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#1
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Why is my dipole low impedance?
Roy Lewallen wrote:
What do you read when you connect a 47 - 75 ohm resistor across the terminals of your measuring device? Plain resistors read appropriately. 27ohms, 56 ohms etc. I don't have a good dummy load. I have a 50 ohm 50w resistor but it's wire wound. It varies all over the range when I sweep the MFJ. Known good piece of coax, end shorted or open, reads 50 ohms when the MFJ is tuned to the quarter wave of the coax. MFJ seems to read a vertical hamstick appropriately - http://digitalcartography.com/n0eq/M...il-600x400.jpg I just made a contact on 5W QRP from Phoenix (here) to St Louis. So I have to guess the antenna isn't open or shorted or something else catastrophic. That's through an LDG tuner. But - Running the dipole into my Yaesu 857 with external tuner bypassed, shows an SWR high enough to cause the transciever to self protect shut down. Engage the tuner, SWR comes down to 1.5:1, and I'm putting out (at least to the back of the radio) very close to the selected 5 watts. Craig 'Lumpy' Lemke www.n0eq.com |
#2
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Why is my dipole low impedance?
Lumpy wrote:
Roy Lewallen wrote: What do you read when you connect a 47 - 75 ohm resistor across the terminals of your measuring device? Plain resistors read appropriately. 27ohms, 56 ohms etc. I don't have a good dummy load. I have a 50 ohm 50w resistor but it's wire wound. It varies all over the range when I sweep the MFJ. Known good piece of coax, end shorted or open, reads 50 ohms when the MFJ is tuned to the quarter wave of the coax. . . . 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. So it appears that either your coax is extremely lossy for some reason (and I can't think of any mechanism which would cause that much loss unless the coax is specifically designed to be very lossy for some special reason) or the meter is oscillating or otherwise misbehaving when connected to coax. Roy Lewallen, W7EL |
#3
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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 |
#4
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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 |
#5
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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 |
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