I thought I had a coax problem. Every time I tried to check the dc
resistance of a length of coax not connected to anything, I always got a
reading around 1.25 meg. This is not a short nor open line so I checked
another length and got similar results, so I was really baffled. The odd
thing is that both of these coaxes work perfectly when connected to an
antenna. I finally found out what I was doing wrong. I was using probes that
I had to hold with my fingers, so I was really measuring the dc resistance
between my fingers. After watching my fingers, I get infinite dc resistance
like I should. This one really baffled me for some time, I don't wish
something like that to happen again.

73 Mike.

VE2CJW wrote:
I thought I had a coax problem. Every time I tried to check the dc
resistance of a length of coax not connected to anything, I always got a
reading around 1.25 meg. This is not a short nor open line so I checked
another length and got similar results, so I was really baffled. The odd
thing is that both of these coaxes work perfectly when connected to an
antenna. I finally found out what I was doing wrong. I was using probes that
I had to hold with my fingers, so I was really measuring the dc resistance
between my fingers. After watching my fingers, I get infinite dc resistance
like I should. This one really baffled me for some time, I don't wish
something like that to happen again.

73 Mike.

Of course if it WAS a megohm shunt at the operating frequency, you
wouldn't even notice it. But yes, your advice goes equally for other
resistance checks. Don't expect to evaluate even a 10k resistor
accurately by holding the test leads on it with your fingers. I have
some voltmeter test leads with "grabber" hooks that I just love for
things like that--these particular ones have points that work like
regular probes too.

If you really do want to check the coax's high resistance with your
fingers, charge up a 30 meter or so length to about a kilovolt, let it
rest with no applied voltage for several minutes, and see if it will
"zap" you when you touch across the conductors. If you're crazy enough
to try it, do NOT do it with two hands, because there's some danger
with the current going through your upper body... If the line stays
charged to, say, 9/10 of the original voltage for an hour, what
resistance is that? Well, it would be roughly a ten hour time
constant, and if it's 50-ohm line with solid polyethylene dielectric,
it will be about 3000pF capacitance, so the resistance would be about
10^13 ohms. Don't be surprised if it's actually better than that,
even...I have a couple metalized polypropylene caps I charged up a few
years ago that I take out and measure every once in a while (very
carefully with a very high impedance volt meter), and they are showing
a self-discharge time constant in excess of 50 YEARS.

Cheers,
Tom

Tom, do you live in the desert?!

You must keep those caps somewhere pretty clean... cool experiment
though... wonder what the record is for capacitor storage over a very
long time.

73,
Dan
N3OX

wrote:
Tom, do you live in the desert?!

You must keep those caps somewhere pretty clean... cool experiment
though... wonder what the record is for capacitor storage over a very
long time.

73,
Dan
N3OX

No; in fact, I live near Seattle! It might actually be worse in a hot
desert, though, because the leakage probably doubles every 10C or so.
Mine are stored with one lead soldered to a PC board, and the outher up
in the air, in a little cardbaord box. Of course, the gates in
isolated-gate PROMS hold their charge a very long time, too. But
modern automated manufacturing in well controlled processes produces
some very consistent materials. I told Bob Pease about my capacitor
'speriment (with two polyester = Mylar caps, and two polypropylene, all
0.1uF, all initially charged to about 12V, the limit of my
high-impedance volt meter) and he told me about being surprised that an
unplugged common printer power supply he had held its charge for many
days. I can tell you that the Mylars have a self discharge time
constant (about 5-10 years) about 1/10 that of the polyprops. The
"supercaps" aren't too bad either--those 1F 5.5V things. Seems to me I
measured a few months for those, though I don't remember for sure right
now.

Way off topic for antennas. Sorry, but some may find it interesting
anyway.

Cheers,
Tom

"K7ITM" wrote in
ps.com:


VE2CJW wrote:
I thought I had a coax problem. Every time I tried to check the dc
resistance of a length of coax not connected to anything, I always
got a reading around 1.25 meg. This is not a short nor open line so I
checked another length and got similar results, so I was really
baffled. The odd thing is that both of these coaxes work perfectly
when connected to an antenna. I finally found out what I was doing
wrong. I was using probes that I had to hold with my fingers, so I
was really measuring the dc resistance between my fingers. After
watching my fingers, I get infinite dc resistance like I should. This
one really baffled me for some time, I don't wish something like that
to happen again.

73 Mike.

Of course if it WAS a megohm shunt at the operating frequency, you
wouldn't even notice it. But yes, your advice goes equally for other
resistance checks. Don't expect to evaluate even a 10k resistor
accurately by holding the test leads on it with your fingers. I have
some voltmeter test leads with "grabber" hooks that I just love for
things like that--these particular ones have points that work like
regular probes too.

If you really do want to check the coax's high resistance with your
fingers, charge up a 30 meter or so length to about a kilovolt, let it
rest with no applied voltage for several minutes, and see if it will
"zap" you when you touch across the conductors. If you're crazy
enough to try it, do NOT do it with two hands, because there's some
danger with the current going through your upper body... If the line
stays charged to, say, 9/10 of the original voltage for an hour, what
resistance is that? Well, it would be roughly a ten hour time
constant, and if it's 50-ohm line with solid polyethylene dielectric,
it will be about 3000pF capacitance, so the resistance would be about
10^13 ohms. Don't be surprised if it's actually better than that,
even...I have a couple metalized polypropylene caps I charged up a few
years ago that I take out and measure every once in a while (very
carefully with a very high impedance volt meter), and they are showing
a self-discharge time constant in excess of 50 YEARS.

I got in some trouble in high school by bringing in some 600volt 8
microfarad oil caps and discharging them loudly against the metal desk
when (I thought) the teacher wasn't looking. I never did find out how
long they would hold a charge, but they would load up to 500-600 volts
and stay that way all day!


--
Dave Oldridge+
ICQ 1800667