The magnetic loop, thus named by the use of the magnetic component of the electromagnetic field, is a parallel circuit LC. The circular form is often met on the commercial models but this antenna can be hexagonal, octagonal or square. The loop has a circumference of 1/10ème of lambda, usually. This loop is closed on a variable capacitor with strong insulation which must be able to support several Kv because of a high Q factor and a very low resistance of radiation. The band-width is thus very narrow.
The antenna is feeded using a coupling loop or a gamma, located opposite to the capacitor.
The difficulty is to design or to get the variable capacitor.It can be motorized and thus operated from the station. Is needed then a very precise gearing down, because if the VC make it possible to cover several bands, The adjustment on each one of them is then very narrow.
Example of execution
Here an execution inspired of an article of F2HN published in Radio-Ref. (10/1988 - a 1 meter square loop antenna provided with one VC and fed via gamma).
Having a variable capacitor whose value is only 10pF, I chose a set of fixed capacitors removable and designed with coaxial cable. Since each change of band even of frequency requires an intervention on the antenna, the interchangeable capacitors provided with banana plugs do not complicate really the operation, they even simplify it: For the 40m band, the small VC make just it possible to sweep the 100kHz band and thus to easily choose a portion of it (band-width with SWR=2.0 on 7MHz: 25kHz).
For the 30m band, the VC make it possible to cover a bandwidth of approximately 300kHz (thus the 100kHz 10.100 to 10.150) and the adjustment remains easy.
On 20m, the VC make it possible to cover 850 Khz. The band (14 to 14.350) is located then on a small portion of the VC, making the adjustment more difficult.
The antenna is envisaged to be used on 40 and 30m band (for the 80m band, it is preferable to double the circumference of the loop. For the 20m band and less, the loop of 4m is too large).
- 4 x 1 meter of copper tubes (diameter 14mm (0.55 in.)
- 4 elbows of connection
- 1 small cut-tube (much more practical than the hacksaw)
- Weldings: etaing used in electronics and little blowtorch
- insulated solid copper wire (2.5 mm2 (0.004 sq in.) even 4mm2 ( 0.0062 sq in.) for the execution of gamma.
- A domino which can slide on gamma (for the adjustment of the SWR - it will be removed from its plastic protection)
- a collar where the outer braid of coaxial cable will be welded. It will be fixed on the frame, in front of the gamma and will slide too for the adjustment of the SWR.
- 1 SO239 (or 50 ohm BNC) chassis mounted connector. (useless if you weld directly, the coaxial cable coming from the station).
support (variable capacitor and connectors for the removable capacitors) :
- a plate of plexi (10cm X 17cm (3.93 x 6.69 in.), according to dimensions of the VC used)
- 2 Chassis Mount Female Banana Plugs.
- Short length of insulated stranded copper wire 2.5 mm2 (0.004 sq in) used to interconnect Chassis Mount Female Banana Plugs and end tubes)
- 4 tapping screws (fixing of the plexi on the tubes)
- 1 variable capacitator 10pF, at least 3mm inter-blades (0.12 in.)
- a button for the variable capacitor (insulating matter. You should not in no case to be in direct contact with the axis of the VC during transmitting).
"coaxial" capacitor (removable) :
- 1 GREY tube PVC (because without carbon) diameter 100mm (3.94 in.) (one slice 5 to 7 cm (1.97 to 2.75 in.) bye capacitor).
- 2 male banana plugs by capacitor.
- A few meters of coaxial cable (That will depend on the model used. For example, for the 40m capacitor, I used TV coaxial (aluminum braid) with approximately 54pF/m = approximately 2m50 (8.2 ft.). you can use RG213 cable (100pF/m and stronger tension of insulation).
- Small clastic cable ties. (strapping banana plugs on PVC).
- about 15 cm plastic cable ties (5.9 in.) (strapping coaxial on PVC).
The antenna could be suspended vertically.
Equalize the length of the copper tubes. Remove the labels of reference and clean, degrease carefully the tubes.
