1、My Loop AntennaStephen E. Sussman-Fort, Ph.D.AB2EWMy Loop Antenna November 20, 2013 2Outline Brief History Characteristics of Small Loop Antennas for HF My Loop for 40m 15m Receive/Transmit Properties of Elect.-Small Loops Loop Equations Measuring Efficiency of a Loop or Vertical Antenna Evaluation
2、of My Loop Comparison to Verticals Conclusions - ReferencesMy Loop Antenna November 20, 2013 3Brief History 1968 Lew McCoy W1ICP attempts using military small-loop antenna adapted for amateur applications12-foot diameter loop Capacitor matching Results from military loop outperforms dipole over 2-5
3、MHz McCoys results disappointing loop is 3 S-units below dipole Reason losses in electrical/mechanical connections overwhelm tiny radiation resistance Military used very careful construction to achieve good resultsMy Loop Antenna November 20, 2013 4Brief History (contd) Even earlier military applica
4、tions have been documented Schutzpolizei 1932 Balkans War 1942Aldershot, England 1937My Loop Antenna November 20, 2013 5Brief History (contd) Since McCoys experiment, many hams have built highly performing “electrically-small” loops (diameterone meter for HF operation see examplesnext page) With pro
5、per materials and construction techniques, the performance of small-loops becomes comparable to full size dipoles Advantages: much smaller footprint, works well a few feet off the ground (in vertical position), does not require radials or counterpoisesMy Loop Antenna November 20, 2013 6Brief History
6、 (contd)JL1BOHAA5TB(see also QST Nov. 2013 p. 67)(much bigger loop)My Loop Antenna November 20, 2013 7Brief History (contd) (Alpha Antennas) Alpha LoopMy Loop Antenna November 20, 2013 8Characteristics of Small Loop Antennas for HF Operatione.g.40m band, C=4m=13 ft., d=1.27m=4.17 ft.(at lowest freq.
7、)Mounting option with loop on topMounting option with loop on bottomMy Loop Antenna November 20, 2013 9Characteristics of Small Loop Antennas for HF Operation Coupling loop 1/5 diameter of main loop for good impedance matching on all bands Antenna is narrowband, operating at resonance of loop L and
8、tuning C Radiation resistance in series with loss res.; power divides between Rrad and Rloss Doughnut-shaped radiation pattern around loop Small loops operate in vertical position; virtually ground independent; elevation above ground can be low (few feet)Equivalent circuit Radiation pattern+ radiati
9、on resistanceMy Loop Antenna November 20, 2013 10Characteristics of Small Loop Antennas for HF Operation Good match results with 1/5 diameter coupling loop, but for best performance must adjust couplingslightly for lowest SWR at resonance on lowest band used Easily done by rotating coupling loop (fi
10、xed loops) or compressing/elongating loop (coax coupling loops)Rotating the coupling loop Elongating the coupling loopMy Loop Antenna November 20, 2013 11Alternative to the Coupling Loop: Gamma Match“The Gamma match is a tapped autotransformer with the coax feed braid connected to the loops central
11、neutral point (opposite the capacitor) and the centre conductor connected via the concentric adjustable Gamma tube to the point on the loop conductor where the voltage to current ratio matches 50 Ohms.”“The geometric parameters and the sliding shorting strap are juggled empiricallyto achieve a perfe
12、ct 1:1 VSWR at loop resonance.”http:/.au/documents/the_underestimated_magnetic_loop_hf_antenna_vers%201.1.pdf .(not tried by us)My Loop Antenna November 20, 2013 12Characteristics of Small Loop Antennas for HF Operation The tuning capacitor experiences very high voltage even at low power levels For
13、my loop: 5W 800v 50W 2500v 100W 3600v Must choose capacitor carefully Sliding contact to capacitors rotor are lossy relative to the very small radiation resistance of loop antenna Preferred capacitors are butterfly, split stator, vacuum typesMy Loop Antenna November 20, 2013 13My Loop for 40m 15m Ma
14、in loop is LMR-400 coax (0.405” diam.) 3.67in diameter (1.12m) high power tuning capacitor taken from a 1.5kW capable MFJ antenna tuner Shielded coupling loop (RG-8X) for noise immunityMy Loop Antenna November 20, 2013 14Receive Properties of Electrically-Small Loops Loop antenna is tuned to a reson
