MFJ-868 SWR/Wattmeter
 

On 8/31/2011 10:55 PM, Jeff Liebermann wrote:
On Wed, 31 Aug 2011 13:57:31 -0500, wrote:

Hi Jeff,

Likewise. alt.internet.wireless appears to be dead. Oh well.
Thanks for posting the info on the coax cable in the MFJ-1800. I
couldn't find the applicable message with Google Groups.

I see MFJ now has a Bi-Quad 12dbi antenna.
http://www.mfjenterprises.com/Produc...uctid=MFJ-1804
If I send the the dimensions....

Ummm.... please don't do that.

Just kidding!

Whew!

Interesting MFJ-1804 biquad antenna. In order to get 12dBi gain out
of something like that, it would either need to be 2 biquads and some
kind of power splitter, or one of those extended biquads as in:
http://www.larsen-b.com/Article/201.html

Ya, I have my doubts about the 12 dbi gain figure. I have built a few
biquads, (with your feed point correction), it has amazed me how easy
it is to make them work (at 2.4 Ghz), I always thought the short
wavelength would cause many dimensional errors. Yes, they work, what
does "work" mean :-)
I've settled on a flat panel antenna for my boat antenna
(15dbi manu. spec.) At one time I had a dish with biquad feed, but it
was to tight, when the wind blew or tide changed it would miss the target.
Do you believe the 20 dbi figure?
http://www.ebay.com/itm/20-DBi-2-4GH...25749399603295

Mikek

MFJ-868 SWR/Wattmeter
 

On Thu, 01 Sep 2011 09:35:37 -0500, amdx wrote:

Ya, I have my doubts about the 12 dbi gain figure.

Everyone lies about gain, but that's ok because few people can measure
the gain (and get reproducible results).

I have built a few
biquads, (with your feed point correction), it has amazed me how easy
it is to make them work (at 2.4 Ghz), I always thought the short
wavelength would cause many dimensional errors. Yes, they work, what
does "work" mean :-)

For a given physical size antenna, high gain antennas imply narrow
bandwidth and critical construction. On the other foot, low gain
antennas, such as the biquad, is fairly broadband, and therefore not
particularly critical to construct. What's fun is to attach the
antenna to a reflection coefficient bridge or directional coupler,
http://pe2er.nl/wifiswr/
http://www.qsl.net/n9zia/rlb/texscan.png
http://www.qsl.net/n9zia/rlb/
sweep generator, and oscilloscope to look at the VSWR curve. Then try
moving things around. On my crowded workbench, location of the
antenna relative to the highly reflective test equipment make a huge
difference. The changes do not really have a big effect on antenna
operation, but they certainly present a different picture as compared
to the nice clean curves on the data sheets.

Do you believe the 20 dbi figure?
http://www.ebay.com/itm/20-DBi-2-4GH...25749399603295

I guess you missed my previous rant on the subject. I bought two of
those yagi antennas (for $6/ea incidentally) just to see what was
wrong with them.
http://802.11junk.com/jeffl/crud/wi-fi-yagi-that-sucks.jpg
Notice the really crude dipole driven element hiding under the
plastic. That isn't going to work. No balun, no matching, exposed
coax wires, and offset from the center line too much. Some tests
showed that it has more gain to the side than towards the front. I
also suspect that the rather large size boom diameter was not
considered in calculating the element lengths. A piece of total junk,
but at the price, probably sells well (which is why I like the antenna
biz).

The above yagi is apparently a cost reduced clone of a similar yagi
that does have a proper feed and a more realistic gain claim. I think
(not sure) that this is the one:
http://www.ebay.com/itm/250847584296
I have no idea if this one works any better.

This is a more reputable source:
http://www.comtelcoantennas.com/yagi_1_8_2_4_ghz.htm
Note the radically higher prices and the 14dBi gain for a 16 element
yagi:
http://www.comtelcoantennas.com/PDF%20Datasheets/Y422416.pdf

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

MFJ-868 SWR/Wattmeter
 

On 9/1/2011 10:11 AM, Jeff Liebermann wrote:

For a given physical size antenna, high gain antennas imply narrow
bandwidth and critical construction. On the other foot, low gain
antennas, such as the biquad, is fairly broadband, and therefore not
particularly critical to construct. What's fun is to attach the
antenna to a reflection coefficient bridge or directional coupler,
http://pe2er.nl/wifiswr/
http://www.qsl.net/n9zia/rlb/texscan.png
http://www.qsl.net/n9zia/rlb/
sweep generator, and oscilloscope to look at the VSWR curve. Then try
moving things around. On my crowded workbench, location of the
antenna relative to the highly reflective test equipment make a huge
difference. The changes do not really have a big effect on antenna
operation, but they certainly present a different picture as compared
to the nice clean curves on the data sheets.


You're building a CW radar, basically.

That's how near field ranges work, too.

