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#11
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Why is my dipole low impedance?
"Lumpy" wrote in message ... I'm a little stumped. I'm sure I'm missing something obvious but I just can't see it. At this point, I think I'd set aside the MFJ and resort to a VSWR meter. Once you verify the antenna, you can get to checking out the MFJ. The front end bridge is very sensitive to overload and damages easily. Dale W4OP |
#12
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Why is my dipole low impedance?
Roy Lewallen wrote:
Something's seriously wrong there. A shorted quarter wavelength of coax should read very high impedance when the far end is shorted and very low when the far end is open. The only time it should read 50 ohms with the far end shorted or open is if it has many dB of loss. And no even half decent coax should have anywhere near that much loss in a quarter wavelength. Maybe I'm not testing correctly. 100' length of new coax. Tests ok for no shorts or open. MFJ set to 2.4MHz gives me a Z (on the advanced menu) of 50 ohms with distal end of the coax either open or shorted. Actually shorting the end with a screwdriver shaft drops the Z from 50 to 49. I assume 1 ohm is within normal limits. Same settings gives me an R (on the main menu) of zero ohms either open or shorted. I get exactly the same results on several pieces of coax, old/new, RG8 or 58. Resistance is zero, impedance is 50, shorted or open at the resonant quarter freq. Craig 'Lumpy' Lemke www.n0eq.com |
#13
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Why is my dipole low impedance?
Harry Lippitz wrote:
1 - Are you using a short test jumper cable or adapter connector between the test instrument and the end of the coax? Could the test lead or connector be at fault? Nothing seems to make a difference. 12" coax to the antenna, 50' RG8, 100' RG8, 50' RG58. 2 - Is there any possibility of a STRONG signal coming back down the cable to stuff up your test instrument readings? Do you live near a source of RF, such as local broadcast radio? I don't think so. I'm receiving (I'm transmitting!) just fine. No local signals clobber me. 3 - Is there any possibility both feeder cables are defective? Perhaps you are using coaxial cable not meant for this purpose? TV coax, or LAN cable? Nope. Several 50 ohm 8/58 coax. Some from cablexperts.com. 4 - Have you wiped out your neighbours TV viewing or something? Checked the cables for sabotage (pins or nails)? No, nothing like that. 5 - Have you climbed the mast and checked the antenna directly at the feed point, without a feeder cable? I can't climb this mast but if I bring it down to 10' and get up there with a ladder, same results. You have probably thought of all these, but I had to ask. Feel free to ask, no matter how seemingly stupid the Q might be. Craig 'Lumpy' Lemke www.n0eq.com |
#14
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Why is my dipole low impedance?
Thanks for everyone's comments
and questions. Based on what several suggest, I think the MFJ analyzer is B/O. I'm still making contacts like crazy, even if the meter says I shouldn't be. I think I'll get the 259 back to MFJ while it's still under warranty. Thanks again - Craig 'Lumpy' Lemke www.n0eq.com |
#15
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Why is my dipole low impedance?
"Lumpy" wrote in message ... Harry Lippitz wrote: 1 - Are you using a short test jumper cable or adapter connector between the test instrument and the end of the coax? Could the test lead or connector be at fault? Nothing seems to make a difference. 12" coax to the antenna, 50' RG8, 100' RG8, 50' RG58. 2 - Is there any possibility of a STRONG signal coming back down the cable to stuff up your test instrument readings? Do you live near a source of RF, such as local broadcast radio? I don't think so. I'm receiving (I'm transmitting!) just fine. No local signals clobber me. 3 - Is there any possibility both feeder cables are defective? Perhaps you are using coaxial cable not meant for this purpose? TV coax, or LAN cable? Nope. Several 50 ohm 8/58 coax. Some from cablexperts.com. 4 - Have you wiped out your neighbours TV viewing or something? Checked the cables for sabotage (pins or nails)? No, nothing like that. 5 - Have you climbed the mast and checked the antenna directly at the feed point, without a feeder cable? I can't climb this mast but if I bring it down to 10' and get up there with a ladder, same results. You have probably thought of all these, but I had to ask. Feel free to ask, no matter how seemingly stupid the Q might be. Craig 'Lumpy' Lemke www.n0eq.com Hi Lumpy I really like your music. You do a terriffic job with that guitar. I'd suggest that you'd do yourself a favor if you recognize that the input impedance of your antenna is not being measured properly by your meter. Instead, try to think of something that is causing the meter to read improperly. Could the error be introduced by a stray (off frequency) signal effecting the meter movement? You know, the antenna picks up the signal and introduces an error. Remember how the impedance looking into a length of coax, that is either opened or shorted, varies all over the place when the length is uncontrolled.. The only time it's impedance is equal to it's Zo is when it's length is 1/8th wave ( or odd multiples). And, then, the impedance is very nearly purely reactancive. Jerry KD6JDJ |
#16
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Why is my dipole low impedance?