15、ance on receive by adjusting the tuning capacitor Atmospheric noise dominates over receiver noise at HF Tuning is narrowband, tending to limit received noise Gain of a loop may be lower than that of other antennas, but what matters is the signal-to noise ratio Small loop antenna can produce a signal
16、-to-noise ratio that is oftentimes better than a dipole for the same bandMy Loop Antenna November 20, 2013 15Transmit Properties of Electrically-Small Loops Small sizeachieved by virtue of construction High efficiencyachieved by proper loop design. Resulting bandwidth is very narrow in an efficient
17、loop Small loop antennas can be almost as efficient as dipole antennas over 20m 10m; over 160m 30m performance within one S-unit High efficiency require larger construction (e.g. 1”copper tubing) Portable loops (e.g. coax loop) less efficient but comparable toother portable antenna solutions (e.g. s
18、hort verticals)Transmit efficiency: the percentage of power applied to the antenna which is transmitted into the air. My Loop Antenna November 20, 2013 16Transmit Properties of Electrically-Small Loops (contd) Radiation resistance Rrad : that part of the antennas feedpoint resistance caused by the r
19、adiation of EM waves from the antenna Rrad is very small Ex: loop diam. 3.67 feet, loop conductor diam, 3 inches Rrad = 0.019 (7 MHz) Rrad = 0.039 (10.1 MHz) Ideal efficiency: 47% on 40m; 76% on 30m Loss due only to RF energy dissipated in the skin-depthof the loop conductor Actual efficiency will l
20、ess because of contact resistance lossMy Loop Antenna November 20, 2013 17Portable Loops Evaluating Efficiency Goal: to build a portableloop: easily transportable and easy to set up Desired power handling ability 50W minimum CW (AlexLoop, Alpha Loop both rated at 10 watts CW) Will be less efficient
21、than fixed-location loop with larger construction Will show that portable loop comparable in efficiency to other small andportableantennasMy Loop Antenna November 20, 2013 18Loop Equations (Circular Copper Loop) (AA5TB)Radiation Resistance, Ohms: RR= (3.3810-8)(fA)2Loss Resistance, Ohms: RL= (9.9610
22、-4)(f)(S/d) (skin resistance only)Efficiency: = RR/(RR+RL)Inductance, Henrys: L = (1.910-8)S7.353log10(96S/pid)-6.386Inductive Reactance, Ohms: XL= 2pif(L106)Tuning Capacitor, Farads: CT= 1/2pif(XL106)Quality Factor: Q = (f106)/f = XL/2(RR+ RL)Bandwidth, Hertz: f = (f106)/Q = (f1-f2)106Distributed C
23、apacity: pF: CD= 0.82SCapacitor Potential, Volts: VC= (PXLQ)f = operating frequency, MHzA = area of loop, square feetS = conductor length, feetd = conductor diameter, inches= decimal value; dB = 10 log10P = transmitter pow r, WattsMy Loop Antenna November 20, 2013 19Loop FormulasThere is additional
24、loss in the loop due to contact resistance. In the formulas, this loss is added to the skin resistance loss.RLtotal = Rskin + RcontactMy Loop Antenna November 20, 2013 20To Measure The Efficiency of A Loop Antenna Measure the 2:1 SWR bandwidth Bmeas Measure the loop dimensions Guess at Rcontact Use
25、the formulas to calculate the bandwidth BWcalc Iterate this process. We endeavor to improve our guess at each turn until the calculated bandwidth BWcalc agrees with the measured bandwidth BWmeas Very simple to do with AA5TBs Excel spreadsheet Once BWcalc BWmeas, we compute the actual antenna efficie
26、ncyvia the formulas (spreadsheet).My Loop Antenna November 20, 2013 21To Measure The Efficiency of a Vertical AntennaPhil Salas AD5DX shows how the efficiency of a vertical can be deduced from a knowledge ofsquare4 the loading coil inductance and placement square4 the length of the antennasquare4 th
27、e ground resistancesquare4 the 2:1 SWR bandwidth http:/ evaluate the efficiency of our loop antenna and compare it tothe performance of various verticalsMy Loop Antenna November 20, 2013 22Evaluation of My Loop Main loop LMR-400 coax (0.405” diam.) LMR-600 coax (0.600” diam.) also tried Main loop 3.
