Google Groups no longer supports new Usenet posts or subscriptions. Historical content remains viewable.
Dismiss
Coax cable calculation
35 views
Skip to first unread message
Trevor Wilson
Apr 10, 2018, 2:42:50 AM4/10/18
to
Here's a question that stumped me back in the old slide rule days:
A coaxial cable, having an inner diameter of 0.0254mm (0.001") and using
an insulator with a dielectric constant of 2.56, is yo have a
characteristic impedance of 2,000 Ohms. What must be the outer conductor
diameter?
You guys can locate the appropriate method of calculation.
--
Trevor Wilson
www.rageaudio.com.au
A coaxial cable, having an inner diameter of 0.0254mm (0.001") and using
an insulator with a dielectric constant of 2.56, is yo have a
characteristic impedance of 2,000 Ohms. What must be the outer conductor
diameter?
You guys can locate the appropriate method of calculation.
--
Trevor Wilson
www.rageaudio.com.au
gghe...@gmail.com
Apr 10, 2018, 9:27:44 AM4/10/18
to
(377 ohms) An air gap would be better.
Jeff Liebermann
Apr 10, 2018, 11:11:44 AM4/10/18
to
On Tue, 10 Apr 2018 16:42:43 +1000, Trevor Wilson
<tre...@SPAMBLOCKrageaudio.com.au> wrote:
>Here's a question that stumped me back in the old slide rule days:
>
>A coaxial cable, having an inner diameter of 0.0254mm (0.001") and using
>an insulator with a dielectric constant of 2.56, is yo have a
>characteristic impedance of 2,000 Ohms. What must be the outer conductor
>diameter?
<https://www.rfcables.org/coax-calculator.html>
<tre...@SPAMBLOCKrageaudio.com.au> wrote:
>Here's a question that stumped me back in the old slide rule days:
>
>A coaxial cable, having an inner diameter of 0.0254mm (0.001") and using
>an insulator with a dielectric constant of 2.56, is yo have a
>characteristic impedance of 2,000 Ohms. What must be the outer conductor
>diameter?
- Select inches.
- Plug in any number for outside dia (D) trial an error.
- Plug in 0.001 for inner diameter (d).
- Plug in 2.56 for permittivity (er) (same as dielectric constant
in this example).
- Punch "calculate"
I used 1,000,000 inches for outside diameter and still obtained only
776 ohms. 1,000,000,000 inches yielded 1035 ohms. I don't think it's
going to make it to 2,000 ohms unless the coax cable is absurdly huge.
--
Jeff Liebermann je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Allodoxaphobia
Apr 10, 2018, 6:11:01 PM4/10/18
to
On Tue, 10 Apr 2018 08:11:37 -0700, Jeff Liebermann wrote:
> On Tue, 10 Apr 2018 16:42:43 +1000, Trevor Wilson
><tre...@SPAMBLOCKrageaudio.com.au> wrote:
>
>>Here's a question that stumped me back in the old slide rule days:
>>
>>A coaxial cable, having an inner diameter of 0.0254mm (0.001") and using
>>an insulator with a dielectric constant of 2.56, is yo have a
>>characteristic impedance of 2,000 Ohms. What must be the outer conductor
>>diameter?
>
><https://www.rfcables.org/coax-calculator.html>
> - Select inches.
> - Plug in any number for outside dia (D) trial an error.
> - Plug in 0.001 for inner diameter (d).
> - Plug in 2.56 for permittivity (er) (same as dielectric constant
> in this example).
> - Punch "calculate"
>
> I used 1,000,000 inches for outside diameter and still obtained only
> 776 ohms. 1,000,000,000 inches yielded 1035 ohms. I don't think it's
> going to make it to 2,000 ohms unless the coax cable is absurdly huge.
Almost sounds like an OLD ("slide rule") homework problem....
> On Tue, 10 Apr 2018 16:42:43 +1000, Trevor Wilson
><tre...@SPAMBLOCKrageaudio.com.au> wrote:
>
>>Here's a question that stumped me back in the old slide rule days:
>>
>>A coaxial cable, having an inner diameter of 0.0254mm (0.001") and using
>>an insulator with a dielectric constant of 2.56, is yo have a
>>characteristic impedance of 2,000 Ohms. What must be the outer conductor
>>diameter?
>
><https://www.rfcables.org/coax-calculator.html>
> - Select inches.