Form the frame on a plane and stable support, a large table for example and maintain the whole by checking that the two diagonals of the frame are equal. The elbows are welded (one needs a little welding, when copper is at sufficient temperature, the welding penetrates by capillarity in the junction tubes/elbow).
The frame being finished, a side will be dedicated to the up of the antenna. In its center, mark a space of 4cm (1.57 in.) which will be removed later.
Prepare the plate of plexi. (see drawing and example).
Fix the two chassis mount female banana plugs.
Fix the variable capacitor by directing these solder contacts towards the tubes.
Important note: The antenna produced here is adjusted without transmitting. If you project to carry out the adjustments with transmitting (case of the use of a ROS-meter; With power the lowest possible, and in any event lower than 10w), make a serious insulation of the axis of the VC. Prolong the axis of the VC with an insulation plug which will support the insulating control knob. It can be then more interesting to fix the VC so that the axis is vertical.
Put the plexi on the place previously marked (center top of the antenna), mark the tubes through the holes of the plexi. punch these marks and pierce the holes which will receive the tapping screws.
Cut out and remove the central part of 4cm (1.57 in.). The top of the frame is now in two part. Weld at the ends of each one of these tubes, 4 in. of flex-wire 2.5mm2 (0.004 sq in) (apart from the zone which will be in contact with the plexi, the welding would form an allowance).
Fix the plexi plate, weld the end of each wire onto the banana plugs. Connect also those at the ends of the variable capacitor.
Form the gamma, weld its end in the center of one frame's side.
Coaxial cable :
The center conductor is welded onto a domino, separated beforehand from its plastic coating. Interconnect the outer braid and the tube collar, welding a small portion of rigid wire.
This device makes it possible to very easily move the power supply of the antenna along gamma, without repeated operations of welding. Once the finished adjustment, the two screws of the domino and the tightening of the collar, are enough to ensure an excellent electrical contact.
It is not necessary to envisage an intermediate support insulating to maintain gamma. Especially if it were envisaged in 4mm2 (0.0062 sq in.), its rigidity and the presence of coaxial at its end is enough to maintain it in place.
For each band, coaxial cable is wound on a PVC support and is used a such fixed capacitor put in parallel on the variable capacitor of the antenna.
The length of cable to be used depends on the characteristics of the coaxial. Here a table concerning two types of cable: TV coaxial cable (very common) and of the RG213 often present in our stations. The significant parameters are the pf/m value and the quality of dielectric conferring a good insulation on high voltages.
|7||114||TV (53 pF/m) (16.1pF/ft)||215 (7.05)|
|RG213 (97pF/m) (29.5pF/ft)||117.5 (3.85)|
|10||48||TV (53 pF/m) (16.1pF/ft)||91 (2.98)|
|RG213 (97pF/m) (29.5pF/ft)||49.5 (1.62)|
|14||15||TV (53 pF/m) (16.1pF/ft)||28.3 (0.93)|
|RG213 (97pF/m) (29.5pF/ft)||15.4 (0.50)|
Do not hesitate to use a bigger length than envisaged. The coaxial cable will be shortened gradually for the adjustment, it is better that it is too long that not enough.
Cut a section of 7cm (for the 40m) or 5cm (for the 30 or the 20m) of PVC tube. remove the plastic external part of two male banana plugs and weld an end of the coaxial cable there, so that the plugs are spaced of 4cm. Fix the two banana plugs on the edge of PVC (the male part exceeds PVC) so as to preserve 4cm between them. They can be maintained each one by a small plastic cable ties passed through PVC (two small holes to be pierced on PVC for that, on both sides of each banana plug).
wind the coaxial cable (the whorls can be disordered) by maintaining it by adhesive tape which will be replaced by plastic cable ties once the finished adjustment.
For that, a SWR-meter (use the lowest possible power), a noise bridge or an antenna analyzer (MFJ259 for example), must be inserted close to the antenna.
Install the capacitor for the 7MHz band and adjust the VC at its middle range. Make slide gradually the domino on gamma and the collar on the frame, until obtaining a minimum SWR.
