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1、WA4MNT Small Transmitting Magnetic Loop Antenna Project 30m, 20m, 17m, 15m, 12m, 10m Bands With the solar cycle improving, I wanted to build an efficient HF antenna for the upper bands that I could easily throw in the trunk of the car and operate quickly with a minimum of setup time. I decided on a
2、small magnetic loop antenna after researching the subject on the web and reading the following statement from VK5KLTs article on small magnetic loop antennas. “A properly designed and constructed small loop of nominal 1m diameter will outperform any antenna type except a tri-band beam on the 10m/15m
3、/20m bands, and will be within an S-unit (6db) or so of an optimised mono-band 3-element beam thats mounted at an appropriate height above ground.” There is a wealth of information on small magnetic loop antenna construction on the web. One of the best sources I found was by AA5TB, http:/ and VK5KLT
4、, http:/ . The antenna I built is a compilation of ideas I borrowed from many sources, gleaned from the work of others, and a little redesign. This project resulted in a three foot diameter copper loop mounted on a small pedestal, with continuous coverage of 30m through 10m bands (10MHz-30MHz). I ha
5、ve been able to obtain an SWR of 1:1 - 1.2:1 over the entire range. Based on the advice of the most successful builders, I chose copper as the metal of choice and all joints are silver soldered to reduce the interconnection resistance. I chose to design my own trombone style capacitor (10pF - 110pF)
6、, and shielded Faraday Loop input. I used copper tubing, readily available low loss dielectric materials, PEX (cross linked polyethylene) tubing for the capacitor insulator, UHMW plastic (Polypropylene) for all other RF exposed parts, and non magnetic hardware for all mechanical fastening. My design
7、 was based on AA5TBs on-line calculator, http:/ and my dielectric spacing exceeds a 2KW rating. I chose to mount my portable loop, a little less than one diameter, off the ground, from the bottom of the loop, with six radials, two loop diameters long, from the base. These antennas are high-Q resonan
8、t circuits. Many kilovolts can be present across the capacitor, and produce concentrated electro-magnetic radiation even at low power levels. For safety, maintain a minimum of 6 feet away from the antenna, while transmitting. I have access to milling and lathe equipment, so my exact approach may not
9、 be suitable for many amateurs; however many good designs are available using butterfly or vacuum capacitors and easier available tools. Even simpler monoband designs may be more appropriate. I incorporated a motor drive for remote tuning with a wired control cable. With the antenna bandwidth being
10、so narrow, tuning for maximum receiver noise yields almost optimum SWR. I use no antenna tuner between the radio and antenna. I am not an antenna theoretician; my expertise is in mechanical design. I have attached all my detailed .pdfs and defer to the complete VK5KLT “An Overview of the Underestima
11、ted Magnetic Loop HF Antenna” article at the end of this document for the theory of operation. Results using my MFJ-259B antenna analyzer: 10m - 28.700 SWR 1.2 : 1 R=53, X=10 28.200 SWR 1.1 : 1 R=46, X=8 12m 24.900 SWR 1.2 : 1 R=56, X=9 15m 21.300 SWR 1 : 1 R=47, x=0 21.060 SWR 1 : 1 R=43, X=0 17m 1
12、8.150 SWR 1.2 : 1 R=43, X=7 20m 14.250 SWR 1.1 : 1 R=43, X=0 14.060 SWR 1.1 : 1 R=44, X=0 30m 10.125 SWR 1.1 : 1 R=50, X=8 Small magnetic loops typically have 5 dBi gain when used with two loop diameter length radials. They exhibit a vertically polarized signal at the horizon and horizontally polari
13、zed signal overhead. Thank you to all the amateurs that have shared their wisdom and made the information public on the internet to make this project a success. You may consider joining the Yahoo groups, MagneticLoopAntenna or MagLoop Ken - WA4MNT E-mail Trombone style capacitor 10pF 110pF Shielde
14、d Faraday input loop Trombone capacitor gear drive Using a , #16816MD, 24 vdc, 40 rpm gearmotor Frequency scale Base with thumbscrews for 6 radials, and surplus fiberglass mast section. Motor controller and cables 12v gel cell transceiver supply, with 12v to 24v switching supply DC-DC module, for t
15、he gearmotor drive Set up for operation Small enough to fit in the trunk of my Toyota Corolla 12v -24v switcher for gearmotor Motor controller, direction and speed , #16816MD, 24v, 40 rpm gearmotor Common Plastics Dissipation Factor Chart This will aid you in selecting suitable plastics for loop con
16、struction. Look for plastics with low dissipation factors. G10 /G11 glass epoxy board have a poor dissipation factor, 0.018, similar to PVC. UHMW (Polypropylene) is one tenth the cost of PTFE (Teflon). 2 B 13 4 7865 21347865 A C D B A C D LOOP_ASSYSHEET 1 OF 3 3RD ANG PROJECT X.XXX 0.001 REVDWG. NO.
