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@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@audiosystemsgroup.com
Cc: towertalk@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@contesting.com] On Behalf Of Wes
N7WS
Sent: Sunday, December 02, 2018 4:05 PM
To: jim@audiosystemsgroup.com
Cc: towertalk@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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