Home |
Search |
Today's Posts |
|
#1
|
|||
|
|||
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!) |
#2
|
|||
|
|||
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 |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
MFJ-868 SWR/Wattmeter | Antenna | |||
MFJ-868 SWR/Wattmeter | Antenna | |||
Wattmeter help | Antenna | |||
WTB: LDG DWM-4 wattmeter | Swap | |||
FS: HF/VHF/UHF wattmeter, URM-120 | Swap |