A loop antenna is called "magnetic loop" if the circumference of the loop is smaller than 0.35 wavelength.
The magnetic loop antenna is an extremely efficient short wave antenna for the small size it constitutes.
It consists of a loop radiator made of copper or aluminium tubing and a tuning capacitor. The size of the antenna is very 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 1/10 to 1/100 of the wavelength.
The antenna works primarily with the magnetic component of the EM field, which extends to the both EM components on larger distance. For that reason the antenna operates well close to ground and radiates a much stronger signal than a dipole when both are close to ground.
Surely, a full size dipole mounted in its optimal hight radiates better than a magnetic loop, but due its efficiency at low height the magnetic loop is an excellent portable antenna or may be used well as indoor antenne when external antennas are not permitted.
The capacitor of the antenna needs to be remotely adjustable to allow a frequency tuning range from 1:2 or 1:3. When properly built, the SWR is below 1.1 on the tuned frequency over the full tuning range.
The bandwidth is always very small and covers only a few KHz. The high Q of the antenna allows a selective receiption and suppresses effectively QRM of nearby BC stations, as well as other QRM.
Here I am using 2 loop antennas, one for 3.5 to 10 MHz and one for
14 to 30 Mhz, both antennas with only 85cm diameter, below the roof.
The 14 to 30 MHZ antenna with 50 W output allows for regular worldwide contacts with good results.
The antenna can be built easily as homebrew project if one can find or build a suitable capacitor.
Below you will find a loop calculation program for your own design together with detailed
instrucions for magnetic loop antennas from 3.5 to 30 MHZ.
Calculation program LOOPABXE.EXE click for free download (new version Dec.31, 2003) with
calculation of multiturn loop, calculating all electric data.
For those who are interested: Formulas used by HB9ABX to design mag. loop
Mag-Loop calculation program (DL0HST) in German
read here: http://www.dl0hst.de/magnetlooprechner.htm
Construction of Mag-Loop (portable / indoor 14 - 30 MHZ)
Picture of loop 14 - 30 MHZ (Photograph thanks HB9DRJ)
Picture of Loop 3.5 - 9 MHZ (Photopgraph thanks HB9DRJ)
Control through Coax Update: Jan1,2005
Magnetic Loop Antenna : Construction hints
The following instructions should be observed
for successful construction and operation of magnetic loop antennas:
DANGER : IMPORTANT NOTES !
The radiated field is very concentrated and may produce health problems.
Therefore, one has to keep distance to antenna of at least 5 meters
if the power exceeds 10 watts.
Coupling to the loop is done mostly at 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 the bottom and the coupling loop on the top without having any change in HF radiation.
There exist several different coupling systems, but the following one
proved to be the best, which was developed by me recently(ABXKOPPEL):
Inductive coupling system using a wire loop, which is led close to the main loop, at the center
of the main loop in the length of half the diameter of the main loop. Then, from both ends leading
to the center, and from there slightly twisted to the output of the 1:1 current balun.
The input of the current balun connects to the 50 Ohm feeding coax.
The wire of this coupling loop is isolated copper wire of 0.6mm diameter (AWG 22). The diameter
may be up to 1mm (AWG 18), and thicker wire is in no way better.
The construction is seen in this picture ABXKOPPEL .
The advantage of this coupling system is, that the SWR remains below 1.1 over the full frequency
range of the antenna! All the other coupling systems require readjustment if you change to an other
frequency band to obtain minimum SWR.
The precise adjustment is done at the final installation location. This is done by lifting or
lowering the center point of the coupling loop where the twisting of the wire begins, by pulling
an isolated guy wire slightly up or down until the SWR is below 1.1.
In free field, the center point is somewhat higher, in a building or below a roof somewhat lower.
The indicated length of half loop diameter is near to the optimal length.
If a sufficient low SWR cannot be reached, then the length of the abxkoppel should be reduced by 1cm and the
SWR measured again. If the SWR now is lower, then the length has to reduced further; if the SWR
is higher, the length should become greater. With a few cm of modification the optimal length can
easily be found. There is no soldering required to do this modification. As a result, the twisted line will
become a bit longer or shorter, which has no influence.
Dimension of ABXKOPPEL for the 3 turn loop of 85cm diameter:
Length of couple loop tightly led aside the main loop: 43cm
Horizontal length: 40cm (2 x 20cm from corner to center)
Length of twisted wires: 12cm (this length is uncritical)
- In general: length of couple loop, tightly aside of main loop: half main loop diameter.