17、 FORMAT - BSCALE: 0.048X.XX 0.01 DO NOT SCALEX.X 0.1 WA4MNT 3 SMALL MAGNETIC LOOP ANTENNA 30m-10m TITLE ENG: WA4MNTX 0.25 WA4MNT P.O. BOX 956 CLARKDALE, AZ 86324 928-639-3481 (VOICE) WWW.QRPBUILDER.COM THE INFORMATION CONTAINED IN THIS DOCUMENT IS THE PROPERTY OF BENT RIVER MACHINE AND SHALL NOT BE
18、USED OR DISCLOSED OUTSIDE OF BENT RIVER MACHINE WITHOUT WRITTEN AUTHORIZATION ANG. 0.5 DIMENSIONS - INCHES UNLESS SPECIFIED TOLERANCES ARE: BYDATEDESCRIPTIONECOREV KL08/08/10INITIAL RELEASE NOTES: ALL COPPER ANTENNA ELEMENTS JOINED BY SILVER SOLDER ANY FASTENERS USED MUST BE 300 SERIES S.S. OPTIMUM
19、HEIGHT IS TWO LOOP DIAMETERS ABOVE GROUND OPTIMUM RADIALS ARE 2 LOOP DIAMETERS LONG THESE ANTENNAS ARE HIGH-Q RESONANT CIRCUITS. MANY KILOVOLTS CAN BE PRESENT ACROSS THE CAPACITOR, AND PRODUCE CONCENTRATED ELECTRO-MAGNETIC RADAITION EVEN AT LOW POWER LEVELS. FOR SAFETY, MAINTAIN A MINIMUM OF 6 AWAY
20、FROM THE ANTENNA WHILE TRANSMITTING. CAUTION ! SEE DETAIL A SEE DETAIL B SEE DETAIL C SEE DETAIL D 1.825“ O.D. FIBERGLASS MAST SECTION (MILITARY SURPLUS) ALUMINUM BASE PLATE SEE DETAIL A SEE DETAIL B SEE DETAIL C SEE DETAIL D 2 B 13 4 7865 21347865 A C D B A C D LOOP_ASSYSHEET 2 OF 3 3RD ANG PROJECT
21、 X.XXX 0.001 REVDWG. NO. FORMAT - BSCALE: 0.048X.XX 0.01 DO NOT SCALEX.X 0.1 WA4MNT 3 SMALL MAGNETIC LOOP ANTENNA 30m-10m TITLE ENG: WA4MNTX 0.25 WA4MNT P.O. BOX 956 CLARKDALE, AZ 86324 928-639-3481 (VOICE) WWW.QRPBUILDER.COM THE INFORMATION CONTAINED IN THIS DOCUMENT IS THE PROPERTY OF BENT RIVER M
22、ACHINE AND SHALL NOT BE USED OR DISCLOSED OUTSIDE OF BENT RIVER MACHINE WITHOUT WRITTEN AUTHORIZATION ANG. 0.5 DIMENSIONS - INCHES UNLESS SPECIFIED TOLERANCES ARE: BYDATEDESCRIPTIONECOREV KL08/08/10INITIAL RELEASE 0.750SCALE ADETAIL WWW.MPJA.COM MOTOR #16816 MD 12 VDC, 40 RPM ROTATION INDICATOR 2 EA
23、. MCM 7297K15 BEVEL GEARS oftentimes with absolutely stunning performance rivalling the best conventional antennas. Easily field deployable and fixed site tuned loops have been the routine antenna of choice for many years in professional defence, military, diplomatic, and shipboard HF communication
24、links where robust and reliable general coverage radio communication is deemed mandatory. On 80m and 160m top-band the performance of a small loop antenna generally exceeds that achievable from a horizontal dipole, particularly one deployed at sub-optimal height above ground. This is a common site l
25、imitation for any HF antenna. So wheres the catch; if the small loop is such a good antenna why doesnt everyone have one and dispense with their tall towers? The laws of nature and electromagnetics cannot be violated and the only unavoidable price one pays for operating with an electrically-small an
26、tenna is narrow bandwidth. Narrow instantaneous bandwidth rather than poor efficiency is the fundamental limiting factor trade-off with small loops. 2 Any small antenna will be narrow band and require tuning to the chosen operating frequency within a given band. Users of magnetic loops must be conte
27、nt with bandwidths of say 10 or 20 kHz at 7 MHz or a little more than 0.2%. They are content as long as the antenna can be easily tuned to cover the frequencies that they wish to use. For a remotely sited or rooftop mounted antenna implementing this tuning requires just a modicum of that ingenuity a
28、nd improvisation radio hams are renowned for. A small transmitting loop (STL) antenna is defined as having a circumference of more than one-eighth wavelength but somewhat less than one-third wavelength which results in an approximately uniform current distribution throughout the loop and the structu
29、re behaves as a lumped inductance. The figure-8 doughnut shaped radiation pattern is in the plane of the loop with nulls at right angles to the plane of the loop. The loop self-inductance can be resonated with a capacitance to form a high-Q parallel tuned circuit. The attainment of a high-Q tells us
30、 that the loop antenna is not lossy and inefficient. When power is applied to the loop at its resonant frequency all of that power will be radiated except that portion absorbed in the lumped I2R conductor and capacitor losses manifesting as wasteful heat. With proper design these series equivalent c
31、ircuit losses can be made negligible or at least sufficiently small compared to the loops radiation resistance that resultantly high intrinsic radiation efficiency and good antenna performance can be achieved. Current through the loops radiation resistance results in RF power being converted to elec
32、tromagnetic radiation. However, since the small loops radiation resistance is very small compared to that of a full sized resonant dipole, getting this favourable ratio of loss to radiation resistance is the only “tricky” and challenging part of practical loop design and homebrew construction. Throu
33、gh utilizing a split-stator or a butterfly style air variable capacitor construction or preferably a vacuum variable capacitor, low loss can be achieved in the tuning capacitor. Conductor loss can then be controlled by optimal choice of the diameter of copper tubing used to form the loop element and
