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Trying to make sense of readings

Mike KK7ER
 

I used a recently purchased Antuino on an EFHW project.  I built a 1:49 transformer, connected it to 135 feet of wire, and hooked up the Antuino.  The wire is too long, obviously, but I'm getting inconsistent readings.  Here is the display when set to plot 25MHz +/- 25MHz:

Since this is +/- 25MHz, I expect that the grid is spaced horizontally in increments of 5MHz.  The vertical numbers do not line up with the grid lines but I assume that the top line is 1:1, second line is 3:1, and the final unmarked line is either 11:1 or infinity:1.  With this interpretation, the best SWRs for this particular choice of wire length are 2.5MHz, 9.5MHz, 16.5MHz, 23.5MHz, 30.5MHz, and 37.5MHz.

The problem is that if I zoom in, the peaks shift!  Changing only the zoom settings (to 13.7MHz +/- 5MHz) I get this:

And at 6.75MHz +/- 5MHz, I see:

If my interpretation of the vertical axis is correct then the reading shown (1.1:1) does not match the graph which shows around 2:1.  And the SWR also does not match at the peak shown below (3.375MHz +/- 5MHz).  The graph shows 3:1 but the reading shows 2.0.


So the SWR readings do not match the graph and the zoomed in peak locations do not match the zoomed out peak locations.  Not even close.  Any ideas what is going wrong?  Thanks!

73, Mike KK7ER

_Dave_K0MBT
 

Would agree with your analysis. The zoomed in readings look like they might be in the ballpark for a 135' end fed half wave. Have you tested the antenna with an SWR meter. 

 

Mike,

Thats due to the low display resolution. At 50MHz span you can expect 1Mhz per dot. Anything in between
falls through and will be displayed as a lower level. When you zoom in the accuracy improves with more resolution.

The antuino is fine by itself but we need even a higher resolution display to get better accuracy.

I am sure someone will hack up the sketch and connect to a better display.

Raj


At 11/10/2019, you wrote:
I used a recently purchased Antuino on an EFHW project.  I built a 1:49 transformer, connected it to 135 feet of wire, and hooked up the Antuino.  The wire is too long, obviously, but I'm getting inconsistent readings.  Here is the display when set to plot 25MHz +/- 25MHz:
[]
Since this is +/- 25MHz, I expect that the grid is spaced horizontally in increments of 5MHz.  The vertical numbers do not line up with the grid lines but I assume that the top line is 1:1, second line is 3:1, and the final unmarked line is either 11:1 or infinity:1.  With this interpretation, the best SWRs for this particular choice of wire length are 2.5MHz, 9.5MHz, 16.5MHz, 23.5MHz, 30.5MHz, and 37.5MHz.

The problem is that if I zoom in, the peaks shift!  Changing only the zoom settings (to 13.7MHz +/- 5MHz) I get this:
[]
And at 6.75MHz +/- 5MHz, I see:
[]
If my interpretation of the vertical axis is correct then the reading shown (1.1:1) does not match the graph which shows around 2:1.  And the SWR also does not match at the peak shown below (3.375MHz +/- 5MHz).  The graph shows 3:1 but the reading shows 2.0.
[]

So the SWR readings do not match the graph and the zoomed in peak locations do not match the zoomed out peak locations.  Not even close.  Any ideas what is going wrong?  Thanks!

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Mike KK7ER
 

Hi Raj,

Unless I'm missing something, the 1Mhz resolution would explain a shift of 1Mhz.  But I'm seeing a peak at 9.5MHz when it should be 6.75MHz, 16.5Mhz when it should be 13.7Mhz, etc.  The error seems too large for limitations of screen resolution to fully explain it.

And screen resolution does not seem to account for the discrepancy between the SWR number shown at the upper right and the peak shown on the graph.

73, Mike KK7ER

 

Hi Mike,

I havent tried SWR with real antennas, just with dummy loads. The SWR readings produced is extremely limited
I find only 1:1 to 1:2 accurate and in reality useful.

Do you know on the graph display screen you can scroll the center marker to both ends of the screen by
rotating the knob?

Raj

At 12/10/2019, you wrote:
Hi Raj,

Unless I'm missing something, the 1Mhz resolution would explain a shift of 1Mhz. But I'm seeing a peak at 9.5MHz when it should be 6.75MHz, 16.5Mhz when it should be 13.7Mhz, etc. The error seems too large for limitations of screen resolution to fully explain it.

And screen resolution does not seem to account for the discrepancy between the SWR number shown at the upper right and the peak shown on the graph.

73, Mike KK7ER

Mike KK7ER
 

I did not know that.  Thank you for pointing that out, Raj.

73, Mike KK7ER