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#121 HAM Tip: Coaxial cable; how to check out 50 Ohm impedance and some coax insights



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if your corks cable storage look like

this over the years and you are not sure

about the impedance of the corks cable

in your store then this video might be

the right one to understand how to check

out the cable impedance and a bit more

about corks cable

welcome to tear expand yeah it is really

not unusual that your storage may look

like this because all the years you

collected different kinds of corks cable

and maybe you bought some at a flea

market or such and then you need your

coax cable for a new project or whatever

and then you are really not sure what is

the impedance of our cable of the cable

we have in the storage and well today

let's talk a little bit about how to

easily determine how to find the

impedance for the cable and secondly I

mean we know that in most cases we need

50 ohm corks cable impedance and then we

have of course a 75 ohm cable more for

our broadband applications like your TV

or so and well then we have this foreign

that 50 ohm cable so I mean this is easy

to see what it is but if you use this

cable or this or maybe this you cannot

be sure if it is the right impedance and

it is not always printed on the

fire so even this year which is a so

this is really a very good brand but you

do not get an information about the

impedance I mean it is easy to look it

up on Google because it is a the IDS

code VZ it is a German brand and it is

the M key 90 so it is easy to look it up

on on Google but if there is nothing

written on the cable then you are really

lost and you might not have an idea what

is it because 50 ohm and 75 ohm are not

the only impedances which are available

with antenna cables so we have the older

norm of 60 ohm and such so sometimes it

is different and so the only idea is to

find really what the impedance is of the

cobble we want to use and the only

device you need is something like this

so this is really a very sheep

capacitance and inductance meter so you

can switch it between you see it already

here far out and yeah so this is now

over range it is clear because this will

show you the inductance so the L and of

course the C so that is what the meter

needs to be able to read and this meters

are very very cheap in the market today

two days so no problem anymore even this

year I guess it is seven years or so

it was a kit and it was for about I

don't know 40 $40 40 bucks so you know

not really expensive and two days you

can get it for 20 bucks from China and

you can buy it on the well-known Bay and

therefore no problem any longer so that

is what you basically need to determine

the Cork's cable impedance and I show

you now how it works and of course for

the wires for the Cork's cables here are

on my bench I know of course impedance

but as long we know the impedance it is

easy to prove that what we are doing is

working just fine I mean this year is I

can't read it so let me zoom it in so

this is a well known cable so this is

the rg-58 and of course you can read it

already it is 50 ohm and therefore we

know this is 50 ohm so very good for us

because then we can prove that what we

are doing will work so I simply cut here

a bit so I do not measure the length so

you have seen I've simply cut it and it

is enough to ass a piece like this so it

can be Leng longer it can be shorter so

no problem at all

so we need a little piece of the wire we

want to test and then let's see what we

can do with it

okay and the only thing I've done so far

is I've taken off the cable insulation

from both ends and that's it so far

and now our little meter here comes into

game and first of all it is switched to

capacitance and we have to zero it out

so that is what it is by now and now

very easy we connect here our wire our

coax cable to our meter all right

so that is all we do

so this end is open nothing happens to

this end and what we can read here on

our meter is 37 and I write it down

thirty-seven point two Pico farad so

that is what the capacitance of the

Cork's cable is here in this length all

right so last that is what we do second

step we flip it over to inductance test

and of course we have to zero it out

once again and we press our zero button

and now it is showing zero micro Henry

and now I just take one end and I twist

it here simply together right so I twist

it simply together that is all I have

done here nothing special and then again

we connect our meter to the wire as we

have done it before and what we read so

far is zero point one Oh Micro

which is equal to 100 nano Henry all

right so that's it so we have now this

both values and now the magic starts all

right so that is what we have so this

here is our C so the capacitance of

exactly this length of our little rg58

and this is our L all right and 0.1

microhenry is equal to 100 nano Henry so

you can use it as it is more familiar to

you so a Bowser is definitely right and

what we are looking for is our cable

impedance and our cable impedance is

that so that is known and we expecting

of course 50 ohm as it is a normal rg-58

corks cable and we know so this is a

very well known wire corks got a cable

for our radio applications and therefore

we of course expect 50 ohm and the nice

thing we can use to determine our cable

our impedance of this cable is a little

formula we have and the formula is that

we have our square root out L over C all

right so that is our formula and if we

put our measured figures here into this

formula then let's see if this works

what we have done here so

just measuring our sea and measuring our

ell and that we are able to determine

with this both figures our that is that

really possible

okay let's try so this is then equal to

square root out of 100 nano farad over

37.2 picofarad

and what now is the outcome let's see

okay and you already may see it we need

a calculator but for all of you who have

passed the ham radio examination

definitely have a little calculator so

let me bring this here into our game and

this is a calculator or maybe a more a

little bit more modern one so I really

can type in the formula as it is and

therefore I have here our square root

and I can fill in our 100 nanofarad

minus 9 so that is 100 nano farad and

divided by 37 point to pick up farad

which is -12 I hope you all can agree on

this so you see now here in the

calculator exactly what I have written

down here so maybe once again 100 nano

farad is equal 110 exponent minus 9

divided 33 point

10-12 all right so that is what it is in

what it is written in a different manner

and that is what I can really try peer

into our calculator and now when I press

the equal button here on my calculator

let's see what the outcome is Wow and

have a look so that is really what we

expected so our calculator is telling us

that it is 51.84 ohm so that is the

impedance we expected and my I mean it

is it is clear that we have some little

variations because our our little meter

here does not work 100% so our C or our

L what we measured maybe a little bit

different to what our meter was

reporting and the wires here are not

really a good so there are a lot of ways

we can simply implement an arrow into

our test setup but after all what we

really can say that we are so close to a

50 ohm that we can say yes this is a 50

ohm wire and you know that is all water

we need we need to be able to

distinguish a 75 ohm cable from a 50 ohm

because this both gorks cables are very

often looks absolutely similar because

75 ohm is used in this local network

setup so

this land coax cable has 75 ohm and as

we already have heard our corks cable

for broadcast applications like your TV

or your broker broadcast radio is using

a 75 ohm as well so that is where you

really need this most to distinguish a

75 ohm cable from a 50 ohm cable and now

let's prove that this really works

because all right we have simply taken

our rg-58 but let me do exactly the same

here with a 75 ohm wire I mean this is

white and simply from the color of the

wire you could assume that it is a 75

ohm wire but we have 50 ohm wires in

white as well so it is sometimes really

tricky and therefore once again I do not

measure the lengths I just cut it here

somewhere all right so that is what I

have let me take off the insulation here

and then let's do exactly the same okay

so let's do exactly the same what we

have done before

okay first let's test our C and it is

alright let's say C is equal to ten

point nine eight whatever pickle florid

so no problem and now let's do the same

for L and of course I've twisted it

together once again just to have here a

short

right so as we have done it before and

our meter is zero already so let's

connect it once again and we read let it

write down L is 0.06 micro Henry which

is equal to 60 nano Henry all right all

right and I've already put the figures

here into the calculator and you see of

course here or 60 nano Henry and here

our ten point nine Pico farad so that

are the values we have tested and let me

know press here our equal button on the

calculator and it's let's see what the

outcome is Wow and again so we are very

very close so seventy four point one

nine so it is clear that this wire here

is for sure a 75 ohm broadband Cox cable

and you see it is really that easy to

determine the impedance of an unknown

quarks wire

okay one more experiment will our little

equation also work for this kind of wire

all right let's check it out

okay so again we connect our C meter to

the wire so exactly the same condition

and we read nine point eight Pico farad

okay

nine point eight Pico farad and now

let's do exactly the same

to get our ell value and when we do our

ELL test we read 1.56 microhenry 1.56

microhenry

so that is what I have written down now

and now let's do our little math and

here is our mas I have already put all

values in and you see we are reading

very close to 400 but it should be a 400

in 250 ohm wire and the error is because

I have done it like this and you simply

see that I really destroy here my values

so I shouldn't do it like this I really

need to put a little bridge over it not

to influence here our values because now

the distances are wrong and yeah so we

