On Mon, 08 Dec 2008 12:32:14 -0500, "Michael A. Terrell"
wrote:


mm wrote:

On Fri, 05 Dec 2008 15:07:46 GMT, "TKM" wrote:


Who knows if it's OK to measure the resistance (using a standard multimeter)
of a coax line connected to an amplified antenna after disconnecting the
amplifier power supply, but not the antenna. I'm concerned about damaging
the amplifier inside the antenna,

You want to measure the resistance of the center conductor, right?

Easy to connect to the coax, but what do you want to connect to at the
other end? The rods that stick out of the antenna?

The voltage from a multimeter, 9 volts usually, isn't going to hurt
anything, but I don't think you can expect a meaningful measurement
either.


What kind of meter are you using with 9 V between the probes?

Well it's been decades since I measured the voltage, if I ever did, so
let me just say that the voltage will be NO MORE THAN the 9 volt
battery provides. It might be less, but there are no voltage raising
circuits in a "standard multimeter", which the OP plans to use.

Ohm
meters are usually current limited, or constant current sdesigns.

How would knowing this help him? His concern was about damaging his
amplified antenna.


There are semiconductors between the input and output of the
amplifier, and with no power, they might be acting as non-conductors.
Maybe I'm wrong about that, but how would you know if the resistance
is good or not, short of finding an identical antenna and measuring
it?

but suspect an open line going from the
lightning arrestor up the roof to the antenna.

So can't you disconnnect the cable at the antenna and measure the
resistance through the line with the arrestor only?

TKM

On Mon, 08 Dec 2008 12:30:47 -0500, "Michael A. Terrell"
wrote:


mm wrote:

On Thu, 4 Dec 2008 15:17:55 -0600, "lurch" wrote:



You have shown the antenna and downstairs TV are good. Something you
bypassed with the 100' cable is bad. Add parts of the bad system, one at a
time, into the good system. Suspect ALL connectors and extension cables
until they work with the good system. The splitter is the main suspect as
all four down cables shouldn't fail at the same time.

You'd really have to work hard to make a passive splitter fail. I
guess applying 110 volts might do it.


Lightning kills them by the millions. As an engineer for a major
CATV MSO years ago, I lost hundreds every month. They are simple three
port RF transformers, wound with wire the size of a human hair.

You're right. Oops.

mm wrote:

On Mon, 08 Dec 2008 12:32:14 -0500, "Michael A. Terrell"
wrote:


mm wrote:

On Fri, 05 Dec 2008 15:07:46 GMT, "TKM" wrote:


Who knows if it's OK to measure the resistance (using a standard multimeter)
of a coax line connected to an amplified antenna after disconnecting the
amplifier power supply, but not the antenna. I'm concerned about damaging
the amplifier inside the antenna,

You want to measure the resistance of the center conductor, right?

Easy to connect to the coax, but what do you want to connect to at the
other end? The rods that stick out of the antenna?

The voltage from a multimeter, 9 volts usually, isn't going to hurt
anything, but I don't think you can expect a meaningful measurement
either.


What kind of meter are you using with 9 V between the probes?

Well it's been decades since I measured the voltage, if I ever did, so
let me just say that the voltage will be NO MORE THAN the 9 volt
battery provides. It might be less, but there are no voltage raising
circuits in a "standard multimeter", which the OP plans to use.

Ohm
meters are usually current limited, or constant current sdesigns.

How would knowing this help him? His concern was about damaging his
amplified antenna.


My point was that the ohm meter was designed NOT to damage solid state
electronics, unless he is using a pre WW-II meter with a high test
voltage & current. A few old meters were still in shops when the first
transistor equipment hit the market. the service manuals warned you not
to use the outdated equipment, or you would destroy the transistors.


--
http://improve-usenet.org/index.html

aioe.org, Goggle Groups, and Web TV users must request to be white
listed, or I will not see your messages.

If you have broadband, your ISP may have a NNTP news server included in
your account: http://www.usenettools.net/ISP.htm


There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

UCLAN wrote:
mm wrote:

Who knows if it's OK to measure the resistance (using a standard
multimeter) of a coax line connected to an amplified antenna after
disconnecting the amplifier power supply, but not the antenna. I'm
concerned about damaging the amplifier inside the antenna,

You want to measure the resistance of the center conductor, right?

Easy to connect to the coax, but what do you want to connect to at the
other end? The rods that stick out of the antenna?

It's easy to test coaxial cable, no matter the length. Simply screw on a
75 ohm termination cap at one end, and measure from center conductor to the
ground shield at the other end. The ohmmeter will read 75 ohms plus
whatever
the impedance of the cable is.

Reword " plus the impedance of the cable is" to "plus the DC resistance
of the is" and you will be correct.

Measure the resistance of the terminator just in case, as I've seen some
that were 91 ohms....

If you see a run of cable that adds more than 10 ohms to the terminator
resistance, suspect a bad run of cable or corrosion at the connectors.

Just remember that DC continuity of coax is just that. Bad or lossy
cable can still show good continuity.

Geoffrey S. Mendelson wrote:

It's easy to test coaxial cable, no matter the length. Simply screw on a
75 ohm termination cap at one end, and measure from center conductor to the
ground shield at the other end. The ohmmeter will read 75 ohms plus whatever
the impedance of the cable is.

Actually it won't. The impedance is at RF frequencies, the resistance is
at DC. The DC resistance of coax is close to 0, being the sum of the DC
resistance of the center conductor and the shield.

The OP's report of NO signal present is more indicative of an open rather
than a high impedance at some RF frequency

Resistance meters measure using DC or relatively low frequency AC, it
will be very close to 0.

Not with the 75 ohm terminating cap. It will be at least 75 ohms unless
there is a short somewhere in parallel with the 75 ohms.