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