Loop Antennas
Loop antennas receive the magnetic component of the electromagnetic wave. There are several ways to build such an antenna, which have different names. But they are all loop antennas.
Loop antennas can be made in many ways - in principle, it is just a conductive wire, wound as a coil.
The shape does not matter as it is the area of the total number of turns that determines the strength of the signal - Circular giving the largest area for a given length of wire, but if it is easier to make a rectangular or any other shape, You are welcome.
Magnetic signals in a loop generate a current, so the wire used should be minimum 0.75mm² and preferably thicker.
Wire Loop / Frame antennas
Wire Loop - “Classic”
Loop antennas has been knows since 30s, so it was obvious to start with this model.
Wire Loop
It is easy to set up a frame antenna for VLF frequencies by yourself. Useful construction manual can also be found in a document written by Gerald Ihninger for the no longer existing http://www.lightningradar.net. For a square frame antenna with a diagonal of 1 m and 8 turns of wire you need
- approximately 4 m of 20 x 20 mm wooden coving,
- 2 x 25 m cable with a single conductor of area 1.5mm² (this corresponds to a diameter of 1.38 mm),
- 8 screw hooks,
- 3 screws/washer/wingnuts, and
- some cable ties.
The assembling takes about two hours. The price for the material is approximately 40 Euro in a German home improvement store.
Figure 15: Assembling details for the frame antenna
Originally made for System GREEN - for System BLUE 3-4 turns is enough
System GREEN: 8 turn, square 100cm ~ Area = 8m²
System BLUE: 3-4 turn, square 100cm ~ Area = 3-4m²
Use two vertical loops at 90 degrees or three vertical loops at 60 degrees. The yellow horizontal loop in the left picture is no longer recommended.
Comment from Egon on vertical antennas: We have implemented three H-field channels to make experiments. Initially, in addition to the two horizontal antennas offset by 90 degrees, we connected a third vertical H-field antenna. We wanted to know if this results into better reception characteristics. This has not been confirmed. The interference signals at the vertical channel are much stronger than the lightning signals. So it makes no sense to position a third vertical antenna in addition to the two horizontal antennas. The detector would work almost permanently in interference mode. If you want to use the third channel, then please also use it as a horizontal channel, for example, with three antennas offset by 60 degrees, or to make experiments.
Multi Turn
20 turn, Ø38cm (38cm diameter) ~ Area ~ 0.19² * Pi * 20 ~ 2,3m² (inductance ~ 500μH)
Use 1mm² lacquer or plastic insulated copper.
Coaxial cable antennas
A cheap loop antenna can easily be constructed using coaxial cable. It has the advantage, that the shield is already integrated in the cable. You can use an arbitrary impedance, but the 75 Ohm version are very cheap to get. With 20 Meter coaxial cable you can form a loop with 3 turns and a diameter of 180 cm (System RED) or 100 cm (System BLUE). Keep 20 cm cable for the connection at both ends. Such a construction will create a loop with an induction of approximately 60 µH and a self-resonance at 2 MHz.
Inner conductor to the terminal 1-2 and the screen from both ends to terminal 3 (shielding).
System RED: 3 turn, circle Ø180 (180cm diameter) ~ Area = 0.9² * Pi * 3 ~ 7,6m²
System BLUE: 3 turn, circle Ø100 (100cm diameter) ~ Area = 0.5² * Pi * 3 ~ 2,4m²
Figure 16: A coax antenna, © Richo Andersen
Cut off the plastic sheath,
remove the wire grid,
put the separated plastic sheath back,
and cover the area by a flexible hose.
Figure 17: Cutting the wire grid in the middle of the coaxial antenna, © Richo Andersen
Figure 18: Connecting a coax antenna, © Richo Andersen and Bernhard Kilimann
Möbius antenna
2 turn, circle Ø100 (100cm diameter) ~ Area = 0.5² * Pi * 4 ~ 3,14m²
The cable is cut midway and the shield from one end soldered to the inner conductor at the other end and vice versa - that way there will be 4 active turns.
Antenna w/Transformers
Whether we use 4 turns of 1mm² or 1 turn with 4mm² is in principle the same result if we use a current transformer.
It may be convenient to use a thick copper pipe or copper bar for external antenna.
This provides a relatively large current to be transformed to adapt the pre-amplifier.
If we use 50-turn, it gives a relative area magnification of approximately 7 times.
A loop of Ø=38cm made of 5mm brake pipe gives a relative area of 0,38² * Pi * 7 ~ 3,2m²
Möbius antennas w/Transformer
Other loop antennas
Several other techniques can be used to build a loop antenna. There is an instruction manual of a copper pipe loop antenna written by Daniel Verschueren for the lightning radar project.
Figure 19: Copper loop antenna, © Daniel Verschueren
General information about antennas (System BLUE)
Preamplifiers are now so good that large antennas are not necessary.
Eg. the H-Field pre-amplifier has differential inputs and common mode signals (electrical) now has much less influence. Moreover, it has a gain of 10 times
(5 times since adaptation to the input of the Controller via cable eats half, while providing excellent adaptation to the transmission cable and Controller input)
Connection to H-Field preamplifier
The preamplifier is supplied with an input impedance of 2k - suits Ferrite antennas and antennas with transformer. Other antennas connected 75 Ohm – use solder bridges on the back side of the PCB.
Coax cable and “coax cable”
For loops without transformer you can use the cheap sat-cable with copper-plated iron wire
For loops with transformer, it must be coax with pure copper and heavy shielding
(75 Ohm 8mm cable as used for radio and TV-set for decades.)
No antennas must be grounded at the antenna.
It may be a good idea to ground the Controller.