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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 |
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!) |
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!) |
"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. |
"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. |
"Reg Edwards" wrote in message ...
"Tom Bruhns" wrote "Reg Edwards" 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? .... =============================== Your para. 1. You can swear till you're appoplectic black and blue in the face - it wasn't me. I'm not THAT stupid. So who was it then? .... ---- Reg. Hi Reg... Well, the Google archive says it was from you. Perhaps you DO have someone else posting under your name. See below. Or is the "VF" column not actually velocity factor? OTOH, I do agree with the posting below, that at low frequencies, beta becomes dependent more on R than on L, and thus the VF changes. Cheers, Tom From: Reg Edwards ) Subject: Coax Cable vs Freqency View this article only Newsgroups: rec.radio.amateur.antenna Date: 2003-08-12 17:41:24 PST For anyone who may be interested. Typical of RG-58 and RG-11 type cables. Zo = Ro - jXo Xo is always negative. Angle of Zo in degrees. Always negative. VF = relative velocity. Freq Ro jXo Angle VF ------ ------ ------ ------ ------ 50 Hz 967 -965 -44.95 0.034 1 kHz 220 -213 -44 0.151 10 kHz 80 -58 -36 0.41 100 kHz 56 -9.3 -9.5 0.59 1 MHz 52.4 -2.4 -2.7 0.63 10 MHz 50.7 -0.76 -0.86 0.65 100 MHz 50.2 -0.23 -0.27 0.66 Smith Chart calculations begin to be inaccurate around 2 MHz and below. So do SWR meters. |
"Reg Edwards" wrote in message ...
"Tom Bruhns" wrote "Reg Edwards" 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? .... =============================== Your para. 1. You can swear till you're appoplectic black and blue in the face - it wasn't me. I'm not THAT stupid. So who was it then? .... ---- Reg. Hi Reg... Well, the Google archive says it was from you. Perhaps you DO have someone else posting under your name. See below. Or is the "VF" column not actually velocity factor? OTOH, I do agree with the posting below, that at low frequencies, beta becomes dependent more on R than on L, and thus the VF changes. Cheers, Tom From: Reg Edwards ) Subject: Coax Cable vs Freqency View this article only Newsgroups: rec.radio.amateur.antenna Date: 2003-08-12 17:41:24 PST For anyone who may be interested. Typical of RG-58 and RG-11 type cables. Zo = Ro - jXo Xo is always negative. Angle of Zo in degrees. Always negative. VF = relative velocity. Freq Ro jXo Angle VF ------ ------ ------ ------ ------ 50 Hz 967 -965 -44.95 0.034 1 kHz 220 -213 -44 0.151 10 kHz 80 -58 -36 0.41 100 kHz 56 -9.3 -9.5 0.59 1 MHz 52.4 -2.4 -2.7 0.63 10 MHz 50.7 -0.76 -0.86 0.65 100 MHz 50.2 -0.23 -0.27 0.66 Smith Chart calculations begin to be inaccurate around 2 MHz and below. So do SWR meters. |
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