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

To: <[email protected]>
Subject: LF: Re: PF per Meter dependant on wire size?
From: "James Moritz" <[email protected]>
Date: Sun, 28 Mar 2010 21:08:56 +0100
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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" <[email protected]>
To: <[email protected]>
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







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