Lumpy wrote:
Roy Lewallen wrote: Something's seriously wrong there. A shorted quarter wavelength of coax should read very high impedance when the far end is shorted and very low when the far end is open. The only time it should read 50 ohms with the far end shorted or open is if it has many dB of loss. And no even half decent coax should have anywhere near that much loss in a quarter wavelength. Maybe I'm not testing correctly. 100' length of new coax. Tests ok for no shorts or open. MFJ set to 2.4MHz gives me a Z (on the advanced menu) of 50 ohms with distal end of the coax either open or shorted. Actually shorting the end with a screwdriver shaft drops the Z from 50 to 49. I assume 1 ohm is within normal limits. Same settings gives me an R (on the main menu) of zero ohms either open or shorted. I get exactly the same results on several pieces of coax, old/new, RG8 or 58. Resistance is zero, impedance is 50, shorted or open at the resonant quarter freq. Craig 'Lumpy' Lemke www.n0eq.com At 2.4 MHz, the line is pretty close to 3/8 wavelength, not 1/4 -- assuming it has solid and not foamed dielectric. At that length with the other end shorted or open, the impedance would be very nearly 50 ohms, all reactive, which means the resistance would be zero as you're seeing. So those measurements are correct. Your meter is showing the correct values when measuring resistors (purely resistive) and the shorted/open coax (purely reactive), so there's nothing obviously wrong with the meter. The remaining possibility for your problem with the antenna is external RF, as someone else mentioned earlier. I find it nearly impossible to use my 259B for antenna measurements at my QTH because of strong AM, FM, and TV signals -- ironically, the "antenna analyzer" is great for a lot of other chores, but not for actually analyzing antennas. It's pretty likely that's what your problem is too. When measuring antennas, I have to use filters which introduce minimal impedance disturbances at the measurement frequency while providing attenuation of the offending signals. Roy Lewallen, W7EL |
#17
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Why is my dipole low impedance?
On Tue, 20 May 2008 10:51:26 -0700, "Lumpy"
wrote: I'm a little stumped. I'm sure I'm missing something obvious but I just can't see it. Built a 40m wire dipole, 33ft per side, symmetrical. Center mounted on an 18' pole. Ends of the legs drop about 2 ft to 16' above ground. Slack/dip in the wire legs is pretty minimal. The mechanical connections of my antenna allow me to get it pretty tight. Without any balun, running coax to the center feed point of the dipole, I get an impedance of around 6-7 ohms. Different lengths of coax, different pieces of coax, make no difference. All the coax pieces I've tried test ok for shorts and continuity. Two of the test coax pieces are brand new. Dipole legs aren't touching anything, including each other. Nearest piece of metal is at least a half wave away from the end of one leg. Mast is an aluminum tubing tripod in sections. The sections aren't particularly bonded, the section connectors are plastic/nylon. The telescopic sections of the tripod/mast are 4' long each. The mast has rubber feet on it's tripod legs, it is not bonded to earth. Same 6-7 ohms shows up on the MFJ meter whether the mast is at 10 ft or 18 ft. Same 6-7 ohms shows up whether I stand at the base of the mast with a 17' piece of coax attached, or if I add a 50 or 100 ft length of coax and stand virtually anywhere. Why isn't my dipole showing a more expectable impedance? Any guesses? Craig 'Lumpy' Lemke www.n0eq.com Have you measured it with an SWR meter from your radio? -- 73 for now Buck, N4PGW www.lumpuckeroo.com "Small - broadband - efficient: pick any two." |
#18
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Why is my dipole low impedance?
Lumpy wrote:
I'm a little stumped. I'm sure I'm missing something obvious but I just can't see it. Built a 40m wire dipole, 33ft per side, symmetrical. Center mounted on an 18' pole. Ends of the legs drop about 2 ft to 16' above ground. Slack/dip in the wire legs is pretty minimal. The mechanical connections of my antenna allow me to get it pretty tight. Without any balun, running coax to the center feed point of the dipole, I get an impedance of around 6-7 ohms. Different lengths of coax, different pieces of coax, make no difference. All the coax pieces I've tried test ok for shorts and continuity. Two of the test coax pieces are brand new. Dipole legs aren't touching anything, including each other. Nearest piece of metal is at least a half wave away from the end of one leg. Mast is an aluminum tubing tripod in sections. The sections aren't particularly bonded, the section connectors are plastic/nylon. The telescopic sections of the tripod/mast are 4' long each. The mast has rubber feet on it's tripod legs, it is not bonded to earth. Same 6-7 ohms shows up on the MFJ meter whether the mast is at 10 ft or 18 ft. Same 6-7 ohms shows up whether I stand at the base of the mast with a 17' piece of coax attached, or if I add a 50 or 100 ft length of coax and stand virtually anywhere. Why isn't my dipole showing a more expectable impedance? Any guesses? Craig 'Lumpy' Lemke www.n0eq.com If I look in the antenna books, the feedpoint impedance of a half-wave dipole less than 0.1 wavelengths above a perfect ground can be less than 10 ohms. You are, say, 17 feet above ground. That's more like 0.14 wavelengths, and the charts in the antenna handbooks show that as you get up to 0.2 or 0.3 wavelengths that you'd expect to be way above ten ohms. And your ground I'm assuming is not "perfect", with a zero-area call it might be better than average, but it's probably not salt-water. So maybe the numbers don't add up perfectly to match the charts in the books, but I think the lesson is that a very low dipole (you're just "pretty low") dipole over perfect ground (maybe you're "very good") can have a very low feedpoint impedance, less than 10 ohms in some circumstances. If your ground really is that good, you're a prime candidate for verticals or an array of verticals. I used to live in the midwest next to the Mississippi, and the ground conductivity was really very very good (if not as good as sal****er) and I was very happy with my 4BTV's performance. Tim. |
#19
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Why is my dipole low impedance?
"Lumpy" wrote in message ... wrote: Craig, I wonder what you are using to measure that 6 - 7 ohms? MFJ Analyzer. Craig 'Lumpy' Lemke www.n0eq.com --------- Check for a short in the connector to the antenna analyzer. Ed, NM2K |
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