Antenna Theory Regular
Joined: 04 Feb 2013
|Posted: Sun Dec 07, 2014 3:19 pm Post subject:
|An effective loop antenna wire length should be in the range one wave length which at 22 Hz is 13000 km. A lot of copper wire.
It is however possible to make the antenna much smaller at cost of effectivity and resulting reduced transmission distance.
Reduce size to 2 meters is the same as listen at shortwave radio with a 2 mm long antenna!
It is your choice to decide how good antenna you need.
TX power and receiver sensitivity are other parameters that effect transmission distance.
Some common diy antennas can be found at www.vlf.it as well how to build matched receivers.
Do not know anything about your actual requirements but guess it is no need for super sensitive devices and what you want is a simple tuned pickup coil, and a effective narrow band filter to avoid picking up 16 Hz (rail road) and 50-60 Hz which else saturates receiver circuit and makes any receiving of 22 Hz impossible.
Probably, as it only is magnetic field that is of interest should pickup coil be well shielded.
A core that not consist of air introduce increased losses, but is preferred as an air core increases loop wire length for an certain inductance.
A effective core material at these frequencies can be taken from an big mains transformer (oriented iron sheets) but core should be open.
Some ferrite material are also possible to use. HV-coil in an old TV do usually have a ferrite core that have high permeability at low frequencies.
Any soft iron can also be used, with somewhat less performance.
My guess is that you want something hand-held, which not is several meters in diameter. A possible simple alternative is to find an old type of ignition coil for cars, the one that looks like a soda-can.
Drill a small hole and pour out the oil. This oil is only needed for improving high voltage isolation and cost weight as hand-held. