Electrical

[Back Forty Bob.]

Measuring the resistance on an RG8X cable.
Using Fluke R115 meter. Meter reads .2ohms.
It should read about 50 ohms + / -
Scale is set on the lower range. Not K or M.
What am I doing wrong?
Cable is known to be good.
Thanks Bob.

 

[Ralph, Ohio]

Your RG8 cable is rated at 50 ohms IMPEDANCE. Impedance is different from resistance even though both are stated in ohms. Impedance can not be measured with your
meter.

 

[Janicholson]

It is a solid core wire, it will read very low resistance in pure resistance value. The shield and core at the radial distance will produce the rated
resistance it is labeled at the frequencies being transferred through it. DC power is often used in those to power an amplifier, or antenna rotator,
(those DC voltages see the Very low resistance). Jim

 

[kcm.MN]

Bob, maybe this article will help:
http://cablesondemandblog.com/wordp...ence-between-50-ohm-and-75-ohm-coaxial-cable/

In short, what you're doing is measuring resistance as if the coax were going to be conducting electricity. And while coax does conduct small amounts of electricity, the 'Ohms' rating given to the cable is more in reference to Impedance.

It's sad, I used to know this stuff pretty well. Even what I wrote I had to now look up. Anyway, as mentioned, you cannot measure the Ohms of coax cable using a multimeter. Just doesn't work that way.

 

[Back Forty Bob.]

Thank you for you help. I need to get my books out again. Resistance is DC impedance is AC.
Radio waves are AC. So by testing with my meter all I am doing is checking continuity.
Thanks again everybody!

 

[MarkB_MI]

Cable impedance has nothing to do with the
resistance of its center conductor.
Impedance is measured between the center
conductor and shield at a high frequency,
typically several megahertz. And it's
done against a length of cable long enough
to be considered infinite.

Most folks don't have the instruments
necessary to measure cable impedance.

 

[Ralph, Ohio]

If you want to verify that the coaxial cable is in good condition you can measure the end-to-end resistance of both the center conductor & the shield. Depending on length the resistance will probably be fractions of an ohm. This verifies that neither conductor is broken. You can also measure resistance between center conductor & shield to be sure the cable is not shorted. This resistance should probably be in million of ohms or megohms. If the copper conductors are green at either end the cable has probably been wet & is no longer top quality but may be usable.

 

[Geo-TH,In]

Mark,
I think Doctors have a pill for male impedance.
Geo

 

[MarkB_MI]

FWIW, cables are usually tested in the
field with a <a href="https://en.m.wikipedia.org/wiki/Time-domain_reflectometer">time domain reflectometer</a>.

 

[David G]

The impedance is to RF at certain frequency ranges, should read 0 ohms end to end and infinite conductor to shield.

 

[David G]

Geo, that is good.

 

[MarkB_MI]

> I think Doctors have a pill for male impedance.

Yes, it can help with transmission line faults.

 

[John T]

Theres a huge difference in Impedance and Resistance, it seems easy now but way back in the sixties at good old Purdue University in EE school it was Greek at first lol

John T

 

[tech4]

Your meter is reading the DC resistance of the cable. I believe the center conductor of RG 8X is #14 and you are reading the DC resistance of the center conductor. The shield surrounds the center conductor and the center conductor insulation and spacing dielectric determine the RF (Radio Frequency) impedance of the cable. Most commercial and amateur radio uses 50 ohm coax while most TV and video
services use 75 ohm coax such as RG6

 

[Geo-TH,In]

John T

Isn't Z = to the square root of XL squared - XC squared + R squared
Which was very easy to do on a slide rule back in the day. Right?
George

 

[JDEM]

RG8-X is supposed to have around 3 to 4 ohms resistance per 1000 feet. If you read .2 ohms, that is a proper reading for 20-30 feet of
cable. Uses a 15 gauge copper wire conductor in the core.

 

[JDEM]

I suppose you are doing a resistance test to diagnose a problem? Like I said, each conductor should read 3 to 4 ohms per 1000 feet of cable.

Your .2 ohm reading is correct for 25-40 feet of cable. Same reading for either the center conductor or the external shield. There should be
an infinity resistance reading when testing between the center conductor and shield. Again shield or center conductor should have around
.004 of an ohm per foot.

 

[MarkB_MI]

> Again shield or center conductor should have around .004 of an ohm per foot.

True, but it's pretty much impossible to accurately measure resistances less than an ohm unless your meter is capable of four-wire measurements. Once you're down to an ohm, that's usually good 'nuff.

