the overall current down the coax is ...

29

Why can't coax, as you see on the side of the this amplifier, be decreased in dimensions? It's all down to characteristic impedance of cable: - If you plug in the numbers, to obtain a centre conductor thickness (d) that is not unfeasibly small, dimension D cannot be to low. For instance if d = 1mm then for a relative permeability of 2.2, D has to be ...

20

So you want to transport that 2.5 GHz (or even 5 GHz ?) Wifi signal over TV COAX cable ? Indeed to the non-RF people you'd just think that would work. And it does BUT there will be almost no signal coming through that cable. The Wifi signal will be attenuated so much in that COAX cable that it will defeat the whole purpose of having an antenna on the roof. ...

17

Unshielded twisted pair (CAT1 and CAT2 Phone cables) at the time (early '80) did not have enough performance, and coax (10Base2, 10Base5) did not require hubs (which were quite expensive then). When 10BaseT was ratified (CAT3), the price of hubs (and switches afterwards) went down due to mass production.

14

The problem is not with the cabling. The problem is with the relative cost of electronics versus cabling. Twisted pair existed at that time, as it had been used for phone lines for a long time. However, the idea of having a separate piece of hardware doing switching in the middle, using the star topology used today for twisted pair, would add cost and ...

14

Yes, this should work fine. Even in its original application, the coax was probably carrying a DC bias to power the LNB on the satellite dish. Assuming it's RG-6 cable, it should be good for at least 3 amps. It's considered conventional to use the shield as ground and the core as +V, though it doesn't particularly matter from a functional point of view.

14

Air lines still exist, with velocity factor very close to 1.0. These are AFAIK mainly used in old-fashioned VSWR measurements. The advantages are that the dielectric constant of air is fairly stable and well-known, and that you can insert a probe (a tiny antenna) into the middle of the transmission line without damaging the dielectric. ePTFE (aka "Teflon ...

13

The R&S instruments are equipped with female test port adapters. Edit: archived pdf You can replace the N type with a more suitable type for higher frequencies.

13

Because the goals are not the same, you are basically comparing a lawn mower with an attack helicopter. IC and components in general have reduced in size due to improvements in the manufacturing processes and technique allowing to make smaller components and improve encumbering or power consumption. However, SMA cables or the bias-tee you show are not ...

12

The thing about coaxial cable is .. it's coaxial. That means there is a core conductor running inside a shield. The shield is usually connected to ground. The filler material inside the cable keeps the core a specific distance from the shield, and with a specific dielectric constant, k. That causes the cable to have a specific, low, well-defined impedance ...

11

RG-6 coax has a 18-gauge center conductor, and exhibits a resistance of 6.385mOhm per foot. Its insulation is suitable for a max of around 10A, but it may get too hot before that. Typical satellite TV operates at voltages between 13v and 18v. At 18v, a 200ft long RG6 will allow up to ~14A. However at 10A, the voltage drop is about 7.8v, so your mileage will ...

10

Soldering directly to the pads of the SMA connector instead of using SMA connectors should work fine electrically. The obvious problem is that the cable is no longer removable. If you're fine with that, then go ahead. It would be good to have some mechanical strain relief if this has a chance of getting flexed. At only 2 inches wavelength, you do need to ...

10

Besides the impedance mentioned in other answers: Because they don't need to, or in other words there is not much market demand. I am mostly referring to items like the one you showed an image of. They are mostly (if for some not exclusively) found in lab or prototyping environments where quality and serviceability is valued more than size. And if you ...

10

how does my signal source see the whole cable? The characteristic impedance ($Z_0$) of any transmission line be it coax or twisted pair (screened or unscreened) is determined by: - $$Z_0 = \sqrt{\dfrac{R + j\omega L}{G + j\omega C}}$$ Where R, L, G and C are the resistance, inductance, conductance and capacitance of the actual line per unit length. ...

8

You must use coaxial cable of the proper impedance. The most common impedance for coax cable is 50 ohms or 75 ohms. If the cable you want to use matches the impedance of the interface AND the antenna, then go for it. But if you use cable of the wrong impedance you will get significant attenuation of the signal to the point where it may not work at all. In ...

