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#11
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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 |
#12
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Jason Dugas wrote: 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 manual states that the "stub" to be measured should be attached with a 50-ohm noninductive resistor in series to that center conductor of the Antenna connector on the analyzer. You may think it says this: MFJ-resistor-coax_center_conductor-- | ---------coax_shield------------ But maybe it means this: MFJ----------------coax_center_conductor-resistor- | ----------------coax_shield-------------------- I don't know what the MFJ documentation says - but if they want a 50 ohm resistor in series from the center conductor and the shield, put it at the far end of the length of coax you are testing. Connect it as shown in the bottom diagram by soldering the resistor directly across the center conductor and shield at the far end. The way I do it is to cut a physical 1/2 wavelength coax for the frequency, install a pl259 on one end and connect it to the MFJ. I install a 51 ohm resistor at the far end of the coax. When that resistor is 1/2 wave away *electrically*, it's impedance will appear across the PL259 plugged into the MFJ. You need to shorten the coax by snipping off some of the length at the far end, then reinstalling the resistor. When you have the thing at an electrical 1/2 wave you'll get Z=50 at the MFJ. Your VF is the length of that piece of coax divides by the original length. Maybe the MFJ documents a better way - I dunno - but I've used my way successfully. What would be a good way of making this connection? I've thought about it quite a bit; the best idea I've come up with so far is to have a small metal enclosure w/ two SO-239's mounted. The resistor would go from center conductor to center conductor of each SO-239. But this requires too many extra connections & lengths. Is there a better way to do this? Would love to see anyone else's experimental setup, particularly if there are pictures or details on the web. Thanks & 73, Jason KB5URQ |
#13
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Jason Dugas wrote: 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 manual states that the "stub" to be measured should be attached with a 50-ohm noninductive resistor in series to that center conductor of the Antenna connector on the analyzer. You may think it says this: MFJ-resistor-coax_center_conductor-- | ---------coax_shield------------ But maybe it means this: MFJ----------------coax_center_conductor-resistor- | ----------------coax_shield-------------------- I don't know what the MFJ documentation says - but if they want a 50 ohm resistor in series from the center conductor and the shield, put it at the far end of the length of coax you are testing. Connect it as shown in the bottom diagram by soldering the resistor directly across the center conductor and shield at the far end. The way I do it is to cut a physical 1/2 wavelength coax for the frequency, install a pl259 on one end and connect it to the MFJ. I install a 51 ohm resistor at the far end of the coax. When that resistor is 1/2 wave away *electrically*, it's impedance will appear across the PL259 plugged into the MFJ. You need to shorten the coax by snipping off some of the length at the far end, then reinstalling the resistor. When you have the thing at an electrical 1/2 wave you'll get Z=50 at the MFJ. Your VF is the length of that piece of coax divides by the original length. Maybe the MFJ documents a better way - I dunno - but I've used my way successfully. What would be a good way of making this connection? I've thought about it quite a bit; the best idea I've come up with so far is to have a small metal enclosure w/ two SO-239's mounted. The resistor would go from center conductor to center conductor of each SO-239. But this requires too many extra connections & lengths. Is there a better way to do this? Would love to see anyone else's experimental setup, particularly if there are pictures or details on the web. Thanks & 73, Jason KB5URQ |
#14
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#17
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"Marco S Hyman" wrote in message ... writes: Jason Dugas wrote: manual states that the "stub" to be measured should be attached with a 50-ohm noninductive resistor in series to that center conductor of the Antenna connector on the analyzer. You may think it says this: MFJ-resistor-coax_center_conductor-- | ---------coax_shield------------ But maybe it means this: MFJ----------------coax_center_conductor-resistor- | ----------------coax_shield-------------------- How does the 259 differ from the 259B? Does the 259 have a distance to fault mode? I ask because the instructions for the 'B say that coax lines must be *open* or *shorted*. Anything in between is verbotten. With the 'B you use the "distance to fault" mode to figure out the electrical length of the line then devide by the physical length to caluclate the velocity factor. // marc (KC7JL) The MFJ-259 does not have a "distance to fault" mode. It has an "SWR" meter and a "Resistance" meter. jason |
#18
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On 31 Oct 2003 13:43:32 -0800, (Tom Bruhns) wrote:
"Reg Edwards" wrote in message ... The velocity factor of ALL solid polyethylene coax cable, regardless of impedance, is 0.665 And this comes from someone who I could swear posted not long ago a table that had velocity factors for solid polyethylene cable that were significantly different from this magic number? But even if we just limit ourselves to HF and above, there's a problem: most "solid poly" cable I've encountered has small gas bubbles in the dielectric, and the v.f. does not measure exactly 0.665. Most of the time, the difference doesn't matter, but sometimes it does, and then it's not safe to assume it's 0.665. And of course a lot of cable these days uses foam dielectric, which can be noticably different from batch to batch. It's quite accurate figure, but the cables are not so accurate. Remember trying to make two RG213/U halfwave stubs for 100MHz, and was surprised to learn that the 'halfwave' length varied about an inch Suppose it wasn't only for the bubbles... 73 Jan-Martin, LA8AK http://home.online.no/~la8ak/ -- remove ,xnd to reply (Spam precaution!) |
#19
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On 31 Oct 2003 13:43:32 -0800, (Tom Bruhns) wrote:
"Reg Edwards" wrote in message ... The velocity factor of ALL solid polyethylene coax cable, regardless of impedance, is 0.665 And this comes from someone who I could swear posted not long ago a table that had velocity factors for solid polyethylene cable that were significantly different from this magic number? But even if we just limit ourselves to HF and above, there's a problem: most "solid poly" cable I've encountered has small gas bubbles in the dielectric, and the v.f. does not measure exactly 0.665. Most of the time, the difference doesn't matter, but sometimes it does, and then it's not safe to assume it's 0.665. And of course a lot of cable these days uses foam dielectric, which can be noticably different from batch to batch. It's quite accurate figure, but the cables are not so accurate. Remember trying to make two RG213/U halfwave stubs for 100MHz, and was surprised to learn that the 'halfwave' length varied about an inch Suppose it wasn't only for the bubbles... 73 Jan-Martin, LA8AK http://home.online.no/~la8ak/ -- remove ,xnd to reply (Spam precaution!) |
#20
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"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. |
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