[TowerTalk] homebrew low voltage surge suppressors?

[K4SAV]

..."Their use of a coil across the line is also questionable."...

-----------------------
Now that you mention it, I may as well give you the answer.

I don't know the value of the inductor they use, but a little circuit 
analysis can ballpark the value.  They claim these suppressors operate 
to 1.5 MHz, so in order to keep the SWR under 1.2 at 1.5 MHz, the 
inductor has to be at least 30 uH.  When you take a 30 uH inductor and 
place it in SPICE and drive it with a typical 8/20 us lightning waveform 
at 20,000 amps, the voltage across the inductor goes to about 74,000 
volts. So the inductor isn't doing much by itself.

If you now add a 1000 volt surge suppressor in parallel, and hit it with 
the same 20,000 amp pulse, the suppressors takes a peak current of 
19,300 amps and the inductor takes 460 amps at the same time, then 
continues to increase to about 1400 amps while it is dumping its stored 
energy back into the suppressor.

So their conclusion that the inductor takes most of the current and the 
suppressor is only there to take the back emf from the inductor is 
wishful thinking. I get the feeling they publish stuff like this to make 
people think their products are a lot better that the competition, 
hoping no one will analyze it.  When you do analyze it they sure look 
stupid.

Jerry, K4SAV

Gary Schafer wrote:

>I agree with you Jerry. What they fail to disclose is the amount of energy
>that is being dealt with. If the "propagation slows down via the coax" that
>doesn't change the amount of energy that has to be dissipated.
>
>Their use of a coil across the line is also questionable. The reason that
>Polyphaser puts a capacitor in series on the equipment side is so that any
>coils in the equipment, like receiver links, doesn't short the line and hold
>the voltage low so the gas tube doesn't fire early. Having a DC path to
>ground there can allow a significant amount of energy to pass before the
>voltage can get high enough for the gas tube to fire. The coil will ring and
>pass energy.
>
>On another note on knowing if the gas tube has failed it can be checked
>easily with a small hi-pot tester on the polyphaser unit connected to the
>antenna side of the suppressor. With a coil to ground in the unit
>disassembly would be required to check it.
>
>73
>Gary  K4FMX
>
>
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