Splain me why a 75 ohm rg6 is rated that way? How can a rg59 that is smaller guage also be rated at 75 ohm? Why is a rg58 50 ohm? How much would an antenna that should be 50 ohm with BNC connectors be effected if it were connected to RG 6?

this is the impedace not resistance of the cable, also the connectors also have an impedance.
the impedance is to do with the constuction and the matrials that make up the cable.

as a general rule recivers have 75ohm cable and transerver have 50ohm cable.. but this is not always the case.

Using 75 Ohm cable when 50 Ohm is needed can mean the difference between working correctly and not working well, if at all. If you have seen Cell phone boosters, the low power ones often use RG6 and the high power ones use RG8- the frequency range and power level demand more separation between the center conductor and shield. Reducing this distance reduces the ability of the shield to work and reduces the upper limit of the frequency range that is critical to the operation of the equipment.

RG59 is rated at 75 ohms as is RG6. Are you thinking of RG58? Usually devices that transmit power use 50 ohm impedance cables. Using 75 ohm cable can cause standing waves and power transmission loss.

As mentioned the impedance is decided by the cable construction. Every cable is made up of capacitance (C), inductance (L) and resistance. At high frequencies the resistance can be ignored and the output impedance becomes sqrt (L/C). For a coax cable an easier way to compute it is using (138 / sqrt(e)) * log (D/d) where D is the diameter of the shield and d is the diameter of the center cable. By changing one or the other you can get whatever impedance you like. Probably more than you wanted to know but you asked :).

It is really not a good idea to mix and match cables this way but in short distances it will probably work.

The reason that you don't mix impedance is to prevent signal reflections. When the impedance levels are matched you have a maximum transfer of signal.

This is all about transmission lines, not resistance. A transmission line, for us. is a kind of cable where the combination of capacitance per foot and inductance per foot combine with the (small) resistance of the cable to end up being a certain impedance.

The cables are designed to have certain impedances so that they can transfer as much energy as possible along the transmission line. Some antennas natively look like a 50 ohm impedance, so the cable for them is made to match. TV antennas have a native impedance of 300 ohms, but flat 300 ohm cable also picks up signals all the way down to the TV, so a nice 4:1 transformer converts the balanced 300 ohm signal to an unbalanced 75 ohm signal. Move ahead from the invention of that 75 ohm cable: lots of things are now designed to have 75 ohm impedance because there's so much cable that will work with it.

And yes, RG59, RG6, and RG11 all are 75 ohm cables. The smaller cables are supple and easy to work with, but they don't transfer really high frequencies as well as the larger cables. The RG11 is horrible to work with but has incredible high frequency response. RG6 is in the middle there somewhere.

There's more info on transmission lines at [Link: thebigger.com].

Herer are a couple of drawings showing how resistance, capacitance, and inductance are spread over a transmission line. The second one is a simplification of the first.





Now that I've looked at it, it looks really complicated. I'm reminded of the book "Bad Science," where the author explains some things and says that if he had to create a T shirt to symbolize the way science should be, but is not, applied to modern issues, the T shirt would say "Actually, it's a bit more complicated than that...."

IIRC, the impedance of the cable is dictated by the construction of the dielectric. I.e., the distance between the center conductor and the outer shield as well as the material between the two.

CJ

The reason that different sizes of coax, can be the same impedance, is that the manufacturer also changes the dielectric material (The foam or gas) in the cable when they do so, in a way that changes the impedance to the wanted value. For example, some very large diameter "hardlines" which look like armored BX cable, are actually filled with nitrogen. This results in a very low loss cable, as there is very little leakage from the center conductor to the shield. The foam in the RG6 cable is actually a combination of air bubbles and usually PTFE. By controlling the size of the bubbles that form, the dielectric constant can be influenced to result in one that gives the impedance wanted.

Actually all the answers are in Maxwell's Equations:


Of course the problem is that using the equations involves integral and differential calculus, and gets really messy for even good mathematicians. I took three transmission theory classes in college where almost everything was based on those equations, and most of the class was about how to decide how to simplify your problems enough to be able to do the math, while still getting accurate answers.

Now you know why I have so little hair! It's all been pulled out solving partial differential equations. Who knew that 4 simple to memorize little equations could cause me to tear my hair out for about 1.5 years!


This sounds like a simple question, but like many of the great questions in science, the answer is really messy!

The answer is that the effect will be different at different frequencies.  When you connect two pieces of transmission line that vary in impedance, you will cause a discontinuity, and reflections begin to occur.  This is similar to feedback in an audio system.  What occurs is that the ability to effectively transfer power is affected.

