The magnetic loop antennais an extremly efficient short wave antenna for the small size it constitutes.
It consists of a loop radiatormade of copper or aluminium tubing and a tunable capacitor.
The size of the antenna isvery small as compared to the size of a traditional antenna as dipole,beam, quad or vertical. The diameter of the loop is in the range of 1/10to 1/100 of the wavelength.
The antenna works with themagnetic component of the EM field, which extends to the both EM componentson larger distance. For that reason the antenna operates well close toground and radiates a much stronger signal than a dipole when both areclose to ground.
Of coarse, a dipole mountedin its optimal hight radiates better than a magnetic loop, but due itsefficiency at low height the magnetic loop is an excellent portable antennaor may be used well as indoor antenne when external antennas are not permitted.
The capacitor of the antennaneeds to be remotely adjustable to allow a frequency tuning range from1:2 or 1:3. When properly built, the SWR is below 1.1 on the tuned frequencyover the full tuning range.
The bandwidth is always verysmall and covers only a few KHZ. The high Q of the antenna allows a selectivereceiption and suppresses effectively QRM of nearby BC stations, as wellas other QRM.
Here I am using 2 loop antennas,one for 3.5 to 10 MHz and one from
14 to 30 Mhz, both antennaswith only 85cm diameter, below the roof.
The 14 to 30 MHZ antennewith 50 W output allows for regular worldwide contacts with good results.
The antenna can be builteasily as homebrew project if one can find or build a suitable capacitor.
Below you will find a loopcalculation program for your own design together with detailed
instrucions for magneticloop antennas from 3.5 to 30 MHZ.
Circuit for motor control Diagram 6th. Aug 1999
Construction of Mag-Loop (portable/ indoor 14 - 30 MHZ)
Picture of loop 14 - 30 MHZ (Photographthanks HB9DRJ)
Construction of Mag Loop 3.5 - 10 MHZ
Picture of Loop 3.5 - 10 MHZ (Photopgraphthanks HB9DRJ)
Magnetic Loop Antenna : Construction hints
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The following instructions should be observed
for successful construction and operation of magnetic loop antennas:
DANGER : IMPORTANT NOTES !
The radiatedfield is very concentrated and may produce health problems.
Therefore, onehas to keep distance to antenna of at least 5 meters
if the powerexceeds 10 watts.
Coupling to the loop is done mostlyat the lower side of the loop and the
tuning capacitor is placed on top.Due to mechanical stability I installed the capacitor with motor on thebottom and the coupling loop on the top without having any deficiency inHF radiation.
There are several coupling loopconstructions. The simplest one is by forming a loop of installation wire(bare or isolated), or using a coax cable (type RG58). The diameter ofthe coupling loop is 20% of the
diameter of the transmitting loop.The coupling loop is soldered at the
end of the coax cable to the centerconductor and the braid.
There are two forms of feeding:either capacitive coupling or inductive
coupling. The easiest one beingjust a simple wire loop as you see here.
Here you see a capacitivecoupling and here you see a gamma coupling.
The position of the coupling loopis inside the loop, exactly opposite
to the capacitor. At this point,the transmit loop voltage is zero.
The distance between coupling loopand transmit loop varies from 0 to 4 cm,
and by changing this distance,the lowest SWR is adjusted. Fine adjustment
can be done by changing the formof the coupling loop, wider or smaller.
A good coupling loop is the symmetricalloop which you see on this picture.
The environment close to the loopinfluences this adustment.
With proper adjustment a SWR betterthan 1.1 can be reached.
Main loop and coupling loop shouldnot be connected directly, as RF
coupling to the feeding coax mayappear easily and can produce RF
interference (TVI/BCI), thereforeno direct connection between center
of main loop and coupling loop.
However, the main loop may be chargedby static electricity and discharges
by producing QRM bursts. This canbe eliminated by inserting a small coil
between center of main loop andthe coax braid of the feeding coax.
If this connection is done withouta coil, radio interference (RFI) may be
produced due to small misbalance.
Coil data: 40 turns of .3 to 0.5mm enameld copper wire at 1cm diameter
wound over 2 cm length.
The main loop may be made of tubing(copper or aluminum) or thick coax cable. If coax cable is used (RG213or RG8 or similar) the inner
conductor and the braid (= shielding/ground)is soldered together
at both ends. These ends are thenconnected to the capacitor.
Very high current flows in themain loop. Therefore thick and short
copper wires are required to connectthe capacitor.
In tubes, the current flows onlyon the surface due to the skin effect,
therefore the use of foil is aninteresting method.
Very efficient and lightwight loopscan be built by using a thick plastic
support and placing copper foilsover this structure. The foil can be placed
in narrow strips in the directionof the loop circumference, as placing
the foil in one step produces crumpling.
The form of the main loop may besquare, n-square or round.
The round form is most efficientas the losses are minimal.
(Best ratio of L:R).
Nearby environment affects the SWR.
In free field, the body of theloop should be 2 loop diameters above ground.
Good are 5 loops diameter hight,higher elevation gives only small signal difference.
If the loop is installed below theroof, then keep 10 to 30 cm space below
roof brick and avoid nearby linesand metallic constructions.
Of coarse, the roof above the loopshould not be a large metallic
construction.
It's important to observe that thefeeding coax below the loop is kept
in the symmetric center betweenthe two half loops straight downward
at the length of one loop diameter.
By not observing this rule, RFImay be generated !
In any case I recommend to inserta broadband current choke into the
feeding coax, about 1m away fromthe transmitting loop, as currents
on the coax nearly cannot be preventedcompletely.
The loop capacitor needs to withstandhigh voltage and high currents.
Butterfly capacitors are a verygood choice as they have no sliding contacts. Also vacuum capacitors maylead to very good results.
100 watts hf power produce about4000 volts on the capacitor
(see program). Required distancebetween plates is ~ 1 mm per 1000 V.
Suitable capacitors are made byHB9TJX (address at end of page).
Variable vacuum capacitors servewell here, however not all types
are suitable for this application.
A DC motor with strong reduction(1000:1 to 2000:1) serves to
control the capacitor. Suitablemotors can be found in airplane
or car model shops.
(E.G: Robbe No.4103 with 2430:1reduction).
Grill motors may serve also fine.
Please note that there is a highvoltage isolation required between
motor and capacitor.
I recommend to use pulsed DC currentto control precisely DC motors.
By adjusting pulse ratio properly,small increments may be controlled perfectly.
A suitable circuit diagram is includedfor that purpose.
(Switch: FAST/SLOW; keys:RUN und BACKWARD).
The remote control motor may befed through the same coax
feeding the loop, hereby no separatecontrol cable is required.
See link under: Control throughcoax.
Butterfly capacitors of differentvalues for 6 KV
are available at:
Markus Reber, HB9TJX
Eystrassse 7, CH-3400 Burgdorf,Schweiz, Tel: ++41 (0)34 423 16 79
See datasheet at http://rmc.ch.vu
Please send comments or questionsby e-mail to: hb9abx@datacomm.ch
or by Packet Radio to: hb9abx@hb9eas.che.eu