Here is the picture of the 160m magnetic loop with ABXKOPPEL at HB9MTN.
(Note the calculated bandwidth and Q and the measured values).
- - -
In the following part, other coupling systems are described, some using inductive coupling,
some capacitive coupling, or galvanic coupling.
The easiest one being justa simple wire loop a wire loop as you see here.
Here you see a capacitive coupling and here you see a gamma coupling.
The position of the coupling loop is inside the loop, exactly opposite
to the capacitor. At this point, the voltage of the transmit loop is zero.
The distance between coupling loop and transmit loop varies from 0 to 6 cm.
By changing this distance, the lowest SWR is adjusted. Fine adjustment
can be done by changing the form of the coupling loop, wider or smaller.
A good coupling loop is the symmetrical loop which you see on this picture.
The environment close to the loop influences this adjustment.
With proper adjustment an SWR lower than 1.1 can be reached.
Main loop and coupling loop should not be connected directly, as RF
could be misbalanced and produce surface waves on the feeding coax,
which in turn can produce TVI or BCI in the building.
However, the main loop may be charged by static electricity and discharges
by producing QRM bursts.
Static charging of the antenna is preventd by connecting a 1 KOhm
resistor to the center of the main loop, and the other side of the
resistor is connected to ground or to the shield of the feeding coax.
The main loop may be made of tubing (copper or aluminum) or thick coax cable. If coax cable is used (RG213 or RG8 or similar) the inner
conductor and the braid (= shielding/ground) is soldered together
at both ends. These ends are then connected to the capacitor.
Very high current flows in the main loop. Therefore, thick and short
copper wires are required to connect the tuning capacitor.
In tubes, the current flows only on the surface due to the skin effect,
therefore the use of foil is an interesting method.
Very efficient and lightwight loops can be built by using a thick plastic
support (ring)and placing copper foils over this structure.
The foil can be placed in narrow strips in the direction of the loop
circumference, as placing the foil in one step produces crumpling.
The form of the main loop may be squared, n-squared or round.
The round form is most efficient as the losses are minimal.
(Best ratio of L:R).
Nearby environment affects the SWR.
In free field, the body of the loop 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 the roof, then keep 10 to 30 cm space below
roof brick and avoid nearby lines and metallic constructions.
Note that the roof above the loop should not be a closed metallic
It's important to observe that the feeding coax below the loop is kept
in the symmetric center between the two half loops straight downward
at the length of one loop diameter.
By not observing this rule, RFI may be generated !
In any case, I recommend to insert a broadband current balun into the
feeding coax, directly before the coupling loop , to prevent any surface currents on the feeding coax line.
The loop capacitor needs to withstand high voltages and high currents.
Butterfly capacitors are a very good choice as they have no sliding contacts. Variable vacuum capacitors are an excellent choice.
100 watts RF power produce about 4000 volts on the capacitor
(see program). Required distance between plates is ~ 1 mm per 1000 V.
Suitable capacitors are made by HB9TJX (address at end of page).
A DC motor with strong reduction (about 2000:1) serves to
control the capacitor. Suitable motors can be found in airplane
or car model shops.
(E.G: Robbe No.4103 with 2430:1 reduction).
Grill motors may serve also fine.
Please note that there is a high voltage isolation required between
motor and capacitor.
I recommend to use pulsed DC current to control precisely DC motors.
By adjusting pulse ratio properly, small increments may be controlled perfectly.
A suitable circuit diagram is included for that purpose.Tuning procedure of antenna in operation:
(Switch: FAST/SLOW; keys: RUN und BACKWARD).
Run motor fast in "receive mode" until the noise or the signal is increasing strongly.
Then go to "transmit low power", run the motor slowly and observe the SWR meter.
Adjust to minimum SWR by moving "forward/backward". Ready.
The remote control motor may be fed through the same coax
feeding the loop, hereby no separate control cable is required.
See link under: Control through coax.
Butterfly capacitors of different values for 6 KV
are available at:
Markus Reber, HB9TJX
Eystrassse 7, CH-3400 Burgdorf, Switzerland, Tel: ++41 (0)34 423 16 79
Please send comments or questions by e-mail to: see here (click)
Manufacturers of magnetic Loops:
- MFJ Loops (good experience, cheap) (Product Catalog, Antennas, MFJ-1786, MFJ-1788)
- Christian Käferlein, Darmstadt, Germany
(SK in October 2005)
- HB9CRU(I3VHF Magnetic Loop Antenna)