[Amps] PCB Trace width for 1500 watts RF?

[Ian White, G3SEK]

Dan Levin wrote:
>Anyone care to opine on the minimum pcb trace width appropriate (i.e. safe)
>to carry 1500 watts between 1.8 and 30 Mhz into an impedance mismatch of
>less than 2.5:1?  Assume 2 oz copper thickness.
>
>Almost as useful would be if someone who knows something about RF (since I
>know practically nothing, which is a separate subject) can tell me the
>maximum current in a wire carrying 1500 watts into a 2.5:1 mismatch.

As a jumper in free air, 6mm (3/16in) copper foil is well safe at 1500W 
into 50 ohms at 28MHz. By touch, you can hardly tell if it's getting 
warm at all.  Thin foil is "the right stuff" for this application, 
because the skin effect keeps the current on the surface, and foil has a 
much higher surface area than a solid wire of the same total 
cross-section.

On PC board, at HF, make the tracks as wide as convenient: 6-8-10mm, 
whatever. On regular 1/16in FR450-ohm stripline probably would be too 
narrow for QRO, but it's not needed anyway for this particular 
application. Dielectric stress in FR4 doesn't seem to be a problem 
either, at HF.

The "SWR up to 2.5" spec would mean that either the voltage *or* the 
current at any given point could be up to 2.5 times higher; so either 
the V^2/R or the I^2*R heating could increase by up to 6.25 times. That 
still shouldn't be a problem.


That reminds me, I'm down to the last few square inches. Any sources of 
very thin copper foil out there? That's copper foil as thin and flexible 
as aluminum kitchen foil - just plain copper, without glue on the back.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek