MFJ-868 SWR/Wattmeter
 

On Wed, 07 Sep 2011 17:46:03 -0400, Mike Coslo wrote:

I have repaired many devices that "I didn't do anything to it - it just
happened!"
- 73 de Mike N3LI -

Well, an admission of guilt would certainly be handy. Before anything
can be repaired, a suitable culprit must be found or assigned. Nothing
can be repaired without first establishing the blame.

If the owner confesses to gross negligence, the repair can proceed
normally. However, if he refuses to accept the blame, someone else
must be found. That's often difficult as one should not blame the
person doing the repairs. It is sometimes convenient to loan the
instrument to a fellow ham, and then blame them for blowing it up. As
a last resort, inflicting inquisition style torture on the owner will
eventually extract a confession, although subsequently getting paid
for the repair might be difficult.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com
# http://www.LearnByDestroying.com AE6KS

MFJ-868 SWR/Wattmeter
 

On Wed, 07 Sep 2011 10:48:55 -0700, Jim Lux
wrote:

Actually, humidity doesn't affect the charging all that much. What
humidity affects is the leakage current across dirty insulators.
P-static is a nice example of static charging in rain at 100% humidity,
for instance.

It's true that dry dust or blowing snow are more notorious for charging,
but just the clear sky current could provide some charging.

Well, what I've noticed is that when the humidity is low, I throw
lightning bolts when I walk across the carpet or slide across the car
seat. When the humidity is higher, then I don't have those problems.
I don't know how this correlates to charge buildup on an antenna, but
my guess(tm) is that there would be some connection.

Also, any antenna design, that
would fry an analyzer, might also fry a radio front end. It's
possible, but unlikely.

I don't know about that. A LNA with a FET front end might be an example
of a ESD sensitive thing, but for HF, where we're usually more concerned
about instantaneous dynamic range and strong signal handling, a more
robust front end is common. One of those +20dBm LO mixers, for
instance, is going to be quite robust.

In a past life, I helped design a marine HF xceiver, which had a +7dBm
mixer (SBL-1) in the front end. Between the antenna and the mixer,
was a low pass filter with no capacitors to ground. There were a few
failures, most of which were due to nearby lightning hits. However
some of these failures were blamed on static buildup.

The higher level mixers will certainly survive a bigger blast through
the antenna. But that's because there's a torroidal isolation
transformer between the antenna and the diode ring. See typical
+23dBm mixer at:
http://www.minicircuits.com/pdfs/RAY-1+.pdf
Also note that the maximum RF power is 350mw, which is well within the
range of what can be delivered by a good solid ESD blast via the
antenna. I don't think it will fry the diodes, but might blow the
tiny wire used in the torroids into a fuse.

I can think of a lot of antennas that won't be too hard on a radio front
end that would cook a delicate detector diode hooked directly up to the
antenna.

Ok. I'll admit that the MJF-269 diodes, which are directly connected
to the antenna terminal, are more easily fried than a receiver front
end, which has a mess of circuitry, and possibly an antenna coupler,
between the antenna and the front end.

I carry a neon lamp line tester in my tool box. It's useful for
checking AC power, but also for detecting high voltages on antenna
terminals. It's amazing what I find at broadcast sites.

There's a whole literature on making neon lamp blinkers with an antenna
and a ground.

These days, it's called "energy scavenging" or "RF energy harvesting".
http://www.eweekeurope.co.uk/news/scavenging-free-green-power-from-radio-waves-35622
I used to have a 4 watt fluorescent lamp on top of my mobile antenna
that flickered with the transmitted envelope.


--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com
# http://www.LearnByDestroying.com AE6KS

MFJ-868 SWR/Wattmeter
 

On 9/7/2011 11:43 AM, dave wrote:
Jeff Liebermann wrote:


ohms-guesser). It therefore seems possible that the owner had
transmitted into the instrument (even though he denies it).

Why would anyone connect that antenna analyzer to a transmitter (unless
using the BNC?)

Oh.. that's an easy mistake to make. Bunch of coax with connectors
lying on the floor, hook up the wrong one. Hook a switch up
incorrectly. Coax switch without enough isolation (100W transmitter with
a 20dB isolation switch is a watt into the analyzer..)

Anyone who has never accidentally cooked a piece of test equipment just
hasn't been working long enough.

MFJ-868 SWR/Wattmeter
 

In article ,
Jim Lux wrote:

Anyone who has never accidentally cooked a piece of test equipment just
hasn't been working long enough.

"Experience is directly proportional to equipment destroyed."

I think I blew up my first piece of test gear at the age of 14 or
so... tried to measure line voltage, with the VOM control still set on
"ohms". POP!

Fortunately it was just one resistor... I eventually replaced it, and
I still have that old 1000 ohm/volt VOM (a 1965-vintage Lafayette)
sitting in a drawer somewhere.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

MFJ-868 SWR/Wattmeter
 

On 9/7/2011 11:47 AM, Richard Clark wrote:
On Wed, 07 Sep 2011 10:48:55 -0700, Jim
wrote:

but just the clear sky current could provide some charging.