8

If it looks anything like the one in this image, it is likely just for historical reasons. At least here in Belguim, they are pretty much always just wired in parallel internally. When everything worked with antennas, you would be able to use a different antenna (and perhaps cable, see note) for the FM radio band than the TV band. The connector for radio ...

8

Energy moves through the cable in both directions simultaneously. Just as different video signals are modulated on different channel frequencies, incoming and outgoing data streams are modulated on different carrier frequencies, and pass each other without interference.

7

The coax cable shielded the signal line between the antenna and the receiving circuit, which is how it reduces extraneous pickup. Note that this does nothing about extraneous signals picked up by the antenna itself. The reason it attenuated signals from the "antenna" is because it adds some capacitance from the signal line to ground. It looks like what ...

7

"Bunny-ears" antenna is a non-technical term for a dipole antenna. At its resonant frequency, a dipole has about 70 ohms impedance at its feed-point. To avoid signal loss and reflections, a matching transformer is needed. This simple bunny-ear dipole should include a 300-ohm-to-75-ohm balun inside, at its feedpoint, if its feedline output is 300-ohm balanced ...

7

The high propagation velocity would be a "side effect" of more desirable property - signal loss/dissipation along a cable. Losses are dependent on properties of isolation material. If you already have the best low-loss material, the next thing is to make it porous, so it will mostly contain dry air. The higher propagation velocity is a side effect of this.

7

You're off by one. Normal coaxial BNC connectors have two conductors: shield (often ground) and signal (or force). The points you have labelled as 1 and 2 are both part of the shield contact -- they both touch opposite sides of the shield on the female BNC connector -- and the point labelled 3 is the signal contact. Triaxial BNC connectors have three ...

6

Unshielded Twisted Pair has existed for as long as phone lines have existed, but it was originally designed to reject noise at audio frequencies. It was not designed with the correct number of twists to reject noise at the high speed digital frequencies of Ethernet. Coaxial cable, however, had long been in use for various RF applications, and its ...

6

Imagine, for a minute that electricity travels quite slowly. When you turn your light switch on what happens? Current starts to flow - it starts working its way down the wire and so does the voltage. The current that flows is determined by two things: - The voltage and An "impression" of what the load resistance might be. Will the current be too small or ...

6

The ratio of inner to outer diameter is set by the desired characteristic impedance and the materials used. For low loss low reflection behaviour you want to tightly control that ratio. You can make coax smaller but it gets harder to tightly control the size ratio, the loss per meter of the cable gets higher due to higher resistance and the hardware gets ...

6

You can easily use 0.81mm diameter coax but it's pretty lossy (3dB/m). Compare with RF-9913 at less than 0.2dB/m, but more like 10mm in diameter. Inside a compact device like a laptop or a wireless router, a few cm of lossy cable is not a problem, but for a larger setup the performance hit is too much. We also use BNC connectors and banana plugs/jacks ...

6

The splitter in your link is for distributing RF cable TV signals - it will seriously attenuate your DC power, if it passes DC at all. If you are only using the coax to distribute DC (or low voltage AC), you can join several coax cables as if they were simple two-conductor cables. There is no need to worry about impedance matching and other RF ...

5

I haven't seen that particular style of adapter but if you don't have any luck tracking one down a solution I use personally is a pair of BNC receptacle to banana plug adapters along with a BNC to banana socket adapter. They are fairly widely available in a variety of styles such as the following: Those particular parts are a Pomona 4435175 and Pomona 1894 -...

5

Noise in a cable shield may or may not be common mode. The idea that it is common mode comes from three-conductor cables, in which there is a shield wrapped around a pair of conductors that carry a differential signal. The signal is taken to be the difference between the conductors. Any potential between the two inner conductors and the shield is common ...

5

I am aware that more length = more resistance so I am wondering if I am good with slightly increasing the voltage on the source side and measuring on the output until I have an exact 5V. NONONONONO! (Did I say "no"?) Current draw is very not constant with electronics. In normal running, you might have enough current to get exactly 5V, dropping 1V on the ...

Only top voted, non community-wiki answers of a minimum length are eligible