> - Plug in any number for outside dia (D) trial an error.
> - Plug in 0.001 for inner diameter (d).
> - Plug in 2.56 for permittivity (er) (same as dielectric constant
> in this example).
> - Punch "calculate"
>
> I used 1,000,000 inches for outside diameter and still obtained only
> 776 ohms. 1,000,000,000 inches yielded 1035 ohms. I don't think it's
> going to make it to 2,000 ohms unless the coax cable is absurdly huge.
Jonesy
Trevor Wilson
Apr 10, 2018, 6:13:06 PM4/10/18
to
I thought someone would have come up with an answer before now. It's not
an overly difficult calculation.
--
Trevor Wilson
www.rageaudio.com.au
Dave Platt
Apr 10, 2018, 7:42:27 PM4/10/18
to
In article <fj4r7g...@mid.individual.net>,
Z0 = 138 log10 ((Dd/Dc)*(1/sqrt(eR)))
it seems to transform to
Dd = 10^(Z0/138) * sqrt(eR) * Dc
with Z0 = 2000, eR = 2.56, and Dc = .001, we'd get approximately
Dd = 10^14.5 * 1.6 * .001
Dd = 1.6 * 10^11.5
Dd = 5.1 * 10^11
or about half a trillion inches. That's if I didn't screw up
something using my slide rule (yes, I have a nice 10" Hemmi here at
my desk) or slip a decimal point somewhere along the way.
This seems a bit impractical for a coaxial cable. The
transmission-line cutoff frequency would be ridiculously low, and
construction of a matching section would be, well, interesting, to say
the least. And, I rather doubt that the classic formula is actually
applicable for impedances (and diameters) this large.
A solid cylinder of dielectric material this large would probably
insist on collapsing under its own gravitational self-attraction,
leaving you with a neutron star or a black hole.
Trevor Wilson <tre...@SPAMBLOCKrageaudio.com.au> wrote:
>>>> Here's a question that stumped me back in the old slide rule days:
>>>>
>>>> A coaxial cable, having an inner diameter of 0.0254mm (0.001") and using
>>>> an insulator with a dielectric constant of 2.56, is yo have a
>>>> characteristic impedance of 2,000 Ohms. What must be the outer conductor
>>>> diameter?
>>>> Here's a question that stumped me back in the old slide rule days:
>>>>
>>>> A coaxial cable, having an inner diameter of 0.0254mm (0.001") and using
>>>> an insulator with a dielectric constant of 2.56, is yo have a
>>>> characteristic impedance of 2,000 Ohms. What must be the outer conductor
>>>> diameter?
>> Almost sounds like an OLD ("slide rule") homework problem....
>
>**Correct. It was.
>
>I thought someone would have come up with an answer before now. It's not
>an overly difficult calculation.
Well, just for grins... using the formula
>
>**Correct. It was.
>
>I thought someone would have come up with an answer before now. It's not
>an overly difficult calculation.
Z0 = 138 log10 ((Dd/Dc)*(1/sqrt(eR)))
it seems to transform to
Dd = 10^(Z0/138) * sqrt(eR) * Dc
with Z0 = 2000, eR = 2.56, and Dc = .001, we'd get approximately
Dd = 10^14.5 * 1.6 * .001
Dd = 1.6 * 10^11.5
Dd = 5.1 * 10^11
or about half a trillion inches. That's if I didn't screw up
something using my slide rule (yes, I have a nice 10" Hemmi here at
my desk) or slip a decimal point somewhere along the way.
This seems a bit impractical for a coaxial cable. The
transmission-line cutoff frequency would be ridiculously low, and
construction of a matching section would be, well, interesting, to say
the least. And, I rather doubt that the classic formula is actually
applicable for impedances (and diameters) this large.
A solid cylinder of dielectric material this large would probably
insist on collapsing under its own gravitational self-attraction,
leaving you with a neutron star or a black hole.
Trevor Wilson
Apr 10, 2018, 8:04:10 PM4/10/18
to
1.59 X 10^20 inches. My instructor ticked my answer as the correct one.
Or, for a more useful figure, 427 light years. That's one big coax. A
few hundred Metres would likely require more mass in it's construction
than exists in our galaxy (maybe the entire universe). I haven't tried
to work that out.
--
Trevor Wilson
www.rageaudio.com.au
0 new messages