|
My figures are correct to a better accuracy than can be determind by a cheap
and nasty antenna analyser. Now go away, leave the newsgroup in peace, and stop your childish nitpicking. |
Clearly, if I had an o-scope and a waveform generator, I wouldn't be farting
with this analyzer!! :) "Tim Shoppa" wrote in message om... "Jason Dugas" wrote in message ... I'm interested in measuring the velocity factor of some coax I have (more of an exercise than necessity). In order to do this, the MFJ-259 Operations ... What would be a good way of making this connection? Do you have a scope and a pulse generator? If so, just make a simple TDR (Time Domain Reflectometry) setup to measure the reflection from a un-terminated or shorted far end of the cable. You will get much more accurate results with substantially less effort. Don't get me wrong, the MFJ-259 is a good instrument. It's just that TDR is so quick and easy and unambiguous for propogation delay measurements. Tim. |
Clearly, if I had an o-scope and a waveform generator, I wouldn't be farting
with this analyzer!! :) "Tim Shoppa" wrote in message om... "Jason Dugas" wrote in message ... I'm interested in measuring the velocity factor of some coax I have (more of an exercise than necessity). In order to do this, the MFJ-259 Operations ... What would be a good way of making this connection? Do you have a scope and a pulse generator? If so, just make a simple TDR (Time Domain Reflectometry) setup to measure the reflection from a un-terminated or shorted far end of the cable. You will get much more accurate results with substantially less effort. Don't get me wrong, the MFJ-259 is a good instrument. It's just that TDR is so quick and easy and unambiguous for propogation delay measurements. Tim. |
Leading eventually to the suggestion that perhaps your MFJ analyzer
isn't worth the powder to blow it up... But I submit that you can make VF measurements on your cable with it, which may be limited by the accuracy with which you can measure the length of line you are looking at and not by the fact that you're using a relatively inexpensive instrument. Here's one way. If you look at a shorted stub 90 electrical degrees long in parallel with a 50 ohm resistor, you'll have to move the frequency by some value to resolve a difference. The amount you have to move the frequency probably determines the resolution you can get in VF. But if you make the line 270 degrees long, you'll have three times the sensitivity. So a hundred foot length of line, measured near 10 meters frequency, would be long enough to get very good resolution--it'd be on the order of 1530 electrical degrees. I'd be surprised if you had trouble seeing the difference between, say, 0.664 and 0.665 VF. But beware that the VF _does_ change with frequency, even over HF. There are times when _I_ care about that, even if some others don't. Cheers, Tom "Jason Dugas" wrote in message ... Asswipe, The question I posed wasn't "What's the velocity factor of ALL solid polyethylene coax cable". Next time read the question and answer it. If you don't know the answer then DON'T POST A REPLY! Too many ignorant people in these groups anymore! "Reg Edwards" wrote in message ... The velocity factor of ALL solid polyethylene coax cable, regardless of impedance, is 0.665 |
Leading eventually to the suggestion that perhaps your MFJ analyzer
isn't worth the powder to blow it up... But I submit that you can make VF measurements on your cable with it, which may be limited by the accuracy with which you can measure the length of line you are looking at and not by the fact that you're using a relatively inexpensive instrument. Here's one way. If you look at a shorted stub 90 electrical degrees long in parallel with a 50 ohm resistor, you'll have to move the frequency by some value to resolve a difference. The amount you have to move the frequency probably determines the resolution you can get in VF. But if you make the line 270 degrees long, you'll have three times the sensitivity. So a hundred foot length of line, measured near 10 meters frequency, would be long enough to get very good resolution--it'd be on the order of 1530 electrical degrees. I'd be surprised if you had trouble seeing the difference between, say, 0.664 and 0.665 VF. But beware that the VF _does_ change with frequency, even over HF. There are times when _I_ care about that, even if some others don't. Cheers, Tom "Jason Dugas" wrote in message ... Asswipe, The question I posed wasn't "What's the velocity factor of ALL solid polyethylene coax cable". Next time read the question and answer it. If you don't know the answer then DON'T POST A REPLY! Too many ignorant people in these groups anymore! "Reg Edwards" wrote in message ... The velocity factor of ALL solid polyethylene coax cable, regardless of impedance, is 0.665 |