Shorten the length of the coaxial cable forming the capacitor gradually until obtaining a resonance frequency of 7050 Khz (or center of desired band). The VC should then make it possible to cover 7MHz band (100kHz).
Once this adjustment carried out, remove the outer sheath and the outer braid at the end of the cable, on 1 cm (0.39 in.). Tighten the plastic collars to firmly maintain the coaxial cable on PVC.
The operation can be repeated with the 30m and 20m capacitor . According to the band, the length of gamma used can be different. (for example 28cm (11 in.) for the 30m and 35cm (13.7 in.) for the 40m). In this case, a compromise can be found around 31cm (12.2 in.) allowing a ROS of 1.2 on both bands.
The -3dB band-width was measured by the method described in the instruction manual of the MFJ66 (dip Meter adapter for MFJ259B).
Any dip-meter which can be used, here an extract of the original instructions manual :
To measure the Q of a circuit you must add a detector circuit (see the figure opposite) and couple it the SWR Analyser (this may slightly alter the Q of the circuit.) Relative Q can be observed by noting the stepness of the dip as you change frequency. A sharp deep dip at resonance is an indication of high Q. A wide shallow dip at resonance is an indication of a low Q.
- Connect a high impedance digital voltmeter across the test circuit in the figure above. Use the lowest range of the voltmeter.
- Couple the SWR Analyzer to the tank circuit. Adjust the Tune control for a maximum voltage reading on the voltmeter. Do not change the coupling during the rest of the test. Record this frequency as F0.
- Find a point above and below F1 that the voltage is at 70% of its max. Record these frequencies as F1 and F2.
- Divide the positive difference between F1 and F2 by F0 to get Q.
The vertical indication graduated into mV corresponds to the measured voltage. The device of measurement remained in place during measurements on the three bands.
Summary gamma/variable cap/removable cap for the three bands.
|Band (MHz)||Gamma cm (in.)||SWR||Fmin Fmed Fmax Var cap (kHz)||Remov.cap. (pF)|
The antenna could be used with 100w if the variable capacitor is sufficiently isolated (inter-blades : more than 3mm) and the coaxial cable from the removable capacitors, of a sufficient diameter.
Like all these small size antennas compared to the wavelength used, the magnetic loop is obviously less efficient than a dipole.
In the particular context of a use in urban environment, inside or near obstacle preventing this time the operation of a dipole, this antenna is surprising. Not very sensitive to the qrm, the signal-to-noise ratio is very interesting. This type of antenna can be also an excellent base for the construction of an active antenna, for an short-wave listener.
The magnetic loop by F6CRP
(Seek the line ?antenne boucle magnétique? in the list on the right)
Examples of constructions
F6EHP. See this 2.20m square loop for 40/80m.
PY1AHD Alex. Several interesting models, of which a hydraulic tuned small magnetic loop. ;o)
AA5TB. Theory, practical, equations. Some antennas for the 70cm band.
Examples of commercial antennas
Ciro Mazzoni Radiocomunanzione Verona, Italy
MFJ. The MFJ-1786 loop.
INAC. The AH-521 loop.
In physics, we often expresses the wavelength with the Greek letter λ (lambda).
The Q factor expresses the quality of an RLC circuit and is defined at its resonance. If this factor is high, the bandwidth is reduced. It also represents the ratio of the measured voltages on the circuit at the resonance and that of the source.
S.W.R.Standing wave ratio
When a transmitter is connected to an antenna, it is hoped that all the RF signal will be radiated from the antenna. In practice, some of the RF energy is reflected back to the transmitter. Reflected energy can damage the transmitter components. The ratio of the forward power and reflected power is the S.W.R. value.
For the development of these antennas, the ideal is to use an antenna analyzer or a noise bridge. Here are some useful links:
MFJ202B from MFJ
RX100 from Palomar (no longer available)
VA1RX from Autek (no longer available)
MFJ259 from MFJ or MFJ-223, MFJ-225
miniVNA HF/VHF (on PC).
AA 908 from american qrp club
A project by VK5JST
A description by VK5BR