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