MFJ-868 SWR/Wattmeter
 

On Fri, 02 Sep 2011 09:38:05 -0700, Jim Lux
wrote:

On 9/1/2011 10:11 AM, Jeff Liebermann wrote:

For a given physical size antenna, high gain antennas imply narrow
bandwidth and critical construction. On the other foot, low gain
antennas, such as the biquad, is fairly broadband, and therefore not
particularly critical to construct. What's fun is to attach the
antenna to a reflection coefficient bridge or directional coupler,
http://pe2er.nl/wifiswr/
http://www.qsl.net/n9zia/rlb/texscan.png
http://www.qsl.net/n9zia/rlb/
sweep generator, and oscilloscope to look at the VSWR curve. Then try
moving things around. On my crowded workbench, location of the
antenna relative to the highly reflective test equipment make a huge
difference. The changes do not really have a big effect on antenna
operation, but they certainly present a different picture as compared
to the nice clean curves on the data sheets.

You're building a CW radar, basically.
That's how near field ranges work, too.

Yep. Also known as a proximity detector and possibly a really bad
interferometer. I can see cars driving by, people moving around in
the house, trees swaying in the wind, and the opening and closing of
doors and windows. These are all easy to identify on the sweep
because they all move around. Only the major dip in the VSWR curve,
near resonance, remains fairly stable.

Back to MFJ bashing... On my desk is yet another MFJ-259B for
repair, probably with the two blown shottky diodes that I previously
mentioned. One of the local hams heard me talking about the problem
over the local repeater, and decided that maybe I could fix it for
him. Usually, they wait until the week before Field Day for such
repairs but he's installing a tower next week and wants it fixed
yesterday. Sigh.



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

MFJ-868 SWR/Wattmeter
 

Jeff Liebermann wrote:


Back to MFJ bashing... On my desk is yet another MFJ-259B for
repair, probably with the two blown shottky diodes that I previously
mentioned. One of the local hams heard me talking about the problem
over the local repeater, and decided that maybe I could fix it for
him. Usually, they wait until the week before Field Day for such
repairs but he's installing a tower next week and wants it fixed
yesterday. Sigh.


Have you discussed this with Martin Jue? I think it's rude to bad-mouth
a fellow ham behind his back. I don't see how the diodes are a problem,
if you follow directions. If you live somewhere dusty or snowy and dry
enough to make static, use a gamma match or an UnUn or some other means
to keep your antenna at DC ground. I have been using germanium diodes
for 50 years and can't remember frying one in a small signal RF
application. What does Martin say about bleeder resistors?

I have a 4" pigtail around the ground lug and a male banana plug on the
end that lives in the middle of the SO-239, unless I am making
measurements. I use the banana plug as a shorting stick to neutralize
any residual capacitive charge in the device/coaxial cable under test.
Center conductor to cable ground. If I am especially concerned I'll
groung the 259B to my extensive safety ground system. When dealing with
little diodes you always want to make sure none of the sparks go through
them. Ground, ground and ground.

MFJ-868 SWR/Wattmeter
 

On 03 Sep 2011 13:56:30 GMT, dave wrote:

Jeff Liebermann wrote:


Back to MFJ bashing... On my desk is yet another MFJ-259B for
repair, probably with the two blown shottky diodes that I previously
mentioned. One of the local hams heard me talking about the problem
over the local repeater, and decided that maybe I could fix it for
him. Usually, they wait until the week before Field Day for such
repairs but he's installing a tower next week and wants it fixed
yesterday. Sigh.

Have you discussed this with Martin Jue?

No. I don't own an MFJ-259B. This is my 3rd(?) repair for what
appears to be exactly the same problem. I don't see how calling MFJ
will prove anything as they are apparently aware of the ESD problem.
See quotes from manual below.

I also reverse engineered the MFJ-1800 antenna, and also decided that
it has a problem. I have not called MFJ on these issues. Too busy
and too lazy.

I think it's rude to bad-mouth
a fellow ham behind his back.

Would you prefer I keep it secret and not tell fellow hams how to fix
it and why I think they blow up? I believe that I clearly labelled my
guesswork as conjecture and not fact. If hams were only allowed to
discuss things that are absolutely certain, the airwaves would be
silent.

I don't see how the diodes are a problem,
if you follow directions.

The units that are failing are not mine. I have no control over how
they are used.

As I vaguely recall, one failed while connected to some HF wire
antenna, the 2nd failed while plugging in a variety of calibrated
loads on the bench, and the most recent failed while attached to a
mobile HF antenna. It was difficult to determine the exact cause of
each failure because the unit did not just die, but instead started
producing insane readings. In all cases, the user thought something
was wrong with the antenna or loads, not the MJF-259B.

If you live somewhere dusty or snowy and dry
enough to make static, use a gamma match or an UnUn or some other means
to keep your antenna at DC ground.

Attach a high impedance voltmeter to a wire antenna blowing in the
wind and note the DC voltage produced. In my area, the humidity
rarely goes below about 30%, so static build up should not be a
problem when attached to an antenna. Currently, the humidity is
80-90% (morning fog), but when we get the hot dry winds from the
desert, the humidity will drop sufficiently low to cause problems for
a few daze.