28、67 in diameter (1.12m) Shielded coupling loop (RG-8X); 1/5 diam. of main loop Three different capacitors evaluated Metal tripod, PVC separates metal of tripod from loop PVC loop frame Used MFJ-259 antenna analyzer and MINI-60 SWR digital analyzer Frequency counter of MFJ-259 unreliable at resonance;
29、 had to use MINI-60 after initial tuningMy Loop Antenna November 20, 2013 23Results for VerticalsAntenna Dimension Counterpoise Band 2:1 SWR BW Rad. Res. Loss Res. Efficiency Base-loaded vertical (AB2EW) 6.67 tall Single wire, 25 long, on ground 40m 185 kHz 0.863 65.3 1.3% 30m 410 kHz 1.78 69.3 2.5%
30、 Base-loaded vertical (AB2EW) 15 tall Single wire, /4 long, elevated 5 40m 248 kHz 4.68 32.1 12.7% 30m 505 kHz 9.02 29.8 coil ground 23.2% Carolina Bug Catcher (Phil Salas) 7 tall Typical good ground, (7-15) 40m 30 kHz 2.45 8.9 ground, 7.9 coil 12.7% Hamstick (Phil Salas data) 7 tall Typical good gr
31、ound, (7-15) 40m 50 kHz 2.45 8.9 ground, 20.5 coil 7.7% My Loop Antenna November 20, 2013 24Results for My Loop MFJ High-Power CapacitorTuning Capacitor Coax for Loop Band 2:1 SWR BW kHz Radiation Res. Total Loss Res. skin resistance + contact loss Max. Theor. Efficiency % for zero contact loss Ante
32、nna Efficiency % High-power LMR-400 40m 16.3 0.0092 0.161 10.9 5.4 100W capacity 0.405” diam.) 30m 30 0.039 0.275 30.4 12.5 20m 63 0.145 0.516 57.8 22.0 High-power LMR-600 40m 15 0.0092 0.135 15.4 6.4 (0.600” diam.) 30m 30.1 0.039 0.249 39.3 13.6 20m 42.3 0.145 0.262 67.0 35.7 My Loop Antenna Novemb
33、er 20, 2013 25Results for My Loop Lafayette Capacitor Tuning Capacitor Coax for Loop Band 2:1 SWR BW kHz Radiation Res. Total Loss Res. skin resistance + contact loss Max. Theor. Efficiency % for zero contact loss Antenna Efficiency % Lafayette LMR-400 40m 16 0.0092 0.159 10.9 5.5 (40W capacity) 30m
34、 29.5 0.039 0.270 30.4 12.7 20m 52 0.145 0.401 57.8 26.6 17m 75 0.403 0.381 77.1 51.5 15m 81 0.736 0.130 85.0 85.0 Lafayette LMR-600 40m 15.2 0.0092 0.137 15.4 6.3 30m 29 0.039 0.240 39.3 14.1 20m 43 0.145 0.269 67.0 35.1 17m 57 0.403 0.141 83.0 74.0 15m 79 0.736 0.088 89.0 89.0 My Loop Antenna Nove
35、mber 20, 2013 26Results for My Loop Hammarlund Capacitor Tuning Capacitor Coax for Loop Band 2:1 SWR BW kHz Radiation Res. Total Loss Res. skin resistance + contact loss Max. Theor. Efficiency % for zero contact loss Antenna Efficiency % Hammarlund LMR-400 20m 51 0.145 0.391 57.8 27.1 (100W capacity
36、) 17m 67 0.403 0.297 77.1 57.8 Hammarlund LMR-600 20m 38 0.145 0.221 67.0 39.6 17m 52 0.403 0.094 83.0 81.0 My Loop Antenna November 20, 2013 27Tuning Capacitor Coax for Loop Band 2:1 SWR BW kHz Radiation Res. Total Loss Res. Max. Theor. Eff. % Antenna Efficiency % High-power LMR-400 40m 16.3 0.0092
37、 0.161 10.9 5.4 100W capacity) 0.405” diam.) 30m 30 0.039 0.275 30.4 12.5 20m 63 0.145 0.516 57.8 22.0 High-power LMR-600 40m 15 0.0092 0.135 15.4 6.4 (0.600” diam.) 30m 30.1 0.039 0.249 39.3 13.6 20m 42.3 0.145 0.262 67.0 35.7 Lafayette LMR-400 40m 16 0.0092 0.159 10.9 5.5 (40W capacity) 30m 29.5 0
38、.039 0.270 30.4 12.7 20m 52 0.145 0.401 57.8 26.6 17m 75 0.403 0.381 77.1 51.5 15m 81 0.736 0.130 85.0 85.0 Lafayette LMR-600 40m 15.2 0.0092 0.137 15.4 6.3 30m 29 0.039 0.240 39.3 14.1 20m 43 0.145 0.269 67.0 35.1 17m 57 0.403 0.141 83.0 74.0 15m 79 0.736 0.088 89.0 89.0 Hammarlund LMR-400 20m 51 0
39、.145 0.391 57.8 27.1 (100W capacity) 17m 67 0.403 0.297 77.1 57.8 Hammarlund LMR-600 20m 38 0.145 0.221 67.0 39.6 17m 52 0.403 0.094 83.0 81.0 My Loop Antenna November 20, 2013 28Summary and Conclusion Small portable loops provide excellent reception and good transmit efficiency in comparison to oth
40、er portable antenna solutions Radials or counterpoises not required with loops Vertical loops work well at low elevations Bandwidth very narrow Very high voltages across tuning capacitor Very small radiation resistance requires great care in construction to avoid contact lossMy Loop Antenna November
41、 20, 2013 29DEMOS Voltage intensity around the loop AA5TBs Excel SpreadsheetMy Loop Antenna November 20, 2013 30ReferencesMany of the figures were taken from the websites below. Certainreferences also provide in depth discussions of loop antennas (and vertical antennas from ref. 3)1. http:/ http:/ http:/ http:/.au/documents/the_underestimated_magnetic_loop_hf_antenna_vers%201.1.pdf .5. http:/ . 6. ($366 for portable loop antenna)7. ($300 for portable loop antenna)
