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G8MNY  > TECH     14.09.19 08:14z 65 Lines 2249 Bytes #999 (0) @ WW
BID : 21492_GB7CIP
Read: GUEST
Subj: Coax Faulting
Path: HB9ON<IW2OHX<IQ2LB<IR2UFV<N3HYM<WH6FQE<ZS0MEE<GB7CIP
Sent: 190914/0803Z @:GB7CIP.#32.GBR.EURO #:21492 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                     (New Aug 10)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

Here is a simple practical method of finding a short on a long run of coax. It
assumes a temporary test wire can be run from end to end & that the coax inner
& outer are continuous, resistive damage in the outer or a corroded short
(as wired below) will not affect the measurements.

CONNECTIONS
                     Faulty shorted coax
       Car    __________________________________
   ÚÄÄLampÄÂÄ()_________________________________)ÄÂÄÄÄ¿
 12V       ³ /                                  \ ³   ³
Floating   V1                                    V2   ³
   ³__________________________________________________³
                    Test wire

Here it is assumed the inner is OK & it has constant resistance per length. (if
this is not true see "DAMAGED INNER" below) 

The 12V supply must be floating so it can be reversed & have no currents to
earth.

The lamp wattage sets a suitable current down the inner conductor so that at
least a few 100mV will be across it. (this may need 5A for thick LDF cables
etc.)  

CIRCUIT
                        Faulty shorted coax
       Car  
   ÚÄÄLampÄÂÄÄÄÄÄÄÄÄÄR1ÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄR2ÄÄÄÄÄÂÄÄÄ¿
   ³       ³            Corrosion_Ü_battery       ³   ³
   ³       ³  ____________________³_____________  ³   ³
 12V       ³ /         No current in outer      \ ³   ³
Floating   V1                                    V2   ³
   ³__________________________________________________³
                    Test wire

With a sensitive DMV measure V1 & V2, then reverse the 12V polarity & repeat.
Average the readings at each end (ignore polarity) to cancel any corrosion
voltage at the short.

Now the total inner conductor voltage drop is just V1+V2

                             V1
And the fault position is  ÄÄÄÄÄ  x  Length,  from the lamp end.
                           V1+V2

DAMAGED INNER
If the inner is damaged more than the outer (water corrosion) then make the
outer the current carrier.


See TECH buls on "Cable Tester", "Coax Feeder Tests" & "A Versatile Pulse
Tester".



Why don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


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