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Coaxial Cable Q factor. - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.

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

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

 Post subject:
Posted: Mon Apr 14, 2008 11:03 am 
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

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

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.


 Post subject:
Posted: Tue Apr 15, 2008 4:41 am 
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

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

Joined: Fri Apr 11, 2008 2:11 am
Posts: 3
Hi Nubbage,

Thanks. have a nice day.

Posted  11/12/2012
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