Re: [Amps] PARALLEL CAPS IN OUTPUT

[Larry Benko <xxw0qe@comcast.net>]

Not true Peter,

Unmatched (assuming 50 ohm output Z in 50 ohm circuit) the max possible 
voltage is (1+p) times the 1:1 SWR voltage.  The reflection coefficient 
p = (SWR-1)/(SWR+1).
In a matched circuit (if the matching has no loss) the maximum voltage 
possible is square root of the SWR times the 1:1 SWR voltage.
Undergrad EE classes cover these topics and programs such as LTSpice can 
show it as well.  Obviously the maximum posible voltage may not be what 
you see depending on electrical distance to the load and the loss in the 
transmission but the above formulas bound the upper limit.
73,
Larry, W0QE

On 12/4/2013 11:49 AM, peter chadwick wrote:
> If one is to believe Philip H. Smith in 'Electronic Applications of the Smith 
> Chart', McGraw-Hill 1969, page 6, Fig 1.3, the maximum voltage appearing on a 
> lossless transmission line with an SWR of infinity is twice the voltage when 
> matched.
>
> So a 1kV rating is adequate.
>
> It makes sense when you think about it, too.
>
> But of course, Smith might have got it wrong.......
>
> 73
>
> Peter G3RZP
>
>
> ========================================
>   Message Received: Dec 04 2013, 06:40 PM
>   From: "Bill Turner" <dezrat1242@wildblue.net>
>   To: "Amps" <amps@contesting.com>
>   Cc:
>   Subject: Re: [Amps] PARALLEL CAPS IN OUTPUT
>   
>   ORIGINAL MESSAGE:          (may be snipped)
>   
>   On Wed, 4 Dec 2013 09:16:06 -0500 (EST), K5GW wrote:
>   
>   >
>   >The voltage rating is not the problem; after all there is less than 300v
>   >rms across a 50 ohm load with 1500 watts power.
>   
>   REPLY:
>   
>   Capacitors don't arc at the RMS voltage. They arc at the peak of the RF
>   cycle. For 1500 watts into 50 ohms, the peak is about 387 VAC. And that's
>   with a 1:1 SWR.
>   
>    A high SWR can cause voltage nodes many times the normal voltage to appear
>   on the coax, and if the coax is just the wrong length, one of those nodes
>   may appear right at your load cap. Have you ever transmitted into the wrong
>   antenna?
>   
>   IMO, padder caps rated at 5 or 6 kV are NOT overkill.
>   
>   Once a capacitor arcs, even if it survives, little blisters form at the
>   point of the arc and, due to corona effect, are prone to arc again but at
>   even lower voltage. It is always best to prevent the arc in the first place.
>   High voltage caps are your friend.
>   
>   73, Bill W6WRT
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