On Mon, 05 Sep 2011 17:21:56 -0700, Jeff Liebermann
wrote:

One was connected to an antenna when it
probably failed, but there were no transmitters active.

Perhaps not intentional Ham transmissions.

Roy often counseled about how his experience included many interfering
commercial sources. And this in a rather remote reach outside
Portland, I believe.

"No one ever expects the Spanish Inquisition!"

73's
Richard Clark, KB7QHC

On Tue, 06 Sep 2011 11:23:53 -0700, Richard Clark
wrote:

On Mon, 05 Sep 2011 17:21:56 -0700, Jeff Liebermann
wrote:

One was connected to an antenna when it
probably failed, but there were no transmitters active.

Perhaps not intentional Ham transmissions.

The first step to solving a problem is to blame someone. I prefer not
to blame hams.

Actually, I lied. Two of those that I've repaired were attached to
antennas. However, the locations are not conducive to having nearby
transmitters. One is in the deep dark forest, where the nearest
neighbor, much less nearest possible RF source, is about 1/2 mile
away. The other lives in a nearby rural area. No forest, but also no
nearby transmitters. However, it's possible that the transmitter in
question may have been one owned by the hams in question. Keying a
nearby HT or mobile might have been the culprit. Dunno.

Roy often counseled about how his experience included many interfering
commercial sources. And this in a rather remote reach outside
Portland, I believe.

Attach a wattmeter to any VHF antenna on a mountain top with FM/TV
transmitters, and you'll get a watt or more of RF indicated. Nice way
to blow up the instrument.

"No one ever expects the Spanish Inquisition!"

The Spanish Inquisition had quite a bit of support from those outside
the clergy. Much of it was about confiscating the property of alleged
heretics. When it grew in size to become a source of regular revenue,
it became an institution (much like our war on drugs). It fizzled out
when they ran out of victims.

At this point, I don't know what is causing the failures. However, I
do know what to do. Add a dummy load and buy some more diodes.

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

Jeff Liebermann wrote:


Actually, I lied. Two of those that I've repaired were attached to
antennas. However, the locations are not conducive to having nearby
transmitters. One is in the deep dark forest, where the nearest
neighbor, much less nearest possible RF source, is about 1/2 mile
away. The other lives in a nearby rural area. No forest, but also no
nearby transmitters. However, it's possible that the transmitter in
question may have been one owned by the hams in question. Keying a
nearby HT or mobile might have been the culprit. Dunno.

Attach a wattmeter to any VHF antenna on a mountain top with FM/TV
transmitters, and you'll get a watt or more of RF indicated. Nice way
to blow up the instrument.



At this point, I don't know what is causing the failures. However, I
do know what to do. Add a dummy load and buy some more diodes.


If you do not short the center conductor (on the DUT) to ground
immediately before connecting to the analyzer you risk discharging a
large "capacitor" directly into the instrument. If I am at an antenna
farm I am grounding all my test gear prior to putting it into service.
There is a Ground lug on the 259B right next to the SO239. They
provide a Type N adaptor with a new meter, too, BTW.

MFJ isn't the only ones that can suffer at the hands of others.

I was trying to locate some interference using a UHF Yagi and
a Motorola R2018 service monitor. Do not key up a hand held in
front of the antenna. At least Motorola anticipated that, and
it took out the picofuse behind the antenna connector instead
of the front end mixer.

Jeff-1.0


--
"Everything from Crackers to Coffins"

On Wed, 07 Sep 2011 08:41:34 -0500, Jeffrey Angus
wrote:

MFJ isn't the only ones that can suffer at the hands of others.

I was trying to locate some interference using a UHF Yagi and
a Motorola R2018 service monitor. Do not key up a hand held in
front of the antenna. At least Motorola anticipated that, and
it took out the picofuse behind the antenna connector instead
of the front end mixer.

Well, do the math. The front end of the service monitor is 50 ohms.
The fuse is probably 0.1A. Power to blow the fuse is:
P = I^2 * R = 0.1^2 * 50 = 0.5 watts = +27dBm
I'll guess a 10dBi yagi and a 3 watt (+35dBm) HT into a 0dBi rubber
ducky.

[Q]: How close to the antenna can you use the HT before you blow the
fuse?

Go thee unto:
http://www.terabeam.com/support/calculations/som.php
and inscribe the tx power (in dBm) and the antenna gains into the
boxes. Use 0dB for coax losses and 440MHz for the frequency. Don't
worry about RX sensitivity. Plug in different values of distance
(miles) until you get an RX signal level of +27dBm. I get:
0.0003 miles = 1.6 ft

So, stay at least 1.6 ft away from your antenna and you won't blow the
fuse. Please change my guesswork to match reality and recalculate.