What I believe is killing the diodes is not RF. It's the user
building up a static charge on plastic seat covers, synthetic clothes,
plastic carpet, etc, and discharging it into the antenna connector
when plugging in the antenna connector. Incidentally, one of my
customers with chronic equipment failures was traced to a negative ion
generator, which produced impressive high voltages on nearby object.

I have been using germanium diodes
for 50 years and can't remember frying one in a small signal RF
application.

How many of these germanium diodes were directly connected to the
antenna connector?

Apparently you missed my previous rant on the topic. See the
schematic extract of the RF section at:
http://802.11junk.com/jeffl/crud/MFJ-259B-RF-section.jpg
Notice the directly connected diodes. The diodes in question are
Avago HSMS-2820 zero bias shottky diodes.
http://www.avagotech.com/docs/AV02-1320EN
15V Max PIV is rather low. It won't take much voltage at the antenna
go exceed 15V. The two 47K resitors going to 0.01uf bypass caps make
an effective ground to any fast risetime voltage spike at the antenna.
An important clue is that BOTH D3 and D4 appear to be blown each time,
which implies an external failure, not a component failure.

What does Martin say about bleeder resistors?

I don't know. I haven't discussed this or any of my allegations with
him or MFJ support.

I have a 4" pigtail around the ground lug and a male banana plug on the
end that lives in the middle of the SO-239, unless I am making
measurements. I use the banana plug as a shorting stick to neutralize
any residual capacitive charge in the device/coaxial cable under test.
Center conductor to cable ground. If I am especially concerned I'll
groung the 259B to my extensive safety ground system. When dealing with
little diodes you always want to make sure none of the sparks go through
them. Ground, ground and ground.

Yep. That's a good way to provide some protection. However, there's
no protection while you're juggling connectors when you run the risk
of a static discharge to the center of the coax connector.

I don't recall reading such a procedure in the user manual. However,
there are plenty of warning:
http://www.mfjenterprises.com/pdffiles/MFJ-259B.pdf
In section 4.1:
WARNING: NEVER APPLY EXTERNAL VOLTAGES OR RF SIGNALS TO THE
ANTENNA CONNECTOR.
and in 5.1:
WARNING: NEVER APPLY RF OR ANY OTHER EXTERNAL VOLTAGES TO THE
ANTENNA PORT OF THIS UNIT. THIS UNIT USES ZERO BIAS DETECTOR
DIODES THAT ARE EASILY DAMAGED BY EXTERNAL VOLTAGES OVER A
FEW VOLTS.
and in 5.2:
WARNING: NEVER APPLY EXTERNAL VOLTAGES OR RF SIGNALS TO THE
ANTENNA CONNECTOR. PROTECT THIS PORT FROM ESD.

Clear enough. It would appear that MFJ is fully away of the fragile
nature of the input circuitry.




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

MFJ-868 SWR/Wattmeter
 

On Sat, 03 Sep 2011 09:25:31 -0700, Jeff Liebermann
wrote:

I have a 4" pigtail around the ground lug and a male banana plug on the
end that lives in the middle of the SO-239, unless I am making
measurements. I use the banana plug as a shorting stick to neutralize
any residual capacitive charge in the device/coaxial cable under test.
Center conductor to cable ground. If I am especially concerned I'll
groung the 259B to my extensive safety ground system. When dealing with
little diodes you always want to make sure none of the sparks go through
them. Ground, ground and ground.

Hmmm.... The problem might be the SO-239 connector. When you plug
something into that connector, it connects the center pin first, and
then the ground. If it were replaced by an N connector, the ground
would make contact before the center pink, thus offering some added
protection. I'll see if the connector can be replaced.

http://www.w8ji.com/mfj-259b_calibration.htm
Most Likely Failures

Other than manufacturing errors, the detector diodes clearly
stand out as the most common problem. They are the most easily
damaged devices in the analyzer. If you have a sudden problem,
it is most likely a defective detector diode. Diode damage almost
always comes from accidentally applying voltage on the antenna
port.

Why are the diodes so sensitive? In order for the detectors to be
accurate within a fraction of a percent (one bit), detector diodes
must have very low capacitance and a very low threshold voltage.
This means the diodes, through necessity, must be low-power
zero-bias Schottky microwave detector diodes. The same
characteristics that make them accurate and linear cause the diodes
to be especially sensitive to damage from small voltage spikes.
ALWAYS discharge large antennas before connecting them to the
analyzer! Never apply external voltages greater than 3 volts to the
antenna port!

and

Because the detector is broadband and because it is dc coupled
to the antenna, any external voltage across the input port
causes measurement errors. It is the accumulated voltage of
multiple sources that is most important, not the strength of
any individual signal. Because of that, large antennas should
be tested at times when propagated signals in the range of the
antenna's response are at minimum strength.

W8JI designed the MFJ-259b.



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

MFJ-868 SWR/Wattmeter
 

On Sat, 03 Sep 2011 09:25:31 -0700, Jeff Liebermann
wrote:

The two 47K resitors going to 0.01uf bypass caps make
an effective ground to any fast risetime voltage spike at the antenna.