 

[John T]

Bob, you ask "What am I doing wrong" my guess is you're measuring or confusing DC Resistance versus AC Impedance, see below"


Here are "Electrical Characteristics" taken from a Belden Data Sheet for RG-8X Cable:

ELECTRICAL CHARACTERISTICS

Nom. Characteristic Impedance 50 Ohms
Nom. Inductance .065 ?H/ft
Nom. Capacitance Conductor to Shield 24.8 pF/ft
Nominal Velocity of Propagation 82 %
Nominal Delay 1.2 ns/ft
Nom. Conductor DC Resistance @ 20 Deg. C 4.3 Ohms/1000 ft
Nominal Outer Shield DC Resistance @ 20?C 3.3 Ohms/1000 ft


NOTE "AC IMPEDANCE" isn't the same as "DC RESISTANCE"

You're not going to measure 50 Ohms "IMPEDANCE" when measuring DC RESISTANCE

Sooooooooooooo measure the length in feet of the piece of cable you're testing, use the Belden specification above (beware, different brand of cable may have different specs) and if there's supposed to be 4.3 Ohms per 1000 feet (That's for Belden) do the math to see how many Ohms you actually have and see if it matches ???? NOTE it takes a good meter to measure extreme low resistance so don't be surprised if your readings aren't same as above

DISCLAIMER No Warranty, its been over 50 years ago since I studied this in Electrical Engineering School at Purdue University and I'm rusty as an old nail BUT STILL UNDERSTAND "AC IMPEDANCE" IS NOTTTTTTTTTTT THE SAME AS "DC RESISTANCE"

Best wishes hope this helps

John T

 

[Indiana Ken]

(quoted from post at 18:09:23 09/28/19) John T

Isn't Z = to the square root of XL squared - XC squared + R squared
Which was very easy to do on a slide rule back in the day. Right?
George

My slide rule did not have "+" or "-" scales. It always amused me that the best thing about engineering calculators was that they included addition and subtraction.

 

[MarkB_MI]

It might be worthwhile to explain why cable impedance even matters. If the impedance of everything in a high frequency transmission line is matched, then the power delivered through it is maximized. This includes the output impedance of the source, input impedance of the load, and all the cables and connectors in between. If there is a mismatch anywhere, energy gets reflected back to the source. This could happen if there's a pinch in the cable, for example, which would change the capacitance at that particular point in the line.

The Wikipedia article I linked to in an earlier post has some good examples of impedance mismatches.

 

[JDEM]

There is an awful lot or technical jargon here not related to your resistance test. I will take you for your work and you ARE checking actual
resistance. Easy test and a good test when a shorted coax or a broken coax is suspected.

 

[John T]

Bob, if as you state "Cable is known to be good" and if the DC resistance is in line with the Belden specs I just posted below, and there's no visible breaks kinks cracks or penetrations, I guess you could have it tested for AC Impedance in the event its not performing satisfactory or the losses are too great, but that takes some serious equipment much more then simple DC resistance you're talking about. Remember 50 Ohms AC Impedance IS NOT the same as the DC Resistance you are measuring.

Again for your convenience here are the Electrical Characteristics

Nom. Characteristic Impedance 50 Ohms
Nom. Conductor DC Resistance @ 20 Deg. C 4.3 Ohms/1000 ft
Nominal Outer Shield DC Resistance @ 20?C 3.3 Ohms/1000 ft

Let us know what you find

John T

 

[oldsarge]

Keesler Afb,Basic Electronics School,1961

 

[Back Forty Bob.]

Wow it is cool to hang out with smart guys! What started all this is having problems
with my radio in my truck. had an antenna stud strip out then found bad coax and first spare was bad.
All of those went into the dumpster. This is a nice 6 footer that i used before I erroneously
believed i had to use an 18' coax. Radio is a Stryker 497HPC with a predator dual coil kick axx
antenna. Radio puts out about 135 watts. Wanted to be sure this coax is good. If I burn up this radio
I doubt the XYL would let me buy a $700 ranger.

Thanks again guys!
Bob.

 

[MarkB_MI]

Hmm. You need a General class ham license to operate that rig. I would have thought antenna impedance was covered in the General test.

 

[55 50 Ron]

RF (radio frequency) IS electricity and think of the radio and television transmitters at 1 KW, 50 KW, 100 KW and on up and know that coaxial cable can conduct VERY LARGE amounts of electricity! The coaxial cable on a typical FM broadcast tower is 3 1/8 inches in diameter at 50 ohms impedance. And you CAN measure ohms (DC type) of a coaxial cable with a multimeter.

 

[John T]

Thanks for the update Bob, now I understand your interest in insuring a good cable !!!!!!!!!!!!!! Assuming the 50 Ohms Impedance is likely satisfactory, any potential shorting current path between the inner conductor and outer shield,,, or perhaps the absence of any necessary load,,, or perhaps impedance mis match, etc. could be detrimental.

John T

 

[sotxbill]

impedance,,,ratio of inner to outer, dielectric being equal. Engineering back in 71.... if the cob webs are working.

 

[Back Forty Bob.]

KD8CLQ

 

[MarkB_MI]

'nuff said.

 

[Tony in SD]

Or First Class Radio Telephony. Can?t remember my number but the license is on my desk in the office.

I think it?s been covered by the collective group well enough to send Back Forty in the right direction.