[TowerTalk] 39 +j0 ohms?

[Chuck Lewis]

Jamie, the impedance reading you are seeing now is (48.3 + j14.4); but your instructions want (46 + j12). So you’re “pretty close”.  The “j“ term and “X” refer to the same thing (reactance).
Impedance has two components: resistance (R) and reactance (X).  Impedance is often shown as   (R + jX), which is shorthand for (Resistance and Reactance). The reactance term (jX) will be positive if the reactance is inductive and it will be negative if the reactance is capacitive. The reactance shown by your analyzer (X=+14.4) is slightly positive, because it’s inductive. You really want it to be X=+12 (a little less inductive).  Your instructions might help you understand whether (48.3 + j14.4) is close enough, or that you still need to tweak it a little. There should be others on this reflector who have experience with these kinds of arrays and can tell you whether, in this case, “pretty close” is close enough.
Don’t sweat the “j” business. For now, just think of “j” as a clue that the following number is the reactance (X).  When you dig into the math, the “j” will make more sense. Please dig into the math!
Here’s the relationship among the three parameters:  Z(impedance) = √(R2 + X2) .  Let’s use your numbers for #2:
Z= 40.8   R= 40.6   X=3.2
R(squared) = 1648.36, and X(squared) = 10.24 .  Add those together to get 1658.6 . When we take the square root of that, we get 40.725. Your analyzer said the impedance (Z) is 40.8. Yay! The math worked! Try it for some other sets of values…
If you think that looks a lot like solving for the hypothenuse of a right triangle, you would be right.
Chuck, N4NM
On 12/2/2018 3:47:49 PM, Jamie WW3S <ww3s at zoominternet.net> wrote:
I'm preparing for the 2 ELE DXE array.....and now, with just about 3 ft of
coax, at the target freq of 7.125 mc, #1 vert measures Z=41.1, R=40.8 and
X=.93 and #2 measures Z=40.8, R=40.6 and X = 3.2. I then attached the 75
ohm 1/4 wave stubs, and the Tee adapter, and measure right at the Tee and
got Z=51.2, R=48.3 and X=14.4...the instructions say it should read close
to 46 ohms +j 12 ohms.....so am I close? I'm stuck on what the j+12 ohms
means in relation to the other readings....

-----Original Message-----
From: Tim Duffy
Sent: Sunday, December 02, 2018 4:11 PM
To: 'Wes N7WS' ; jim at audiosystemsgroup.com
Cc: towertalk at contesting.com
Subject: Re: [TowerTalk] 39 +j0 ohms?

Jamie:

Is this a full size vertical (1/4 wavelength) with a proper radial field?

If the VSWR is 1.3:1 at 7125 KHz - what is the problem you are trying to
solve? What frequency is X=0?

73,
Tim K3LR

-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Wes
N7WS
Sent: Sunday, December 02, 2018 4:05 PM
To: jim at audiosystemsgroup.com
Cc: towertalk at contesting.com
Subject: Re: [TowerTalk] 39 +j0 ohms?

No, you just need to calibrate at the far end of the coax.

Sent from my iPhone

> On Dec 2, 2018, at 1:27 PM, Jim Brown wrote:
>
> Where are you measuring? You'll only get the actual impedance measuring
right at the feedpoint with a very short piece of coax connecting the
analyzer.
>
> Now, it IS possible to transform a measurement made at the end of a long
piece of coax to the feedpoint Z, but to do that you need to save the data
to a file and export to software like AC6LA's ZPlots or AE6TY's SimSmith.
AND you need to know the length of the feedline, know the exact type of
coax, and plug both into the software.
>
> 73, Jim K9YC
>
>> On 12/2/2018 12:07 PM, Jamie WW3S wrote:
>> trying to understand a 40m vertical, my Rigexpert analyzer has Z, X, R
and C.......when I measure the vertical at 7.125 (target freq) the SWR is
1.3:1, but the Z is 50 ohms, I'm told the impedance needs to be 39+j) ohms?
How can I read that with my rigexpert.....I can get around 39 ohms at
7.400.....is my vertical too long, or too short?
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