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Coaxial Cable Q factor. - RF Cafe Forums

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Simpang
 Post subject: Coaxial Cable Q factor.
Posted: Fri Apr 11, 2008 2:42 am 
 
Lieutenant

Joined: Fri Apr 11, 2008 2:11 am

Posts: 3

Can anybody recommand which type of coaxial cable to use to meet the following specification.

The length of cable : 2.85 meter.

OD of cable : 5.0mm +/- 0.3mm

Capacitance : 150pF +/-10pF

Q factor : More than 200 @ 1Mhz


 
   
 
nubbage
 Post subject:
Posted: Mon Apr 14, 2008 11:03 am 
 
General
User avatar

Joined: Fri Feb 17, 2006 12:07 pm

Posts: 218

Location: London UK

Hi Simpang

There are RF coaxial cables that meet some of the parameters, but not others.

I feel the capacitance per m figure is too high for a cable with 5mm OD.

If you look at RG83 or RG88 they are bigger OD but have this high value of capacitance you specify.

I have used a mineral insulated fire-rated instrument cable as an antenna feeder in high temperature fire/safety applications. This met the 5mm OD constraint and came close to the capacitance per m figure of 150pF.

However this is not intended for RF use, but because you only need a short length this might be OK.

It was a Pyotenax single core inner (approx 0.8mm diameter) solid soft copper outer but I do not know the part number. The filling permittivity was around 5, the outer diameter was 5mm and the capacitance about 146pF/m


 
   
 
Simpang
 Post subject:
Posted: Tue Apr 15, 2008 2:18 am 
 
Lieutenant

Joined: Fri Apr 11, 2008 2:11 am

Posts: 3

Hi Nubbage

Thanks for the advice.

The capacitance of 150pF is the total capacitance of 2.85 meter length og cable. This work out to be 52.6pF per meter length of cable.

The q-factor of more than 200 is also refer to the 2.85 meter length of cable.

Will appreciate very much if you can recommand again.

Thanks


 
   
 
nubbage
 Post subject:
Posted: Tue Apr 15, 2008 4:41 am 
 
General
User avatar

Joined: Fri Feb 17, 2006 12:07 pm

Posts: 218

Location: London UK

Hi Simpang

It seems it would need to be a 75 ohm characteristic impedance cable.

The nearest to the spec you give is RG6, Belden 46462, Taylor H109 or Volex-Raydex CT100.

This has a diameter over the braid outer of 5.15mm, and a capacitance of 56pF/m which would give a total capacitance of 160pF.

The Q is easy to meet for all of those cables at VHF/UHF


 
   
 
Simpang
 Post subject:
Posted: Wed Apr 16, 2008 12:44 am 
 
Lieutenant

Joined: Fri Apr 11, 2008 2:11 am

Posts: 3

Hi Nubbage,

Thanks. have a nice day.


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