Hi Jeff,

I'm sure you are perfectly aware of the single point of failure in
that generality.

Few Caps exhibit 0.01uF (when so marked) to transients (where it is
presumed they will exhibit 1/2*pi*f*c reactance to the risetime).

When we (silverbacks) got into this game, (the preferable) mica caps
were available, snipped out of the nearest sacrificial TV or radio.
Trying to read those several styles of color coding was the biggest
hurdle, but I had plenty in my junk-box.

Ceramic is ubiquitous, now, and far from choice in these matters,
unless you do deep research (maybe).

You got any favorites that respond to this?

73's
Richard Clark, KB7QHC

MFJ-868 SWR/Wattmeter
 

On Sat, 03 Sep 2011 09:51:00 -0700, Jeff Liebermann
wrote:

(...)

K0TO is collecting voltage measurement of the MFJ-259b for the purpose
of identifying blown diodes:
http://www.k0to.us/HAM/MFJ%20Diode%20measurements/Gather_MFJ_Data.htm
Estimates of the correct values:
http://www.k0to.us/HAM/MFJ%20Diode%20measurements/MFJ-259B%20Test%20Point%20Voltages.htm

Incidentally, his schematic at:
http://www.k0to.us/HAM/MFJ%20Diode%20measurements/11-17-sch_mfj259b-BW.pdf
looks better than most of the scans I've seen.

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

MFJ-868 SWR/Wattmeter
 

On 9/3/2011 1:32 PM, Jeff Liebermann wrote:
On Sat, 03 Sep 2011 09:51:00 -0700, Jeff
wrote:

(...)

K0TO is collecting voltage measurement of the MFJ-259b for the purpose
of identifying blown diodes:
http://www.k0to.us/HAM/MFJ%20Diode%20measurements/Gather_MFJ_Data.htm
Estimates of the correct values:
http://www.k0to.us/HAM/MFJ%20Diode%20measurements/MFJ-259B%20Test%20Point%20Voltages.htm

Incidentally, his schematic at:
http://www.k0to.us/HAM/MFJ%20Diode%20measurements/11-17-sch_mfj259b-BW.pdf
looks better than most of the scans I've seen.


Good resources. Thanks, Jeff.

John - KD5YI

MFJ-868 SWR/Wattmeter
 

On Sat, 03 Sep 2011 11:31:51 -0700, Richard Clark
wrote:

On Sat, 03 Sep 2011 09:25:31 -0700, Jeff Liebermann
wrote:

The two 47K resitors going to 0.01uf bypass caps make
an effective ground to any fast risetime voltage spike at the antenna.

I'm sure you are perfectly aware of the single point of failure in
that generality.

I am? Ummm... well, I guess so.

Few Caps exhibit 0.01uF (when so marked) to transients (where it is
presumed they will exhibit 1/2*pi*f*c reactance to the risetime).

True. They all have some internal resistance to overcome. However,
that's negligible resistance when compared to that of a static blast.
Static electricity has lots of potential (volts), but is only able to
deliver small amounts of current. That's why we don't get
electrocutes by the potential (voltage) difference between our head
and our feet. Dividing the large voltage, by the tiny current,
results in a fairly substantial source resistance. I'm too lazy to
look it some real numbers, but I'm sure it's in mega ohms. The 47K
resistance, and whatever ESR the 0.01uF contributes, has little effect
on the energy delivered to the shottky diode.

Incidentally, if the source resistance of the static blast was much
less, then the diode would not simply be fried. It would probably
explode.

When we (silverbacks) got into this game, (the preferable) mica caps
were available, snipped out of the nearest sacrificial TV or radio.
Trying to read those several styles of color coding was the biggest
hurdle, but I had plenty in my junk-box.

Dumpster diving in Henry Radio's trash can in West Smog Angeles was
one of my favorite after skool exercises. Salvaging old TV chassis
and dead tubes were the grand prizes. Silver mica caps came a close
second.

Ceramic is ubiquitous, now, and far from choice in these matters,
unless you do deep research (maybe).

Ceramic is cheap. I was a big fan of porcelain caps from AVX in big
power amps. If you wanna handle current, there's nothing better.
Silver mica would get hot, ceramic would explode, and everything else
was either too big or too expensive.

Incidentally, I don't think they make 0.01uF silver mica caps. The
biggest I played with were in antenna tuners at 4700pF (or should I
say uuF for nostalgia purposes).

You got any favorites that respond to this?

I don't understand the question. Favorite what?

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

MFJ-868 SWR/Wattmeter
 

On Sat, 03 Sep 2011 15:51:10 -0700, Jeff Liebermann
wrote:

Few Caps exhibit 0.01uF (when so marked) to transients (where it is
presumed they will exhibit 1/2*pi*f*c reactance to the risetime).

True. They all have some internal resistance to overcome.

It goes beyond that. Extrapolating from power applications hides the
defects of ceramic.

