[Heathkit] [Heath] Fw: Heathkit SB-610

[Eddy Swynar]

Hi Morris,

I think you may be a tad confused here---nothing to worry about, though...!

The capacitance IS, indeed, expressed as that "C" betwixt the centre conductor & the braid---you are correct! But then one has to take into account the actual PHYSICAL LENGTH of the coax itself needed for a specific "C".

As Bob stated, in the example of RG-58 coax, a length of that coax one metre long has 100-pfd. of overall capacitance---ONE THIRD of a metre's length of RG-58 would have one third of 100 pfd., or, 33.3-pfd.

Think of a length of coax as being just one, LONG capacitor---trim it, & the "C" diminishes: lengthen it, & the "C" increases. Just use 100-pfd.-per-metre as a starting point for RG-58...

I hope that's clear...I know only too well that this can be rather confusing to anyone who has never actually used a run of coax as an actual capacitor, before...I have here, as a "padder" to an air variable capacitor that turned-out to be under-sized as a base tuner for an extended inverted "L" antenna for 160-meters. I inserted the "coax capacitor" in parallel with the variable cap, then physically trimmed it with a pair of shears to resonance!

~73~ de Eddy VE3CUI - VE3XZ



**********************************************************************************************************************


On 2012-03-08, at 2:31 PM, G3OOU at aol.com wrote:

> 
> In a message dated 08/03/2012 12:24:49 GMT Standard Time, mdilli at nnwifi.com 
> writes:
> 
> I may be  wrong but I've always expected the capacitance of coax to be
> expressed from  shield to center conductor, not serially.  In this case you
> would not  have a simple series connection of two capacitors and the
> equipment in  question.
> 
> Morris 
> 
> 
> 
> Hi All
> 
> If the length of the coax is significantly less than 1/8 wavelength at the  
> operational frequency then I would suggest that it can be treated as a  
> capacitor. 93ohm coax has a significantly lower capacitance per unit length 
> than  an equivalent 50ohm coax.
> 
> RG58 is 100pF per metre, RG62 is 44.6pF/m, RG8X is 75pF/m and RG174  is 
> 101pF/m. 
> 
> 73
> 
> Bob
> 
> Bob F Burns
> C Eng,  FIET, MSE
> Amateur Radio Callsign: G3OOU
> Registered RSGB  Instructor
> G-QRP Member No: 6907; Zone: 14, QRA: IO91WH; WAB:  TQ25
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