.... and I repeat .... is it acceptable to enclose it in a
non-conductive enclosure, on a multiband antenna, say resonant on 160 or
80, especially at high power (1500w)?
73 Dwight NS9I
On 2/1/2011 7:53 PM, jimlux wrote:
> On 2/1/11 2:24 PM, Steve Hunt wrote:
>> Here are a couple of simple calculations based on EZNEC predictions of
>> the impedance at the end of the G5RV ladderline - where the 1:1 balun
>> operates:
>>
>> 40m: Z = 27-j78
>> At the 1500W level, I = 7.5A rms and Vpk = 875v
>>
>> 30m: Z = 37 + j291
>> At the 1500W level, I = 6.4A rms and Vpk = 2,655v
>>
>> Are we sure that the cables being recommended for the bifilar winding
>> can withstand those sorts of voltages? I'm not familiar with the US THNN
>> cable, but from what I see on Internet pages THNN #12 is rated at 600v.
>>
>
> rated at 600V means that it can be used in a 600Vrms system, where it
> will have to withstand surges of more than that without breakdown. The
> actual breakdown voltage is substantially higher.
>
> In reality, it will stand off more than that. THHN is nylon over PVC
> insulation, and at least one catalog says that the nylon thickness is 4
> mils over 15 mils (about 0.4mm) of PVC. Typical breakdown strength for
> PVC insulation as used in wires has been reported at around
> 600-900V/mil, with some folks reporting lower, depending the amount of
> plasticizers and such. There's also the issue of small cracks, etc.
>
> A safe number for breakdown might be 100V/mil (which leaves a lot of
> margin) and still gets you to 1500V breakdown, without considering the 4
> mil thick nylon jacket, which is much tougher and probably has a 1000V
> breakdown in and of itself.
>
> A chart in a paper by Babrauskas(2005) shows a breakdown at 60 Hz in .4
> mm thick PVC of around 30kV.
>
> That paper, by the way, has more information on breakdown and fire in
> PVC insulated wire than you could possibly need.
>
>
> I wouldn't worry about HV breakdown in this application. I would worry
> about heating.
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