>From my practical experience fabricating small matching networks empirically ,
>I would say Jim's analysis is right on the money. I think a lot of folks get
>carried away and use "gigantic " components when reasonably large will do.
Sent from my iPhone 5
> On Aug 1, 2014, at 8:45 AM, Jim Lux <jimlux@earthlink.net> wrote:
>
>> On 7/31/14, 11:19 PM, Matt wrote:
>> Could someone help me out with some advice.
>>
>>
>>
>> How much separation between circuit board traces in an antenna switch box
>> would be recommended to reliably handle the voltage associated with legal
>> limit at 2:1 SWR?
>
> You can start by looking at the underlying physics:
> http://home.earthlink.net/~jimlux/hv/paschen.htm
> http://home.earthlink.net/~jimlux/hv/sphgap.htm
>
>
> A good rule of thumb is that the breakdown voltage across an insulator's
> surface is 1/3 that of the breakdown in air. for uniform fields: 70kV/inch in
> air. This is "sharp edge to sharp edge" and another rule of thumb says that
> the needle gap distance is 3 times the "almost uniform" gap of big spheres.
> So overall, I'd say a factor of 10.
> In air, uniform field, 0.014 inches/kV
> across the board, without coatings, maybe 0.140 inches/kV?
>
>
> Conformal coating (with no voids) greatly increases the breakdown voltage,
> because you're not sparking across a surface. Now, you're looking at the
> breakdown strength
>
> I'm sure there's some "design rules for HV PWBs" out there..
> http://www.smps.us/pcbtracespacing.html
> gives a UL test of 40V/mil (this is about half the air gap breakdown for
> uniform fields), and discusses various industrial standards..
>
> http://www.smpspowersupply.com/ipc2221pcbclearance.html is a handy table
>
>
> 1.5 kW into 50 ohms is 273Vrms, or 383V peak.
> 5.4 Arms, or 7.67Apk (for current handling, though, rms is more appropriate)
> 2:1 VSWR is double that, or around 800Vpk (for breakdown), and 11 Arms (for
> thermal limits)
>
> So, by my "rule of thumb, physics based", I'd say about 0.112 inch spacing.
>
> The above site gives spacings ranging from 0.1811" down to 0.0677" (for 800V)
> depending on which rule system you use.
> For 500V, the graph shows 2.5mm (very close to 0.100") for IPC2221A which is
> for uncoated PC boards.
>
> http://www.smps.us/pcb-calculator.html has a handy trace width calculator
> it gives widths ranging from 50 to 85 mils for 11 Amps..
> I'd be real careful, though.. at RF, skin effect is something worth
> considering. At 30 MHz it's about 0.5 mil. 1 oz copper is 1.4 mils thick. 3
> oz would be three times that, or 4.2 mil. For 1 oz copper, then, the RF
> current will fill the conductor, but for the thicker 3oz copper, at least at
> the top of the HF bands, the current will be flowing mostly in the surface of
> the conductor. That said, the thicker copper will give a better thermal path.
>
>
>
>>
>> Just tinned or conformal coating?
>>
>> How wide would you recommend the trace to handle the same signal using 3oz
>> board stock - longest trace is about 2", most less than 1"?
>
>
>
>>
>>
>>
>> Thanks in advance
>>
>> Matt
>>
>> KM5VI
>>
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