For example you may have seen instances with Amateur Radio or CB equipment where you need to "tune" the antenna.  This is usually done by changing the electrical length of the antenna using a capacitor.  What you are doing is changing the impedance so that it closely matches the impedance of the cable, so that these reflections (standing waves) are reduced.  The capacitor adds an impedance that allows for getting the impedance correct at a narrow range of frequencies, but messes up the impedance at frequencies outside that range. 

noot98, bcf's incredible answer shows why you don't understand the idea, and illustrates just how far it is from reasoning that makes sense to us.

bcf, thank you so much. You have just provided relief to me about a question I've had for at least thirty years, but never got into the math enough to do more than make similes or metaphors. I actually feel physical relief after reading your answer!



This is really important for everyone to know about. As I've said, it's really really helpful to know much more about a thing than is required to solve the problem at the moment. Such knowledge gives you the ability to solve totally new problems. Remember this; some day it will help you out.


Oh. I see. I get it. NOT!


I had a really great career going in Physics in college until I hit Differential Equations. I was stopped cold by the math.  The part I italicized is the part I could not get -- why would you teach a class where you don't have the students understand a damn thing, but require them to be able to work out how to transmogrify formulas so the formulas look like ones  given in the book?  It's like teaching a language using only half-filled out crossword puzzles!


I totally understand! I never could get it.  I had absorbed German, and my kind of understanding of systems made Linguistics a natural change of major.  So now I have a degree in Linguistics, and lots of hair.


Like I said about the T shirt.


That's a nice simple explanation. Thanks for that.


...and that's why we don't tune any of the antennas we use: they are either meant to be used over a wide range of frequencies or they are cut to the correct size to tune them physically.

Thanks again.

Actually, the professor teaching the class was excellent, but it was quite entertaining at times. He had a very heavy Russian accent, so often the whole class would listen intently, and ask very strange questions, then someone would pipe up in the room, and say a single word that had been butchered by his pronunciation, and the whole room would go Owwwww? For example, we kept hearing "registor", and eventually someone said, "resistor", and everything became clear.

Anyway, the professor was very good about getting to the meaning behind the equations. The equations are all about how electric and magnetic fields and waves are physically related, and oriented to each other. To this day, I often will write the equations and figure out how they affect a quantity, because I need to have an idea of how the fields are related, but don't need a numerical answer. So I think the courses were a big help, in that I did end up understanding. The math is less of an obstacle today, as much of these courses is now taught using mathematical software, which lets the engineers worry less about solving the math, and more about understanding how to use the equations, and what they mean.

I do break out software to solve such problems on occassion, but most of the time I simply get a general feel for how things are altered, using the math and some rudimentary short cuts, and then build something up quickly in the lab, using my calculations, and do some small alterations until I get the results I want.

Perfect short post that is exactly the answer./

Well, yes, just as "why does a bus take a route that is ten miles round trip?" is answered by "because if you add up the individual bits of street route, that's how far it has gone." This says nothing about how the bus route is designed to serve the public in the best possible way, and that the route was made longer here and there to include more customers, etc."

Here are some other possible answers:


Because the units we use are ohms, and it's 75 of them. At particular frequencies we usually call RF.


Because it also is.


Because that's what it was designed to be.


The antenna would be just fine. The signal, however, whether transmitting or receiving, would be attenuated in comparison to a setup with a 50 ohm cable and 50 ohm connectors. When all impedances match, all the power inserted at one end goes out the other end. Wherever there is a mismatch of impedances, some of the energy reaching that point is reflected back along the cable. Thus, not all the energy that goes in comes out. Worse, that energy then reflects back from the source to the output end, diminished and later in time than the original, making an echo.

Would it have been a help to you to know this back in analog TV days? To know that a TV image to the right of the main image is caused by such reflections, either of signals in the air, or of signals in the cable? I think so -- that knowledge helped me work out several problems very rapidly. This is why I don't want to limit these discussions to the simplest answer. Like that answer about the bus route: it's true and it's simple. But actually, it's a little more complicated than that.

Looking confused on the protocol for the forum Ernie. You are saying folks should not ask questions if they don't already understand the answer??? And that Maxwell level equations should be presented that you didn't understand but admired anyway?

Yes his post was excellent and good info. But the answer was given five different ways and so this kind of commentary putting down OP and all the others who offered answers doesn't seem to be in good form :).

Thanks Amirm,
This guy is so cryptic i didn't even realize he was making a off handed remark.