34、 paying very careful attention to low ohmic interconnections to the capacitor such as welded or silver soldered joints, etc. With 100 Watts of Tx drive power there are many tens of Amperes of RF circulating current and Volt-Amps-Reactive (VAR) energy flowing in the loop conductor and tuning capacito
35、r. In the case of an air variable, capacitor losses are further minimised by welding the rotor and stator plates to the stacked spacers to eliminate any residual cumulative contact resistance. When connected across the loop terminals the butterfly construction technique inherently eliminates any los
36、sy rotating contacts in the RF current path. The configuration permits one to use the rotor to perform the variable coupling between the two split stator sections and thus circumvent the need for any lossy wiper contacts to carry the substantial RF current. Since the fixed stator plate sections are
37、effectively in series, one also doubles the RF breakdown voltage rating of the composite capacitor. In view of the fact the loop antenna is a high-Q resonant circuit, many kilovolts of RF voltage can be present across the tuning capacitor and appropriate safety precautions must be taken. Small trans
38、mitting loop antennas capable of handling a full 400 Watts PEP or greater are readily achievable when appropriate construction and tuning components are selected. Feeding and matching: Although loop antennas have deceptively simple appearance, they are complex structures with radiation patterns and
39、polarisation characteristics dependent on whether theyre fed in a balanced or unbalanced fashion. The method of feeding and matching the loop resonator, 3 ground plane configuration, as well as the geometric form factor and physical proportions of the loop element itself are all fertile ground for e
40、xperimentation. Various matching methods include series capacitor, transformer coupled subsidiary shielded-Faraday loop, and gamma-match, etc; each with their respective merits. The choice really boils down to personal preference as both the gamma and Faraday feed techniques work well. However, the
41、Faraday shielded auxiliary loop located at the bottom central symmetry plane yields better loop electrical symmetry and balance that can in turn provide sometimes beneficial deeper front-to-side ratio and pattern nulls. In addition to imparting slight pattern asymmetry the Gamma match method can als
42、o result in some deleterious common-mode current flow on the outer braid of the feed coax that might need choking-off and isolating with ferrite decoupling balun to prevent spurious feeder radiation and extraneous noise pick-up on Rx. Much also depends on the site installation set up in respect of c
43、onductive objects in the loops near field that can disturb symmetry. With the elegantly simple transformer-coupled Faraday loop feed method the 50 signal source merely feeds the auxiliary loop; theres no other coupling / matching components required as there are no reflected reactive components to d
44、eal with (the main loop appears purely resistive at resonance with just the core Rradand Rlosscomponents in series). The impedance seen looking into the auxiliary feed loop is determined solely by its diameter with respect to the primary tuned resonator loop. A loop diameter ratio of 5:1 typically y
45、ields a perfect match over a 10:1 or greater frequency range of main loop tuning. Simple transformer action occurs between the primary loop and the feed loop coupled circuit due to the highly reactive field near the resonant primary loop which serves to greatly concentrate magnetic flux lines which
46、cut the small untuned feed loop. The degree of magnetic flux concentration is a function of the Q of the tuned primary which varies with frequency, i.e. the highest Q occurring at the lowest frequency of operation and the lowest Q exhibited at the highest frequency. This variation in Q results from
47、the variation in the sum of the loss resistance and the complex mode radiation resistances of the primary radiator loop as a function of frequency. The effective feed impedance of the secondary loop is controlled by its diameter / ratio of area and by the number of flux lines cutting it; thus the im
48、pedance seen looking into the secondary loop will be essentially independent of frequency. One can intuitively see this because when the feed loop is extremely small in relation to a wavelength at the lowest frequency of operation, the number of magnetic flux lines cutting it is large because of the
49、 very high Q, whereas when the feed loop becomes a larger fraction of a wavelength as the frequency of resonance is increased, the concentration of flux lines is reduced due to the lower Q. If one seeks mode purity and figure-8 pattern symmetry with deep side nulls, the fully balanced Faraday transformer c