Coax
power handling
Updated September
14, 2008
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New for September 2008!
Information on this topic was a request by a Microwaves101 reader
Max way back in oughtseven... about time we did something with
it!
Coax power handling is a complicated
subject, but it can be broken down into two phenomena. High peak
power can cause failures due to arcing, while high average power
can cause failure due to heat.
Peak power handling
Power handling of air coax is
a topic that is related to atmospheric
breakdown.
Once breakdown occurs, a short
circuit is provided across the coax, and Hell breaks loose.
Arcing is caused when the electric
field E exceeds a critical value which we will denote E_{d}
for electric field at discharge. In air, the critical field is about
1,000,000 volts/meter, in PTFE it is raised to about 100,000,000.
These numbers are approximate, there's no sense trying to be exact
in calculating breakdown, just be sure you avoid it by an order
of magnitude or more and you'll have little to worry about.
The electric field of a coax
varied as a function of position along the radius (denoted ""
in the radial coordinate system). You'd have to use calculus to
derive this, but we just looked it up in Pozar's
Microwave Engineering.
Here, b is D/2 and a is d/2,
the radii of the outer and inner conductors. The peak Efield obviously
occurs right at the surface of the center conductor. If this isn't
obvious to you, consider becoming a program manager!
Rearranging the equation for
the maximum peak voltage,
The peak power you can put into
a coax under wellmatched conditions (low VSWR) is calculated from
the peak voltage it can withstand:
The equation for characteristic
impedance of coax can be found somewhere on this web site, we'll
save you from searching for it:
Recall that 0
is the impedance of free space, or 377 ohms.
Plugging it all together, we can can come up with the equation for
peak power handling as a function of coax geometry.
That's the final equation for
maximum peak power handling of coax, ready for us to do some analysis.
Remember that this result is only true for a matched load. If you
accidentally broke a connection to a highpower transmitter, you'd
see a very high VSWR, in that case the peak voltage could double.
If you need to consider this type of mishap, you want to further
derate your power handling by 6 dB.
Now let's look at some coax examples...
how about the air dielectric 50ohm connectors? The breakdown strength
of air 3,300,000 volts/meter according
to Wikipedia, but that is at "dry air" at standard
temperature and pressure, between spherical electrodes. Let's use
1,000,000 volts/meter.
Connector species
(outer diameter) 
Inner diameter 
Voltage at breakdown 
Max power (low
VSWR load) 
Maximum power
(high VSWR load) 
3.5 mm 
1.52 mm 
634 V 
4023 W 
1005 W 
2.92 mm 
1.27 mm 
529 V 
2098 W 
524 W 
2.4 mm 
1.04 mm 
435 V 
1886 W 
471 W 
1.85 mm 
0.80 mm 
335 V 
1118 W 
279 W 
1 mm 
0.43 mm 
181 V 
325 W 
81 W 
How about PTFEfilled coax? The
breakdown field strength of PTFE is about 10,000,000 volts per meter!
So an "049" cable (0.049 inches "D", 0.015 inches
"d") can withstand 2260 volts and pass almost 50,000 watts
peak. This seems to good to be true, doesn't it? It is. The problem
is that with voltage breakdown, the limitation of the weakest link
in the chain is what you need to focus on. Your semirigid cable
might be able to pass thousands of watts, but as soon as that signal
crosses a path where the PTFE dielectric fill is interrupted by
air, it will spark. At the end of the cable, where the connector
is soldered on, there is surely going to be a gap in the dielectric.
You need to revise the calculation for air dielectric, in which
case you'll see 256 volts is the maximum voltage, 358 watts is the
maximum power into a good load, and 89 watts is the maximum into
an unmatched load. Note that at this interface the coax presents
71 ohms impedance.
Before we move on to average
power handling of coax, let's look at power handing as a function
of line impedance for air coax, which is part of the
"coax compromise" that led to the fifty ohm standard.
If you allow the center diameter freedom to move away from 50 ohms,
you'll see that maximum peak power handling occurs at ~30 ohms.
By the way, if anyone
wants a copy the spreadsheet that generated this curve, just ask.
Eventually we will add peak power handling to our coax spreadsheet
that you can grab in download
area.
Average power handling
Coming soon!
