December 20th 06, 07:39 PM
posted to rec.radio.amateur.antenna
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First recorded activity by RadioBanter: Jul 2006
Posts: 296
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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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