There is a earth-sky current in the femtoamperes per cm² that has a
constant potential gradient on the order of 600V/m (or something like
that). This current is the return path for all lightning strike
charge transfers, world-wide.

a few pA/sq meter and a kV/meter field is how I always remember it (in
round numbers) It's small, that's true.


Few dipoles are co-planar, parallel wrt ground.

Actually, humidity doesn't affect the charging all that much. What
humidity affects is the leakage current across dirty insulators.

Which could easily overwhelm this femtoampere charge where a gigaOhm
leakage is trivial (zealous Hams using teflon technology?).

Well.. Consider a 20 meter long wire hanging 10 meters above the
ground (half a 80m dipole) so now you're talking tens of pA, 100 Meg
isolation isn't hard to get with clean insulators. That 20 m wire is
like a 850 pF capacitor. If assume, say, 25 pA charging current, you
get a volt every 42 seconds. After half an hour or so, you're up to 40
volts or so..

Granted it's not a lot of Joules.. (heck, probably not even a microjoule)..

So fair weather charging isn't likely to kill your MFJ..

Back to the dust/rain/induction charging, which can certainly get the
levels needed.

MFJ-868 SWR/Wattmeter
 

Dave Platt wrote:

I think I blew up my first piece of test gear at the age of 14 or
so... tried to measure line voltage, with the VOM control still set on
"ohms". POP!

Fortunately it was just one resistor... I eventually replaced it, and
I still have that old 1000 ohm/volt VOM (a 1965-vintage Lafayette)
sitting in a drawer somewhere.


Were you involved with THE pinball machine?

Geoff.

--
Geoffrey S. Mendelson N3OWJ/4X1GM
Making your enemy reliant on software you support is the best revenge.

MFJ-868 SWR/Wattmeter
 

On 9/7/2011 10:01 PM, Jeff Liebermann wrote:
On Wed, 07 Sep 2011 10:48:55 -0700, Jim
wrote:

Actually, humidity doesn't affect the charging all that much. What
humidity affects is the leakage current across dirty insulators.
P-static is a nice example of static charging in rain at 100% humidity,
for instance.

It's true that dry dust or blowing snow are more notorious for charging,
but just the clear sky current could provide some charging.

Well, what I've noticed is that when the humidity is low, I throw
lightning bolts when I walk across the carpet or slide across the car
seat. When the humidity is higher, then I don't have those problems.
I don't know how this correlates to charge buildup on an antenna, but
my guess(tm) is that there would be some connection.


It has to do with the leakage currents discharging you when it's more
humid. Interestingly, humid air has a higher breakdown field than dry air.



In a past life, I helped design a marine HF xceiver, which had a +7dBm
mixer (SBL-1) in the front end. Between the antenna and the mixer,
was a low pass filter with no capacitors to ground. There were a few
failures, most of which were due to nearby lightning hits. However
some of these failures were blamed on static buildup.

And, you can take ESD hits that degrade, but don't destroy, the part.
I've got some LNAs in my desk drawer that appear to work just fine,
except the NF is a bit higher than it used to be.

MFJ-868 SWR/Wattmeter
 

On 9/8/2011 1:17 PM, Dave Platt wrote:
In ,
Jim wrote:

Anyone who has never accidentally cooked a piece of test equipment just
hasn't been working long enough.

"Experience is directly proportional to equipment destroyed."

I think I blew up my first piece of test gear at the age of 14 or
so... tried to measure line voltage, with the VOM control still set on
"ohms". POP!

Fortunately it was just one resistor... I eventually replaced it, and
I still have that old 1000 ohm/volt VOM (a 1965-vintage Lafayette)
sitting in a drawer somewhere.


So, you only did it *once* in all this time? Every few years I do the
same thing, except the fuse catches my error.

John KD5YI

MFJ-868 SWR/Wattmeter
 

On 9/8/2011 1:30 PM, Jim Lux wrote:

And, you can take ESD hits that degrade, but don't destroy, the part.
I've got some LNAs in my desk drawer that appear to work just fine,
except the NF is a bit higher than it used to be.


How do you know it was ESD that caused the degradation?

MFJ-868 SWR/Wattmeter
 

In article ,
Jim Lux wrote:

Well.. Consider a 20 meter long wire hanging 10 meters above the
ground (half a 80m dipole) so now you're talking tens of pA, 100 Meg
isolation isn't hard to get with clean insulators. That 20 m wire is
like a 850 pF capacitor. If assume, say, 25 pA charging current, you
get a volt every 42 seconds. After half an hour or so, you're up to 40
volts or so..

Granted it's not a lot of Joules.. (heck, probably not even a microjoule)..

So fair weather charging isn't likely to kill your MFJ..

Back to the dust/rain/induction charging, which can certainly get the
levels needed.

I've observed clear-air charge on a TV antenna (about 30' above ground
level) which was sufficient to create a visible spark when I touched
the center connector of the coax to a grounded static-drain block.

No rain at the time, no appreciable amount of dust (SF Bay area)...
just clear air with a mild breeze.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!