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December 20th 06, 06:39 PM
posted to rec.radio.amateur.antenna
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Determining if reactance is capacitive or inductive.
wrote in message oups.com... Its not that I am trying to get the exact feedpoint impedance values from the antenna. I understand how 50 Ohm coax affects the values that I've measured. I was originally trying understand how to determine if a reactance value was capacitive or inductive. I do see now that when I use NEC2 to plot the values for this antenna, the resistance and reactance appear more linear across the frequency range I measured. I see if I apply the logic of the MFJ manual to the plot from NEC2, it makes sense and that my data is all over the place. For what reasons I do not know. I do not think the feedline is radiating. Maybe it is just the meter. Or perhaps some larger deviation from 50 ohms in the coax and connectors. And Richard - the antenna doesn't stay outside, that is why its not sealed. This is just rigged up for testing. -Scott, WU2X wrote: Your coax is what, 10-12 feet long? That's pushing a couple of wavelengths on 2m, but isn't quite there. To read the impedance of the antenna directly and get a good set of resistance and reactance curves clearly, you need probably less than a foot of coax, or an exact multiple of an electrical half wavelength (so exactly 2 wavelengths * velocity factor would do it). You can use your MFJ-259 to cut the line. It's possible to transform your impedance readings into the antenna's impedance if you know the (random) electrical length and loss of the coax and are handy with a Smith chart or have some other calculating ability. Exact multiple of a half wave is easier... it's an impedance repeater, aside from the loss in the line. To that end, I'd use an exact multiple of half wave of better coax... RG-8 or 9913 or something. 73, Dan It may be YOU that is causing the vaules to be so erratic. Your pressence can detune the antenna. 10- 12 feet is too close. |
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