have definitely a higher higher arrow

doing it like this all right so I have

simply improved this short here a little

bit to get it here better

yeah matched if you like and you already

see that our value is improving already

so maybe our capacity is a little bit

different to what we have measured and

you see this already is improving it a

lot so that it already gets closer so

you see even with a wire like this a

this little formula is working and so

you really can be sure that you are able

to

determine your wires

whatever you need you can do it like

this and you already may wonder why it

doesn't matter what the length is of our

yeah piece where we tested our you know

values so it really doesn't matter so

you can take it longer or shorter so

maybe if it is a bit longer you have a

more accurate reading but basically it

really doesn't matter how long it is it

I mean it is it may be dependent on your

on your tester because as you see it

really needs to go down into the nano

Henry and down into the Pico farad and

as longer the virus as bigger are this

figures so that might be have an impact

of what you are doing but basically if

your tester is very accurate and is able

to go down to the deepest values without

a huge arrow then it really doesn't

matter so the arrow we tested here on

this wire is most likely due to the

accuracy of our meter here and therefore

you know um if it would if it would have

been longer maybe our accuracy would

have been better but anyways basically

no matter how long it is it always works

and the question is why and the answer

is really as simple as it can be so this

year is the equivalent circuit diagram

of course cable all right so what you

see here is nothing else than like we

have a lot of inductance here in line

with the wire and we have all these

capacities okay

and now dependent where you're where you

cut your cable here or there or there or

there you see that the ratio of your

inductance and your capacitance is

always the same so what we are looking

here in our formula is really the ratio

and as the equivalent circuit diagram is

like this and even it is longer the

ratio of L and C is always the same no

matter how long your test piece is so

you know it is always the same ratio so

that is what you need to know and as we

already talked about corks cable let me

share some additional informations about

corks cable the graph we are looking at

here is telling us the loss versus

impedance and research has found that

this an impedance of 77 ohms we have

really the lowest loss if we compare the

different impes impedances so we see for

instance that here with lower impedance

I mean we could really do corks cable in

any desired impedance on value but

questions are always our what is the

best for all purposes and we see with 77

ohm we have really as a minimum ever

possible loss compared with other

impedances and that is what this graph

is showing us so with the lower

impedance we would have a higher loss

and with an higher impedance we have

again the same so for yeah let me say ah

for receiving devices 77m would be the

optimal impedance and now you get maybe

the first clue why we really use 75 ohm

impedance corks cable for broadcast

application where we only have to manage

receiving signals and therefore as this

graph is telling us 75 ohm is perfect

well alright so if 75 ohm is so

brilliant so why don't we use it with

our radios and now our transmitter

transmitter come into our consideration

and what you can see here is the maximum

power handling of a coax cable and what

you simply see is that with the 3000

corks cable we would be able to handle

the highest power until we have a

voltage break drop down so that means

now we have not only received we have

also transmitted and with transmit we

have a complete

different problem because as you can see

if we go along here our maximum power

you see at let me say 75 ohm we are not

really able to handle our high power all

right hmm

so now we are a little bit in a problem

as for receiver we know we should have

75 ohm because then we have the lowest

loss and that is brilliant but on the

other hand we can't use our 75 ohm

brought a cast corks cable as 75 ohm is

not really able to handle high power and

as we use a transmitter so we want to

transmit our signals right we need a

corpse cable with which is able to

handle high power but on 30 ohm where we

have really the peak on maximum power

handling we would have really a bad

receive performance so what have we done

we put our our impedance or radio

impedance somewhere around 50 ohm and

with 50 ohm so this is a kind of

compromise so we have still a high power

handling on the other hand we have still

an acceptable receive so that means loss

handling so therefore that is the reason

why our radio corks cables are 50 ohm

and why our radios themselves are all

matched to 50 ohm because for a

transmitter and receiver it is really

the best impedance to handle our

receive an hour transmitted all right so

that's it for today I hope this

information was of any value for you and

yeah if you like this video please give

me a big thumbs up and to catch you next

time bye