At HF/VHF and above, successful applications comes from throwing uF
solutions at pF problems. Ceramic's performance reveals inductive
reactance above 1-10 MHz. ESR also exhibits the same turn-around in
the same frequency range. Ceramic temperature coefficient is (Y5V)
goes into the toilet in weather that most of the south and eastern
seaboard has seen this summer. XR7 voltage coefficient causes
capacity to plummet at the voltages you offer for static. Over time,
ceramics lose capacity for simply having been in service for a while.

Aside from that, they work fine.

I was a big fan of porcelain caps from AVX in big
power amps. If you wanna handle current, there's nothing better.

However, those ceramics are 1,000 times (min.) larger than what you
have recommended. They serve an entirely different agenda.

AVX discusses these issues in much the same terms (for those larger
caps too) at:
http://www.avx.com/docs/techinfo/mlc-tant.pdf

Incidentally, I don't think they make 0.01uF silver mica caps. The
biggest I played with were in antenna tuners at 4700pF

Where there is every chance that one silver mica head-to-head with the
ceramic actually exhibit better performance (protecting the diodes).
Perhaps with the scarcity of silver mica, however, 10uF ceramics would
make do (it is not like any precision is demanded to force a selection
of 0.01uF which is boilerplate recommendation from the 50s).

73's
Richard Clark, KB7QHC

MFJ-868 SWR/Wattmeter
 

Jeff Liebermann wrote:



Yep. That's a good way to provide some protection. However, there's
no protection while you're juggling connectors when you run the risk
of a static discharge to the center of the coax connector.

I don't recall reading such a procedure in the user manual. However,
there are plenty of warning:
http://www.mfjenterprises.com/pdffiles/MFJ-259B.pdf
In section 4.1:
WARNING: NEVER APPLY EXTERNAL VOLTAGES OR RF SIGNALS TO THE
ANTENNA CONNECTOR.
and in 5.1:
WARNING: NEVER APPLY RF OR ANY OTHER EXTERNAL VOLTAGES TO THE
ANTENNA PORT OF THIS UNIT. THIS UNIT USES ZERO BIAS DETECTOR
DIODES THAT ARE EASILY DAMAGED BY EXTERNAL VOLTAGES OVER A
FEW VOLTS.
and in 5.2:
WARNING: NEVER APPLY EXTERNAL VOLTAGES OR RF SIGNALS TO THE
ANTENNA CONNECTOR. PROTECT THIS PORT FROM ESD.

Clear enough. It would appear that MFJ is fully away of the fragile
nature of the input circuitry.

I learned to ground everything working on transmitters the size of
houses. The B+ is bled and grounded when you open the door, but you
still ground anything metal before you touch it. It translated nicely to
CMOS procedures on the bench. I am a grounding fool because I know any
conductor can store a charge lethal to solid state and that any friction
produces a charge. (I humidify, too!)

MFJ-868 SWR/Wattmeter
 

On 04 Sep 2011 14:05:13 GMT, dave wrote:

Jeff Liebermann wrote:
Yep. That's a good way to provide some protection. However, there's
no protection while you're juggling connectors when you run the risk
of a static discharge to the center of the coax connector.

I don't recall reading such a procedure in the user manual. However,
there are plenty of warning:
http://www.mfjenterprises.com/pdffiles/MFJ-259B.pdf
In section 4.1:
WARNING: NEVER APPLY EXTERNAL VOLTAGES OR RF SIGNALS TO THE
ANTENNA CONNECTOR.
and in 5.1:
WARNING: NEVER APPLY RF OR ANY OTHER EXTERNAL VOLTAGES TO THE
ANTENNA PORT OF THIS UNIT. THIS UNIT USES ZERO BIAS DETECTOR
DIODES THAT ARE EASILY DAMAGED BY EXTERNAL VOLTAGES OVER A
FEW VOLTS.
and in 5.2:
WARNING: NEVER APPLY EXTERNAL VOLTAGES OR RF SIGNALS TO THE
ANTENNA CONNECTOR. PROTECT THIS PORT FROM ESD.

Clear enough. It would appear that MFJ is fully away of the fragile
nature of the input circuitry.

I learned to ground everything working on transmitters the size of
houses. The B+ is bled and grounded when you open the door, but you
still ground anything metal before you touch it. It translated nicely to
CMOS procedures on the bench. I am a grounding fool because I know any
conductor can store a charge lethal to solid state and that any friction
produces a charge. (I humidify, too!)

Such an extreme RF environment is not necessary to blow up the diodes.
None of the 3ea MFJ-259B boxes that I replaced required a transmitter
the size of a house to blow up. Much as the protective procedures
that you are recommending are genuinely useful, the instrument first
has to protect itself.

Assuming they were all fried by ESD, I tried to conjur a method that
would protect the existing design. As simple bleeder resistor to
ground will only help under trivial situations. The worst case
senario, of holding the instrument in one hand, and plugging in a
PL259 that is connected to an ungrounded antenna with a large static
charge, is all too common. Changing to an type-N connector will help
because the grounded shield is connected first, instead of the center
conductor, as in the SO-239.

