Tom wrote:
>>> A resonant 160 meter loop around 20-30 feet above the ground has a
>>> feedpoint impedance of around 50 ohms on 160, and about 80 ohms on 80
>>> meters. The antenna impedance isn't really high until 40 meters, where it
>>> would be resonant far outside the band and have a terrible mismatch to any
>>> feedline.
Expanding a bit on what Tom said, the source resistance at resonance will also
depend on the type of ground.
With AutoEZ, I first used this dialog to create a 4-sided horizontal loop
having a perimeter of "P" ft at a height of "H" ft above ground.
http://ac6la.com/adhoc/160mLoop1.png
I then created a series of test cases with "H" ranging from 10 to 90 ft (~0.02
to 0.16 λ at 1.8 MHz) in 5 ft steps and with "P" having an initial value of 560
ft (~1005/Freq). Then the "Resonate" button was used to automatically adjust
the loop perimeter "P" for resonance at each height.
http://ac6la.com/adhoc/160mLoop2.png
Repeating that process for three different ground types gives this result for
the source resistance.
http://ac6la.com/adhoc/160mLoop3.png
With the perimeter frozen at 560 ft, on 40m the loop will not be resonant, as
Tom said. The source *reactance* will be a few hundred ohms negative. The
source *resistance* will look like this.
http://ac6la.com/adhoc/160mLoop4.png
On the other hand, if you calculate the source resistance for a 40m *resonant*
loop (*not* the scenario that the OP described nor which Tom addressed but just
to illustrate), where the loop perimeter is ~145 ft and where 90 ft above
ground is ~0.65 λ instead of ~0.16 λ, you'll get this.
http://ac6la.com/adhoc/160mLoop5.png
If you'd like to duplicate these results, or run calculations for a different
band, the following model file is suitable for use with the free demo version
of AutoEZ (http://ac6la.com/autoez.html). You'll also need EZNEC and Microsoft
Excel.
http://ac6la.com/adhoc/160mLoop.weq
Download that file, use the AutoEZ "Open Model File" button to open it, tab to
the Calculate sheet, select all the "P" values (cells D11-D27), and click the
"Resonate" button. Or change all the frequencies to your new choice and set
all the initial "P" values to ~1005/Freq (initial value not critical), then
click "Resonate". Or just set all the frequencies and "P" values as desired
and click "Calculate All Rows" instead of "Resonate". In any case, make sure
EZNEC is already open in a separate window beforehand.
When the calculations finish tab to the Custom chart sheet. Select "R at Src"
for the chart Y axis and "Variable 1" (which is "H", height) for the chart X
axis.
Dan, AC6LA
http://ac6la.com
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