SOME MEASUREMENTS of a SML
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Consider:
1)?
The DC resistance of the loop is constant.? It's independent of
frequency.? That's one of the distinguishing characteristics of pure
resistance vs. reactance.
2)?
The radiation resistance of the loop varies with frequency.? However,
it is a very small fraction of an ohm, typically around 0.001 ohms
(that's a mΩ or milliohm using proper abbreviations) or less.? This is
what spoils tuned loops for efficiency.
The
DC resistance of my 0.5-inch copper tubing loop of 36-inches diameter
(circumference of 113.10-inches) can be calculated.? I measured 7.007
Amperes through the loop with a voltage drop across the loop of 46.3 mV
(millivolts).? So the DC resistance is (Ohm's Law):
? ?? R = V / I = 0.0463 volts / 7.007 Amperes = 6.61 mΩ (6.61 milliohms) or 0.00661 ohms.? ?
Another
measurement yielded 4.123 Amperes through the loop with a voltage drop
across the loop of 29.5 mV (29.5 millivolts).? Once again:
? ?? R = V / I = 0.0295 volts / 4.123 Amperes = 7.17 mΩ (7.17 milliohms) or 0.00717 ohms.
And
for statistical significance:? measured 1.60 Amperes through the loop
with a voltage drop across the loop of 9.98 mV (9.98 milivolts) or
0.00998 volts.? Again:
???? R = V / I = 0.00998 volts / 1.60 Amperes = 6.24 mΩ (6.25 milliohms) or 0.00624 ohms.
The numerical average of these three readings is:
???? R AVG = [6.61 + 7.17 + 6.24] mV / 3 = 6.66 mΩ (6.67 milliohms) or 0.00667 ohms.
???? AVG R = [ 0.00661 + 0.00717] / 2 = 6.89 mΩ (6.89 milliohms) or 0.00689 ohms.
This
compared to a radiation resistance of 0.001 (1 milliohm) or less ohms, I
believe makes a reasonable first-order argument for constant current
vs. frequency.
Now
we need a good measurement of radiation resistance to validate this
argument.? Using a VNA for such low resistances is challenging.? I'll
think of something.? Any other ideas?
I
could feed a known amount of RF energy and actually measure the current
induced through the loop.? I have several home made current probes that
I can easily calibrate.? Other ideas and/or techniques are welcome
along with contributions of others in measuring various parameters of
SMLs.? .?? .?? .?? Please?
Dave - W?LEV
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