LF: Re: PF per Meter dependant on wire size?

["James Moritz" <james.moritz@btopenworld.com>]

Dear Paul, LF Group,

One formula for C per unit length:

C = 24/Log(4H/d) picofarads/m , H= height, m, d = diameter, m

So the capacitance depends on the logarithm of the ratio of height over wire 
diameter, which only varies a little for a large change in diameter, e.g for 
1mm wire at 10m high = 5.2pF/m, 10mm diameter wire at 10m high 6.7pF/m. 
Actually this formula only applies to an infinitely long, uniform, 
horizontal, straight wire. In practice, the presence of ends, downleads, 
things on the ground, insulation on the wire, etc. etc. will all have an 
effect, and are difficult to calculate, so 6pF/m is usually as good an 
estimate as you are likely to get.
Adding multiple wires will increase capacitance. If the wires are many 
metres apart (spacing large compared to height), you can multiply the 
capacitance by the number of wires. But usually, the wires are more closely 
spaced, and there is less increase in capacitance. The figures I have to 
hand are for two 1mm wires 100mm apart, C is higher by 39% compared to a 
single wire, 1m apart and C is 68% higher.
Cheers, Jim Moritz
73 de M0BMU

----- Original Message ----- 
From: "Paul A. Cianciolo" <paulc@snet.net>
To: <rsgb_lf_group@blacksheep.org>
Sent: Sunday, March 28, 2010 8:24 PM
Subject: LF: PF per Meter dependant on wire size?


> Hello,
>
> I see from previous posts that a number of  aprox 6 PF what size wire is
> this for?
>
> Increasing wire size should in "C"  as should several conductors in 
> parallel
> spaced a few feet apart.
>
> PauLC
>
> W1VLF