> What is being said here, I'll say it again, operating at 1.5 kW on an
> N connector, particularly HF, is tight, there's no reserve. If you're
> a contester, you're baiting Murphy to show up at the worst possible
> time.
>
> You may get away with it. Others have not.
That is very true Guy. There is no headroom at all.
I'm amazed everyone keeps discussing and using RMS voltage,
which has NOTHING to do with voltage breakdown. It is the peak
voltage, and not even the repeating peak voltage but the
ABSOLUTE peak voltage that causes the problem.
You would be amazed at how high that can be with typical
systems, when all factors are considered. For example, early Icom
775DSP's outputed about 300 watts or more on a leading edge
transient no matter what the actual power was set for.
If you use that radio with an amplifier that drives to 1500 watts with
65 watts of drive and set the power control for 65 watts, you can
count on a spike of up to 350 watts banging the amplifier.
Most 1500 watt amps have the headroom to handle that on pulse,
and if the tank is loaded heavy enough peak output power during
the transient can exceed 5000 watts. (This transient is a major
source of switch and component failures in amplifiers.)
The line must handle the SWR factor+the transient power and we
must consider PEAK...not RMS.... voltage.
When you strike an arc in the closed connector, the ionized air
remains in the connector and following voltages, even when much
lower, will sustain the arc.
I had a radio here (you can guess what one) that, when I watched
the PO of an AL1500 on a fast storage power meter, reached over
5 kW. It not only hammered the heck out of the PA, it stressed
everything in line. That radio, used with the AL1500, would trigger
an arc in a KW Johnson matchbox that normally can handle over
4kW!
When the Matchbox flashed, the SWR on the line went up to
almost infinity. You can imagine the connector voltage if I had a
connector 1/4 wl back from the tuner, where the voltage would be
maximum.
While that radio was an extreme case, MOST radios today have
some overshoot on the closure. It is a common problem, not a rare
event, and it won't show on anything but a fast peak storage
measurement system....but it will cause arcs and other problems.
That's one reason why you need at least a 2:1 safety factor in
voltage (not power) over the highest expected PEAK voltage on the
line. Another is (if you do stuff like I do) you might just grab the
wrong antenna on occasion.
N connectors have a place, but it is in SWR critical applications or
at UHF. Anyplace where you want a mechanically tough BNC
connector is a good place for a N.
73, Tom W8JI
W8JI@contesting.com
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