DIY line level cable recommendations

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#21
Johnny2Bad said:
It might be simpler for our Original Poster (OP) if we say dielectric outer diameter affects characteristic impedance, and given identical dielectric constant, alters coaxial cable capacitance.
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It would be simpler, but wholly untrue. The capacitance and the characteristic impedance is not set by the outer diameter but the ratio of the outer and inner diameters.

In practical terms for this application, reducing the dielectric outer diameter raises capacitance;
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No.

So he should seek high quality dielectric material if the goal is lower cable capacitance, and will sacrifice capacitance if he seeks smaller diameter cable.
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No.

To get lower cable capacitance you need higher characteristic impedance or higher velocity factor. The highest velocity factor comes from air dielectric, but even that only gives you 2/3 of the capacitance of a typical cable.
 
#22
DF96 said:
For analogue audio use, the RF impedance tells you the cable capacitance. This is because Zrf=sqrt(L/C) and Vrf=sqrt(1/LC).
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This is a simplified formula for radio frequencies well above 100 kHz. If you want to build a one mile long audio frequency transmission line, there is a more complex formula.

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But of course the simplified formula will tell you the cable capacitance.
 
#24
kinku said:
I have bought few coax cables before but they all are very rigid due to the kind of shielding and it is hard to wire PCBs with them. ( for short distances ). Also looking for a flexible ( very) shielded coax too.
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If you insist on thin, flexible coax for in-cabinet wiring, try RG-174. IIRC it's a 50 Ohms characteristic impedance cable used in IF and RF applications. If you want to get even more anal, use Sub-miniature A connectors for a screwed down connection.

For shielded twisted pair, I like Helusound/Helukabel AES/EBU DMX cable. I know it's 110 Ohms but for short runs from RCA sockets on the rear panel of amps/preamps/buffers to circuit boards or source selection pot or volume pot, it sounds very good.
 
#27
DF96 said:
It would be simpler, but wholly untrue. The capacitance and the characteristic impedance is not set by the outer diameter but the ratio of the outer and inner diameters.


No.


No.

To get lower cable capacitance you need higher characteristic impedance or higher velocity factor. The highest velocity factor comes from air dielectric, but even that only gives you 2/3 of the capacitance of a typical cable.
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The inner diameter of coax cable suitable for this application varies little. The vast majority of Coax cables available in the market don't fall into his desired criteria; the remainder fall into a narrow range of specifications. Technically the ratio is the specific factor, but practically, it's the outer diameter and the dielectric.

There are many useable online calculators that take into consideration every factor and the resulting specifications if those are desired to be known. No math necessary.
 
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#29
Johnny2Bad said:
The inner diameter of coax cable suitable for this application varies little.
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A coax cable may be thin (maybe 2.5mm outside) or thicker (maybe 6mm outside) and yet still be 75 ohms RF impedance. The inner diameter of the thinner cable will be thinner.

Technically the ratio is the specific factor, but practically, it's the outer diameter and the dielectric.
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No, practically it is also the ratio not the diameter - because theory and practice agree.

No math necessary.
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True, provided someone else has already done the necessary sums. The key point is that you can have a desired level of capacitance with various diameter cables or a different level of capacitance with the same diameter cable - which is the opposite of what you said.
 
#31
kinku said:
I have bought few coax cables before but they all are very rigid due to the kind of shielding and it is hard to wire PCBs with them. ( for short distances ). Also looking for a flexible ( very) shielded coax too.
Which will be the least hefty coax cable that will work in 3-4 feet distance? I bought a couple but they are pretty rigid so take much more length than needed as they can't loop in short radius
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For short runs inside equipment I use RG-174U. It's flexible and has low capacitance per foot. Also silver plated copper clad center, Teflon dielectric, silver plated copper shield. For longer runs it's RG-180B/U. Similar to above with even less capacitance per foot and just a tiny bit larger, but still small compared to many others.

eBay source
Specification 180
Specification 174
 
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