Back to back diodes might work if the RF levels are low enough. The
non-linearity of the diodes will cause measurement accuracy problems
and rectify any off frequency RF going into antenna connector.

I'm tempted to try 1:1 RF broadband transformers which should work
over octave frequency ranges. Better yet, a tuned 1:1 RF xformer, to
improve the front end selectivity so that it can be used in an RF
polluted environment[1]. That would work, but will also be very
clumsy and expensive.

I know of several devices where failure is sufficiently common, that
spare parts are included with the instrument, and the components are
in easily accessible sockets. While not a great solution, it does
make some sense.

Other than attaching a grounded anchor chain to the MFJ-259b, spraying
holy water around the area to increase humidity, or carrying various
anti-static protection devices, do you have any suggestions as to how
the instrument could better protect itself from ESD?






[1] Attaching a wattmeter to the typical VHF antenna on a mountain
top that is colocated with FM/TV xmitters will show a watt or three of
RF. The front end of the MFJ-259b is not going to like that.

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

MFJ-868 SWR/Wattmeter
 

On Sat, 03 Sep 2011 22:29:49 -0700, Richard Clark
wrote:

At HF/VHF and above, successful applications comes from throwing uF
solutions at pF problems.

That's not a problem. In order to get obtain decent bypassing across
5 octaves of bandwidth (2-30MHz), one needs to have multiple capacitor
values and types in parallel. The self-resonant characteristics of
the capacitors is the limiting factor. At some frequency, every
capacitor, and its associated lead inductance, will exhibit an
impedance dip commonly known as series resonance. Below this
frequency, the capacitor will look ummm... like a capacitor. Above
this frequency, it will be more like an inductor.
http://www.ecircuitcenter.com/Circuits/cmodel1/cmodel1.htm

None this has anything useful to do with the 0.01uf caps in the
instrument. The diodes are in series with 47K resistors, which are
much larger than any inductive reactance that the 0.1uf bypass
capacitor might present. Since the MFJ-259b only works well up to
maybe a 10:1 VSWR or 5Kohms, the 47K is sufficiently larger than
whatever reactance is presented by the 0.01uf to make the capacitor
characteristics to not be an issue.

While component selection and circuit design are interesting topics,
the current problem is MFJ design quality, MFJ-259b, ESD protection,
and chronic detector diode failures.





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

MFJ-868 SWR/Wattmeter
 

On Sat, 03 Sep 2011 09:25:31 -0700, Jeff Liebermann
wrote:

Apparently you missed my previous rant on the topic. See the
schematic extract of the RF section at:
http://802.11junk.com/jeffl/crud/MFJ-259B-RF-section.jpg
Notice the directly connected diodes. The diodes in question are
Avago HSMS-2820 zero bias shottky diodes.
http://www.avagotech.com/docs/AV02-1320EN
15V Max PIV is rather low. It won't take much voltage at the antenna
go exceed 15V. The two 47K resitors going to 0.01uf bypass caps make
an effective ground to any fast risetime voltage spike at the antenna.
An important clue is that BOTH D3 and D4 appear to be blown each time,
which implies an external failure, not a component failure.

Argh. I just started working on the analyzer, and found a few errors,
all of which are my mistakes.

1. The antenna analyzer that arrived (3 days late) today is not the
expected MFJ-259B but an MFJ-269. The difference is that the MFJ-269
goes up to UHF frequencies. The front end is similar, but not
identical.

2. The MFJ-269 has a type-N connector, while the MFJ-259 has a UHF
SO-239 connector. So much for the idea of substituting a type-N
connector.

3. Despite my idea of installing a bleeder resistor on the antenna
connector to drain off the static charge, the antenna connector
already shows 50 ohms resistance to ground. I missed this path
because of the rather difficult to read schematic of the MFJ-259 RF
section at:
http://802.11junk.com/jeffl/crud/MFJ-259B-RF-section.jpg
The path is from the antenna connector, through R24, L11, and then to
ground.

The MFJ-269 schematic is easier to read at:
http://802.11junk.com/jeffl/crud/MFJ-269-RF-section.jpg
which goes through R88, L12, and then to ground.

What bugs me is that the diodes are blowing up despite this rather low
resistance to ground. Either hams are finding some rather high power
ESD sources with which to blow up their analyzers, or some other
failure mechanism is involved.

I haven't finished working on the MFJ-269 quite yet. A report and
photos when I'm done.

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

MFJ-868 SWR/Wattmeter
 

On 9/5/2011 2:58 AM, Jeff Liebermann wrote:

What bugs me is that the diodes are blowing up despite this rather low
resistance to ground. Either hams are finding some rather high power
ESD sources with which to blow up their analyzers, or some other
failure mechanism is involved.

I've had more than one device damaged while operating around High power
equipment, with probable RF in the shack excursions. My MFJ analyzer, an
old Sony camera that had it's floppy drive head destroyed, and a couple
other things I cannot remember.

I haven't had any problem with the analyzer since installing my dummy
load on it.

