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G8MNY > TECH 21.05.18 09:21l 65 Lines 2249 Bytes #999 (0) @ WW
BID : 20444_GB7CIP
Read: GUEST
Subj: Coax Faulting
Path: IW8PGT<IR2UBX<F1OYP<ON0AR<GB7CIP
Sent: 180521/0809Z @:GB7CIP.#32.GBR.EURO #:20444 [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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