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[TowerTalk] Connectors

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Subject: [TowerTalk] Connectors
From: (Tom Rauch)
Date: Wed, 4 Apr 2001 13:02:15 -0400
> 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 

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 

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 

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