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G8MNY > TECH 27.05.26 17:24l 65 Lines 2249 Bytes #8 (0) @ WW
BID : 60868_GB7CIP
Read: GUEST
Subj: Coax Faulting
Path: JH4XSY<IW0QNL<HB9ON<DK0WUE<PD0LPM<PI8ZTM<CX2SA<ED1ZAC<GB7CIP
Sent: 260527/0817Z @:GB7CIP.#32.GBR.EURO #:60868 [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 __________________________________
ﾚﾄﾄLampﾄﾂﾄ()_________________________________)ﾄﾂﾄﾄﾄｿ
12V ｳ / \ ｳ ｳ
Floating V1 V2 ｳ
ｳ__________________________________________________ｳ
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
ﾚﾄﾄLampﾄﾂﾄﾄﾄﾄﾄﾄﾄﾄﾄR1ﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾂﾄﾄﾄﾄﾄﾄﾄﾄR2ﾄﾄﾄﾄﾄﾂﾄﾄﾄｿ
ｳ ｳ Corrosion_ﾜ_battery ｳ ｳ
ｳ ｳ ____________________ｳ_____________ ｳ ｳ
12V ｳ / No current in outer \ ｳ ｳ
Floating V1 V2 ｳ
ｳ__________________________________________________ｳ
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 ﾄﾄﾄﾄﾄ 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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