Drivel: Yes, I know we're close to the near field and the numbers
aren't very accurate.

Jeff-1.0

Jeff 2.0 (the upgraded version).

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

On Wed, 07 Sep 2011 09:19:56 -0700, Jeff Liebermann
wrote:

Well, do the math. The front end of the service monitor is 50 ohms.
The fuse is probably 0.1A.

I don't suppose the reference to a "picofuse" is literal to amperage
(pA), but the introduction of a 0.1A fuse is equally a supposition (I
suppose, because from my experience with precision power measurement
equipment, they often used a commercially available, specialty fuse
rated at 10mA - a Wollaston wire).

Power to blow the fuse is:
P = I^2 * R = 0.1^2 * 50 = 0.5 watts = +27dBm
P = I^2 * R = 0.01^2 * 50 = 0.005 watts
I'll guess a 10dBi yagi and a 3 watt (+35dBm) HT into a 0dBi rubber
ducky.

[Q]: How close to the antenna can you use the HT before you blow the
fuse?

Getting too close (less than 5 to 10 wavelengths, and for "gain"
antennas, 20 wavelengths or more) negates directivity.

Go thee unto:
http://www.terabeam.com/support/calculations/som.php
....
I get: 0.0003 miles = 1.6 ft

Which is extremely suspect.

Drivel: Yes, I know we're close to the near field and the numbers
aren't very accurate.

Which makes the exercise rather pointless.

73's
Richard Clark, KB7QHC

On 07 Sep 2011 13:27:08 GMT, dave wrote:

If you do not short the center conductor (on the DUT) to ground
immediately before connecting to the analyzer you risk discharging a
large "capacitor" directly into the instrument.

Good point except that none of the 3 analyzers that failed were
(allegedly) attached to equipment. They were attached to antennas.
Antennas normally do not have BFC's (big fat capacitors) attached, but
do build up static charges. With the humidity currently at 60% or
more, I don't think that's likely. Also, any antenna design, that
would fry an analyzer, might also fry a radio front end. It's
possible, but unlikely.

If I am at an antenna
farm I am grounding all my test gear prior to putting it into service.
There is a Ground lug on the 259B right next to the SO239. They
provide a Type N adaptor with a new meter, too, BTW.

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.

The plot thickens. I checked the accuracy of the MFJ-269 and is was
off on the real part of anything it was measuring about +10%. A good
50 ohm load would read about 56 ohms. It didn't take long to find the
culprit. Instead of 51.1 ohms (1%), R88 showed 54 ohms, while R85 and
R86 read 52 ohms (or there about within the limits of my cheapo
ohms-guesser). It therefore seems possible that the owner had
transmitted into the instrument (even though he denies it). That also
might explain how 3 out of 4 diodes were found fused open. Most
diodes I've seen end up shorted when the junction is punctured. I'll
be replacing all the 51.1 ohm resistors.

Meanwhile, I added a grounded load cap, returned the instrument to its
owner, extorted a free lunch, applied the requisite instrument safety
lecture, and warned him of the accuracy problem. The dip in the VSWR
curve is all he needs at this time, which is unaffected by bridge
imbalance.

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

On 9/7/2011 8:54 AM, Jeff Liebermann wrote:
On 07 Sep 2011 13:27:08 GMT, wrote:

If you do not short the center conductor (on the DUT) to ground
immediately before connecting to the analyzer you risk discharging a
large "capacitor" directly into the instrument.

Good point except that none of the 3 analyzers that failed were
(allegedly) attached to equipment. They were attached to antennas.
Antennas normally do not have BFC's (big fat capacitors) attached, but
do build up static charges. With the humidity currently at 60% or
more, I don't think that's likely.


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.



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.

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.




If I am at an antenna
farm I am grounding all my test gear prior to putting it into service.
There is a Ground lug on the 259B right next to the SO239. They
provide a Type N adaptor with a new meter, too, BTW.

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.

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

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?).

Actually measuring this current (ca 1970s) required using two
polonium-210 coated probes (what are still available as static brushes
for vinyl records) feeding FETs.

The following link:
http://www.angelfire.com/electronic/...rad/brush.html
illustrates what the brush is like, and its electrical attributes.
Another source, the manufacturer (which deeply hides the polonium
reference):
http://www.nrdstaticcontrol.com/doc/microbalance.pdf

73's
Richard Clark, KB7QHC

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.