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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 |
#2
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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 - |
#3
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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 |
#4
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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 |
#5
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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 - |
#6
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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 |
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