Hmmm, I would have sworn the manual covers this, or is that just on the B
model?.... I'll have to try this, but how about just connecting a length of coax, open at the other end and looking for the min Z. The longer it is, the less effect of any "end effect". TO get around end effect, go to the shorted stub method...but... I don't understand the 50 ohm in parallel. I'd put some large R in parallel when trying the shorted-end method such that there will be a reading. If I recall, mine stops above 1500 ohms. So, I'd put, say, 1 K in parallel then find the two freqs where I see some specific value lower than the resistor alone, then take the geometric mean for the stub's resonant freq. Steve No U's in my email addr... "Tom Bruhns" wrote in message m... Leading eventually to the suggestion that perhaps your MFJ analyzer isn't worth the powder to blow it up... But I submit that you can make VF measurements on your cable with it, which may be limited by the accuracy with which you can measure the length of line you are looking at and not by the fact that you're using a relatively inexpensive instrument. Here's one way. If you look at a shorted stub 90 electrical degrees long in parallel with a 50 ohm resistor, you'll have to move the frequency by some value to resolve a difference. The amount you have to move the frequency probably determines the resolution you can get in VF. But if you make the line 270 degrees long, you'll have three times the sensitivity. So a hundred foot length of line, measured near 10 meters frequency, would be long enough to get very good resolution--it'd be on the order of 1530 electrical degrees. I'd be surprised if you had trouble seeing the difference between, say, 0.664 and 0.665 VF. But beware that the VF _does_ change with frequency, even over HF. There are times when _I_ care about that, even if some others don't. Cheers, Tom "Jason Dugas" wrote in message ... Asswipe, The question I posed wasn't "What's the velocity factor of ALL solid polyethylene coax cable". Next time read the question and answer it. If you don't know the answer then DON'T POST A REPLY! Too many ignorant people in these groups anymore! "Reg Edwards" wrote in message ... The velocity factor of ALL solid polyethylene coax cable, regardless of impedance, is 0.665 |
Hmmm, I would have sworn the manual covers this, or is that just on the B
model?.... I'll have to try this, but how about just connecting a length of coax, open at the other end and looking for the min Z. The longer it is, the less effect of any "end effect". TO get around end effect, go to the shorted stub method...but... I don't understand the 50 ohm in parallel. I'd put some large R in parallel when trying the shorted-end method such that there will be a reading. If I recall, mine stops above 1500 ohms. So, I'd put, say, 1 K in parallel then find the two freqs where I see some specific value lower than the resistor alone, then take the geometric mean for the stub's resonant freq. Steve No U's in my email addr... "Tom Bruhns" wrote in message m... Leading eventually to the suggestion that perhaps your MFJ analyzer isn't worth the powder to blow it up... But I submit that you can make VF measurements on your cable with it, which may be limited by the accuracy with which you can measure the length of line you are looking at and not by the fact that you're using a relatively inexpensive instrument. Here's one way. If you look at a shorted stub 90 electrical degrees long in parallel with a 50 ohm resistor, you'll have to move the frequency by some value to resolve a difference. The amount you have to move the frequency probably determines the resolution you can get in VF. But if you make the line 270 degrees long, you'll have three times the sensitivity. So a hundred foot length of line, measured near 10 meters frequency, would be long enough to get very good resolution--it'd be on the order of 1530 electrical degrees. I'd be surprised if you had trouble seeing the difference between, say, 0.664 and 0.665 VF. But beware that the VF _does_ change with frequency, even over HF. There are times when _I_ care about that, even if some others don't. Cheers, Tom "Jason Dugas" wrote in message ... Asswipe, The question I posed wasn't "What's the velocity factor of ALL solid polyethylene coax cable". Next time read the question and answer it. If you don't know the answer then DON'T POST A REPLY! Too many ignorant people in these groups anymore! "Reg Edwards" wrote in message ... The velocity factor of ALL solid polyethylene coax cable, regardless of impedance, is 0.665 |
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