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G8MNY > TECH 21.10.26 10:33l 65 Lines 2257 Bytes #2 (0) @ WW
BID : 8650_GB7CIP
Subj: Coax Faulting
Path: DB0FFL<OE2XZR<OK1TOR<OK0NAG<DB0RKB<DK0WUE<PD0LPM<PI8ZTM<CX2SA<ED1ZAC<
GB7CIP
Sent: 261021/0757Z @:GB7CIP.#32.GBR.EURO #:8650 [Caterham Surrey GBR] $:8650_GB
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
and 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 and 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 and 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 and 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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