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Changing reactance and Impedance

W

W6HSA

W9WDX Amateur Radio Club Member
May 8, 2009
63
2
18
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I purchased a LP-100A computing SWR/Watt meter that displays reactance and impedance in real time. I have an Imax 2000 and a Sirio 3 element. On the beam, the R & Z are 48 and the SWR is 1.1. On my ground plane, the R & Z are 54 and the SWR is 1.3. These measurements have been rock steady.

Today I noticed the ground plane R & Z now equals 64 with the same SWR. I was at a loss to account for this but it rained pretty hard yesterday and today so I'm assuming this caused the change. If so, I would expect that the SWR would have risen as well. The beam has been unchanged.

What say the experts? The drain hole is not plugged up but perhaps it is wet inside?
 

R and Z are two different readings.You seem to be treating them as one by saying that "R&Z equals....".The only way R will equal Z is when there is a perfectly pure resistance present without any reactance (value of X) present.You also need to know the value of X. The R value is the resistive component of the antenna, X is the reactive component, and Z is the overall impedance of the antenna that is determined by both R and X. The same value of SWR can be obtained using different values for R and X. Ideally R=50 and X=0.
 
Thank you for your reply. I'm trying to learn antennas a bit better. I wasn't trying to equal the values of R & Z but stating they were equal in value on the meter. Both R & Z changed from 55 to 65 on the vertical and I was trying to understand the impact. Here are the stats. I'm not sure they are good readings or bad readings per se nor whether the increase is a good thing or bad thing. I can surmise that since it is greater than the ideal of 50, it is not a good thing but not sure how it affects the performance.

I appreciate your expertise here, Captain Kilowatt.

With 4 watts input:

Vertical originally (change)
Z = 55 (65)
R = 55 (65)
X = 1.2
SWR = 1.3

Beam
Z = 46
R = 46
X = 2.0
SWR = 1.1
 
This doesn't say anything about the change in R and Z. But what you are seeing because of the rain is 'normal'. I would guess that when things dry a bit, those R and Z numbers will go back to what they were, or close anyway.
I do think that seeing this is a fairly good indication that SWR alone isn't exactly the best indicator of how 'well' an antenna is doing. There are just too many combinations of R and X that can show as a good SWR (Z matches/ratios). The biggy in all that is the X's, which do not contribute to signal radiation. (Which isn't really telling you anything if you don't know about X's (reactances) to start with. Which is some of that 'theory' stuff that people seem to think isn't any good for anything. Which just means you haven't learned enough yet, for it to make sense. A PITA thing, ain't it?)
- 'Doc


Oh yeah. All X's aren't a bad thing, if they are in the right place. The antenna just isn't the right place. Another one'a them PITA thingys.
 
From your readings I would say that your antennas are resonant as evidenced by the low and barely changing value of X. Either that or there is a problem with the meter. If there is no reactence (X) then the value of Z will be equal to the value of R since Z is a product of both R and X and if X=zero then it becomes a non-issue. I still wonder why however that the values change without affecting X. Usually rain will change the capacity of an antenna and that appears as a change in capacitance reactance (X). Either way the changes seen are very small and to be expected from wet to dry. See what happens when things dry out. I had a tribander that would go from 1.2:1 up to 2:1 simply when it rained. With a coating of ice and snow it was over 3:1. Everything returned to normal when it dried off.
 
Ah, this is VERY helpful. Thank you both for taking the time out to reply. I bought the LP-100A (and an HO-10 for which I'm trying to locate a manual) to get a better handle on what is going on with my station. With respect to SWR, I was reading that it is not the entire story. It appears from your replies that my X readings are good and the other values should revert back to normal when things dry out, but either way are not a concern.

With the meter, I can keep an eye on it as needed. The antennas seem to perform well.

Edit: Actually, X did change when the values of Z & R changed. X dropped down. I wrote the number down in my station notebook. Now that I think of it, I did go through the connections in the shack - including the antenna connections at the switch, as I was re-arranging things to accomodate the scope. I didn't associate that event with the drop in X. Perhaps that was a good thing? That's assuming, of course, that X dropped due to this event.

Thanks again, fellas.
 
Last edited:
I changed the coax on the beam from LMR400 to 1318 (9913) and the readings are now: R = 49.5 and X = 50. I am very pleased with that. It makes me wonder what can be done to change the values on the vertical? I have adjusted it for lowest SWR. I'm thinking it is as good as the electrical characteristic of the antenna can get?

Anyway, good stuff this. Antennas have always been a dark hole with SWR the only window into their health until now (for me).
 
W5LZ said:
Which is some of that 'theory' stuff that people seem to think isn't any good for anything. .
Click to expand...

Hearing that does get old, doesn't it? ("Don't confuse me with theory, my Antron is a half wave over a quarter wave because the box says so.")
 
I'd like to ask a more fundamental question: Is having an impedance of 68, a reactance of 1.0 and a SWR of 1.4 necessarily bad? Put another way, what impact would the impedance of 68 have over an ideal impedance of 50?

Thanks for your input...
 
W6HSA said:
Ah, this is VERY helpful. Thank you both for taking the time out to reply. I bought the LP-100A (and an HO-10 for which I'm trying to locate a manual) to get a better handle on what is going on with my station. With respect to SWR, I was reading that it is not the entire story. It appears from your replies that my X readings are good and the other values should revert back to normal when things dry out, but either way are not a concern.

With the meter, I can keep an eye on it as needed. The antennas seem to perform well.

Edit: Actually, X did change when the values of Z & R changed. X dropped down. I wrote the number down in my station notebook. Now that I think of it, I did go through the connections in the shack - including the antenna connections at the switch, as I was re-arranging things to accomodate the scope. I didn't associate that event with the drop in X. Perhaps that was a good thing? That's assuming, of course, that X dropped due to this event.

Thanks again, fellas.
Click to expand...

W6HSA, if you saw a shift in the center frequency, going down a bit, and you added some length to your feed line during the re-arranging, then you might be seeing the effects of feed line transformation. I wouldn't worry about it too much if your analyzer readings are still close to R=50, X=0, in the frequency range you work most.
 
Not pointing fingers, but this "syndrome" is becoming more and more prevalent: a new piece of technology produces results that somehow don't square with what the owner expects. The owner now believes that his station is about to melt down, or at least isn't effectively using every available microwatt.

Those readings are well within what most of us would consider a "normal" tolerance within a "normal" range. If your transmitter isn't cutting back its output automatically, it's pretty safe to say that all is well.

As I've said before, you can lose sleep over a change in "x" from 1.2 to 2, or you can make a note of it, get on the air and have fun. Your choice.
 
It was really a learning exercise for me. I just got the meter, saw the changes but couldn't account for them nor assess their impact. Your comments summarize what I learned from this. I understand better the characteristics of this antenna relative to the beam. I have two additional antennas that I can now understand better. SWR is but one of the parameters. I now understand reactance and impedance much better.

Thanks to you all for your input.
 

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