RG58 and RG174 capacitance

[Fantastic]

I picked up some RG 58 and RG 174 locally .
I started a new thread for this and hopefully guys will put up the measured capacitance of cables that they sourced locally.

Often I found local cables having fairly high capacitance per meter. About 120 to 150 pF per meter. Usually the shields are not so good and the insulation of the core used all types of PVC. Some are soft and some are hard. I haven't yet got one with teflon insulation.

The RG58 mentioned above costs Rs 25/- a meter . It's capacitance is 87pF per meter. Insulation is a denser PVC that's harder to cut through than usual.

The RG 174 costs Rs 10/- per meter and it's capacitance is 77pF per meter. The inner insulation is also like the RG58. One has a name Deepcab and the other one has no name. I think it's just labelled here or they just buy it out as an OEM part.

Both have a multi stranded tinned copper core . The shield is not very good but for short distances and places with low external interference it should be OK. The braided shield looks poorer than on some local cables.

Will be trying them out soon in a 1 meter long interconnect.

 

[jls001]

I use Belden 1855A which is RG 58. Rated 16.3 pF per foot. Dual shielding - braid and foil. Core is solid, 23 AWG.

PS: which meter is good for capacitance measurement? Mine can go down only to 2 nF range and so totally useless for measuring cable capacitances.

 

[Fantastic]

The Belden will be about 54 pF per meter.

However shielding is far superior to the local cables. As mentioned earlier , it's not just locally generated interference but also the constant presence of RFI from a lot of sources . The mobile phone possibly is the worst ! Also all active Bluetooth and Wi-Fi devices.
Some equipment are sensitive to RF ingress and and could affect the sound in many ways.

Does the meter have a 3 1/2 digit display ( 1,999 nF ) ? Use short test leads ( 6 inches or so ) and check open circuit capacitance . Then measure the cable. Deduct the open circuit reading from this ( if it isn't zero ). But you already know this !
Some meters zero automatically on an open circuit or have a REL button to automatically cancel out the stray capacitance.
I've read that some people use a 1000pF (1nF) capacitor on the input to try and reduce inaccuracies on the lowest readings. You could try this.
I'm not quite sure if I'm addressing your question !

 

[keith_correa]

Mine can go down only to 2 nF range and so totally useless for measuring cable capacitances.

Why don't you parallel a known large value cap to what you are measuring. You can then minus that known value from the reading to get the value of what you are measuring.

 

[omishra]

I use Belden 1855A which is RG 58. Rated 16.3 pF per foot. Dual shielding - braid and foil. Core is solid, 23 AWG.

PS: which meter is good for capacitance measurement? Mine can go down only to 2 nF range and so totally useless for measuring cable capacitances.

Where can i buy this?

BTW I have Tenma 72-7730A which measures correctly few pFs too.

 

[Fantastic]

The Tenma meter and the JLS001 meter have the same display resolution of 1 pF. The measurement resolution is poorer. Worst case is the open ckt stray capacitance +/-20 count+/- 1.2% error on the actual value ! That's quite a bit.
Chances are that JLS001's meter is similar. Just add a 1000pF capacitor ( value not too critical) and measure the capacitance with and without the capacitance to be measured.The difference should be the capacitance you want measured .

 

[jls001]

Why don't you parallel a known large value cap to what you are measuring. You can then minus that known value from the reading to get the value of what you are measuring.

I tried this with some known capacitor values in series. Should it be in parallel? In series, the reading kept varying so I didn't trust it.

BTW, my meter (Meco 45CF) is 4 and half digit, so when set at 20 nF range (which I wrongly mentioned as 2 nF), resolution as per the data sheet is 1 pF, but I just couldn't measure the value of the Belden 1855A, which ought to be about 56-57 pF.

 

[jls001]

Where can i buy this?

BTW I have Tenma 72-7730A which measures correctly few pFs too.

It's available from Promedia, Mumbai at Rs 100/m. Number +912267021711.

Is the Tenma inductor meter too?

 

[omishra]

It's available from Promedia, Mumbai at Rs 100/m. Number +912267021711.

Is the Tenma inductor meter too?

thanks Joshua,
For inductance have other meter.

About varying reading for low capacitance, because probes are moving. I fix them against flat surface with tape in 'V' shape. Then make ends near to small cap and measure with hands off. Mostly for less tha 100pF. I successfully measured 2.2pF.

 

[Fantastic]

I tried this with some known capacitor values in series. Should it be in parallel? In series, the reading kept varying so I didn't trust it.

BTW, my meter (Meco 45CF) is 4 and half digit, so when set at 20 nF range (which I wrongly mentioned as 2 nF), resolution as per the data sheet is 1 pF, but I just couldn't measure the value of the Belden 1855A, which ought to be about 56-57 pF.

The 1000pF should be in parallel....right across the meter input or at the end of the leads. Use short wires and keep your hands away from the leads. Things like 2pF etc. can't be reliable measurements.
However you can make a relative comparison between cables . Measure the capacitance of well known brands which will have fairly accurate capacitance. At least you will know how far or close you are to the actual value.

But +/- 20pF error would not really matter in practice for cables ! I've measured 100pF 2KV disc caps ( blue in colour) and found they were within 5% of the value ! You could try them.

 

[Fantastic]

Just found a site with a free download for LCR measurement using a sound card. Capacitance supposedly goes down to 0.1pF !
Can do it with the demo software. The registered one costs only Euro 10/- !
It's in German , but you can get it translated fairly easily I think.

The measurement jig shown in the handbook pdf is very simple and easy to make.

LCRMeas - LCR measurement program

Here is an English version. I have not checked it. Might be the same !
http://icom.hsr.ch/fileadmin/user_upload/icom.hsr.ch/publikationen/RLC_Meter_EN.pdf

 

[sachu888]

I have used RG-174 Toyo brand cable which Joshua sent me in Amp CAmp and CNC Phonostage. I liked this cable as it has low capacitance, flexible and Ok shielding.
I would like to try some better cables. I have read that Silver plated copper is very good for signal.

Regards
Sachin