A 100 Watt Small Loop:
Description and Measurements
for Loop 4
This is a supplement page for "Measuring the Efficiency of a QRP Small Loop Antenna."
Loop Diameter = 3.3 ft.
Conductor: 5/8 in copper tubing
Center Height = 6.0 ft.
Capacitor: MFJ-19 Butterfly Type (12-67 pFd)
Voltage Rating: 5.9 kV peak voltage
Coupling Loop: 8.5 inch diameter loop of RG-8 coax
Mast: 1 inch diameter PVC pipe
Frequency Range: 15 through 20 meters (11.4-24.8 MHz)
Power: No arcing in tests up to 100 watts CW.
Sliding the small coupling loop up and down on the mast, as recommended by Matt Roberts, KK5JY, enables you to match the impedance to the feedline. 
Figure 5. Loop 4.
Figure 6. MFJ-19 capacitor with 6:1 planetary drive reducer and a "handle" (a chopstick) glued to the knob for more precise tuning.
Measurements and Calculations
This table shows the measurements and results for Loop 4 (the AE7PD loop) that are in Figures 3 and 4 of the article.
Band f1 f2 f0 BWU QL QRAD QL/QRAD
[MHz] [MHz] [MHz] [kHz]
20m 14.152 14.174 14.163 22 322 1055 0.305
17m 18.093 18.130 18.111 37 245 462 0.530
15m 21.131 21.180 21.155 49 216 268 0.806
>12 24.781 24.852 24.816 71 175 151 1.159 !
f1 = Lower -3 dB bandwidth frequency (at VSWR = 2.62)
f2 = Upper -3 dB bandwidth frequency (at VSWR = 2.62)
Both frequencies are for an unloaded loop, measured with a RigExpert AA-230 antenna analyzer as described below.
f0 = Resonant frequency = SQRT(f1 * f2)
BWU = Unloaded bandwidth = f2 - f1
Unloaded Q = f0 / BWU
QL = Loaded Q = 0.5 * Unloaded Q
QRAD is from the formula in Ref. 2 of the main article,
or the Efficiency Calculator
I connected a RigExpert AA-230 antenna analyzer to the loop with a 50 foot length of RG-8 coax and subtracted the effect of the coax using the Open-Short-Load (OSL) calibration feature in the RigExpert Antscope program. Figure 7 shows an example of a calibrated VSWR plot for measuring the bandwidth.
Figure 7. Calibrated Antscope Plot on the 15 m band
The unloaded, half-power bandwidth is the difference between the two frequencies where VSWR = 2.62. Hovering the mouse cursor over the curve opens a text box with precise readouts for the frequency, SWR, and impedance information. The bandwidth in Figure 7 is 49 kHz, as shown in the table.
AAplot is W7AY's Mac OS X application that works with Rig Expert analyzers.  AAplot includes an optional calibration procedure that is more accurate than the commonly used Open-Short-Load method. This high accuracy procedure is used in HP and Agilent network analyzers. From W7AY's site :
“As it is usually used, the Rytting calibration is most often performed using three 'known' impedances that are (1) a short circuit, (2) an open circuit and (3) a known load resistance that is close to the terminal impedance of the analyzer (e.g., 50 ohms).
“However, the bridge topology that is implemented in the Rig Expert antenna analyzers produces very large errors when measuring impedances in regions where the VSWR is large (i.e., when the absolute value of the reflection coefficient is near 1.0). Because of that, AAplot lets you use a moderately low resistance (e.g., 15 to 25 ohms) in place of a short circuit load, and a moderately large resistance (e.g., between 150 to 300 ohms) in place of the open circuit load.”
I repeated the bandwidth measurements for Loop 4 using AAplot and the “Lo-Hi-Load” calibration procedure with 22 ohm, 270 ohm, and 47 ohm terminations on the RG-8 coax line. In this case there is no significant difference between the results and those shown in the table.
Figure 8. AAplot user interface for VSWR measurements.
Peter DeNeef, AE7PD, is an Extra Class amateur radio operator in the U.S. This Web site has not ads or conflicts of interest.
Email: HamRadioAndVision "at" gmail "dot" com.