But that brings up something interesting. You're reading 50 Ohms on the
input connector? Isn't that going to make *all* readings somewhere near 1:1?


- 73 de Mike N3LI -

MFJ-868 SWR/Wattmeter
 

Mike Coslo wrote:

On 9/5/2011 2:58 AM, Jeff Liebermann wrote:

What bugs me is that the diodes are blowing up despite this rather low
resistance to ground. Either hams are finding some rather high power
ESD sources with which to blow up their analyzers, or some other
failure mechanism is involved.

I've had more than one device damaged while operating around High power
equipment, with probable RF in the shack excursions. My MFJ analyzer, an
old Sony camera that had it's floppy drive head destroyed, and a couple
other things I cannot remember.

I haven't had any problem with the analyzer since installing my dummy
load on it.

But that brings up something interesting. You're reading 50 Ohms on the
input connector? Isn't that going to make *all* readings somewhere near 1:1?


- 73 de Mike N3LI -

Is it possible that 50 Ohms at DC is much higher Z at MF/HF/VHF?

MFJ-868 SWR/Wattmeter
 

Mike Coslo wrote in
:

But that brings up something interesting. You're reading 50 Ohms on
the input connector? Isn't that going to make *all* readings somewhere
near 1:1?

If you look at the circuit, there is a DC path from the centre pin, via the
50 ohm resistor that forms part of the measurement bridge, and an RFC to
ground. That would look like 50 ohms at DC, but it will not prevent the
measurement bridge working in the way you suggest.

Owen

MFJ-868 SWR/Wattmeter
 

Jeff Liebermann wrote in
:

What bugs me is that the diodes are blowing up despite this rather low
resistance to ground. Either hams are finding some rather high power
ESD sources with which to blow up their analyzers, or some other
failure mechanism is involved.

But the reality is that people do damage these things... and so the
method you suggested earlier is not likely to be sufficient to protect
them.

I am careful to avoid connecting an instrument of this type to an
antenna system unless I have drained any static charge first, and avoid
other transmitters on air nearby.

An easy trap is to connect the analyser or the like to an antenna, then
start working on the antenna without considerig the risk of introducing
a spike during the work.

Painful, but worth disconnecting immediately after each measurement. Not
as painful if BNC connectors are used.(Why did they build those things
with SO239... don't answer!)

Owen

MFJ-868 SWR/Wattmeter
 

On 9/5/2011 6:47 PM, Owen Duffy wrote:
Jeff wrote in
:

What bugs me is that the diodes are blowing up despite this rather low
resistance to ground. Either hams are finding some rather high power
ESD sources with which to blow up their analyzers, or some other
failure mechanism is involved.

But the reality is that people do damage these things... and so the
method you suggested earlier is not likely to be sufficient to protect
them.

I am careful to avoid connecting an instrument of this type to an
antenna system unless I have drained any static charge first, and avoid
other transmitters on air nearby.

I think you are correct. I do think that more care needs to be taken
with these devices than with a lot of other electronic items Hams use.
I liken them to RF attenuator blocks. You see a lot of them in company
repair shops, because people are often trying to put too much power
through them.

After getting the new replacement analyzer, and using the dummy load,
and I do the static drain too, plus I keep it away from other people,
it's lasted FB.

- 73 de Mike N3LI -

MFJ-868 SWR/Wattmeter
 

On Mon, 05 Sep 2011 19:46:16 -0400, Mike Coslo wrote:

I liken them to RF attenuator blocks. ... people are often trying to put too much power
through them.

This leaves me wondering about intentions:

"Must - get - more - power - out - of this attenuator!"

73's
Richard Clark, KB7QHC

MFJ-868 SWR/Wattmeter
 

On Mon, 05 Sep 2011 11:47:21 -0400, Mike Coslo wrote:

On 9/5/2011 2:58 AM, Jeff Liebermann wrote:

What bugs me is that the diodes are blowing up despite this rather low
resistance to ground. Either hams are finding some rather high power
ESD sources with which to blow up their analyzers, or some other
failure mechanism is involved.

I've had more than one device damaged while operating around High power
equipment, with probable RF in the shack excursions. My MFJ analyzer, an
old Sony camera that had it's floppy drive head destroyed, and a couple
other things I cannot remember.

I used to have a Sony Mavica MVC-FD73 camera. Nice camera for 640x480
closeups. I gave it as a present to a friend, who bashed in the LCD
display. I repaired it, and she bashed it in again. I gave up.

Per one of my previous postings, none of the failures coincided with
any high power RF nearby. One was connected to an antenna when it
probably failed, but there were no transmitters active. I've tried
interrogating the owners and none claim that they did anything
potentially destructive. I've been assuming it was ESD, not RF that
is killing the diodes, but I'm not sure.

I just tested the diodes. 3 diodes were open, one was ok.

I haven't had any problem with the analyzer since installing my dummy
load on it.

Probably a good idea.

But that brings up something interesting. You're reading 50 Ohms on the
input connector? Isn't that going to make *all* readings somewhere near 1:1?

