Playing around with this on-line loop evaluation tool:

http://www.66pacific.com/calculators/small_tx_loop_calc.aspx

Immediately below are the outputs for a 17 gauge (.045 inch diameter) wire just under one-quarter wavelength in length for the 40 meter band loop antenna. Compare what the loop antenna calculator calculated versus what was seen for ‘actual real-world results’ shown further down:

Input Values:
Length of conductor: 32 feet
Diameter of conductor: .045 inches (17 AWG)
Frequency: 7 MHz
RESULTS:
Antenna efficiency: 21% (-6.9 dB below 100%)
Antenna bandwidth: 217 kHz
Tuning Capacitance: 149 pF (real world result = 24 pF)
Capacitor voltage: 702 volts RMS
Resonant circulating current: 4.60 A
Radiation resistance: 0.484 ohms (real world = 22.2)
Loss Resistance: 1.87 ohms
Inductance: 3.46 microhenrys
Inductive Reactance: 152 ohms
Quality Factor (Q): 32.3
Distributed capacity: 26 pF
Antenna "circumference": 32 feet
Comments:
1) The specified conductor length of 32 feet is OK.
**2) Conductor length should be between 17.0 and 34.1 feet at
the specified frequency of 7 MHz.
**3) For highest efficiency, the conductor length for a small
transmitting loop antenna should be greater than 1/8 wavelength
(greater than about 17.0 feet at the specified frequency of 7
MHz).
4) To avoid self-resonance, the conductor length for a small
transmitting loop antenna should be less than 1/4 wavelength
(less than about 34.1 feet at the specified frequency of 7 MHz).

– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –

Below are the actual, real-world results obtained:

Antenna efficiency: ~100% (on a par w/dipole)
Tuning capacitance: 24 pF
Antenna bandwidth: 50 kHz 1.5:1, 110 kHz 3:1
Radiation resistance: 22.2 ohms (fed w/transformer next to
tuning capacitor)
AC and DC wire resistance calculations at 7 MHz:
Freq Gauge Wire Len DCR ACR Rrad Ratio
----- ------ ----- ---- ------- ----- ---- -----
7 MHz 17 ga. Al 32' 0.265 2.452 22.5 9.17
7 MHz 10 ga. Cu 32' 0.0318 0.835 22.5 26.9

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