Owen answered that question. L12 (100 uH) is in series with the 50
bridge "load" and therefore allows measurements to be made without
padding the antenna with an additional 50 ohms.

I threw together a web page on the MFJ-269 repair:
http://802.11junk.com/jeffl/MFJ-269-repair/
Hopefully, this will help anyone following my succession of mistakes
and screwups. I'm not really done as I want to verify that it's
working correctly and hopefully not require recalibration.

Incidentally, no lock washers on any of the screws inside.


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

MFJ-868 SWR/Wattmeter
 

Mike Coslo wrote in news:j43n06$ds52$1
@tr22n12.aset.psu.edu:

;... plus I keep it away from other people,
it's lasted FB.

That is probably key, don't trust other people... and be pretty wary of
one's self!

It is interesting you mention the Other Persons thing... I am in the
process of drafting a web article commenting on the advice that is so
common these days on online fora, "can you borrow an analyser?..."

This is almost always offered to some newbie who is having trouble
"tuning up his antenna".

Most don't understand complex numbers, impedance, transmission lines,
etc etc and someone tells them get an analyser and tune for resonance. I
can only guess that the advisers are equally lacking in knowledge and
experience, but I will concede that the analyser dumbed down to a VSWR
meter with self contained RF source than can be tuned outside band
limits, can be a pretty handy thing.

But despite the fact that adjusting many, if not most antennas for
minimum VSWR is the sensible objective, the resonance brigade chimes in
with make X zero looking into some arbitrary length of feed line, a
condition which isn't always associated with minimum VSWR and may
actually not be achievable. Some newbies have clipped their mobile whip
away to make that discovery, but perhaps without understanding that it
was a likely outcome from that strategy.

So Mike, it is likely that someone borowing an analyser to fix a problem
wont obtain real benefit, but may damage it in the process.

Owen

MFJ-868 SWR/Wattmeter
 

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

MFJ-868 SWR/Wattmeter
 

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

MFJ-868 SWR/Wattmeter
 

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-868 SWR/Wattmeter
 

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"

MFJ-868 SWR/Wattmeter
 

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

MFJ-868 SWR/Wattmeter
 

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

MFJ-868 SWR/Wattmeter
 

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

MFJ-868 SWR/Wattmeter
 

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.

MFJ-868 SWR/Wattmeter
 

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?)

MFJ-868 SWR/Wattmeter
 

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

MFJ-868 SWR/Wattmeter
 

On 9/5/2011 8:21 PM, Jeff Liebermann wrote:

I've tried
interrogating the owners and none claim that they did anything
potentially destructive.

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


- 73 de Mike N3LI -

MFJ-868 SWR/Wattmeter
 

On 9/7/2011 11:54 AM, Jeff Liebermann wrote:

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).


Yeah, denial is de rigueur.

- 73 de Mike N3LI -

MFJ-868 SWR/Wattmeter
 

On 9/7/2011 2:43 PM, 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?)

People do odd things. Some times it is noobs, sometimes someone connects
the wrong cable and for some weird reason keys the transmitter,
sometimes it is someone just not thinking.

I've seen too much stuff to not believe really crazy things don't happen.

- 73 de Mike N3LI -

MFJ-868 SWR/Wattmeter
 

On 9/6/2011 12:58 AM, Owen Duffy wrote:

That is probably key, don't trust other people... and be pretty wary of
one's self!

It is interesting you mention the Other Persons thing... I am in the
process of drafting a web article commenting on the advice that is so
common these days on online fora, "can you borrow an analyser?..."

It is a good lesson, (be wary) and I think that MFJ makes another
"analyzer", the 207, which will suit the average user just fine, as long
as they are not needing V/UHF. It's cheap, simple, small, and I used one
before I got into HF mobile. It doesn't show anything but SWR. Given
that that is what most people are interested in, it suits well.

Then after a bit of reading, and talking they might start to look for
the interesting details, and the instruments to measure those details.

- 73 de Mike N3LI -

MFJ-868 SWR/Wattmeter
 

Mike Coslo wrote in :


Then after a bit of reading, and talking they might start to look for
the interesting details, and the instruments to measure those details.

Absolutely, if acquiring an MFJ259B or the like is a stimulus to learn more
of impedance, transmission lines and antennas, that is great.

Owen

MFJ-868 SWR/Wattmeter
 

On 07 Sep 2011 18:43:40 GMT, 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?)

Good question. I asked the same question when we discovered that the
radio clubs Kenwood something HF transceiver had most of the
components in the receiver front end vaporized or incinerated after a
contest weekend. My best guess(tm) is that someone plugged one of the
tangled mess of coax cables lurking behind the bench, into a live
transmitter, thinking it was an antenna cable. Few of cables are
labeled. Of course, nobody will admit to doing it.

My guess(tm) is that something like that happened with the MFJ.

Incidentally, in the box with the MFJ-269 was an N-male to UHF-female
adapter, which brings back the possibility of an ESD discharge when
the center pin was connected first.

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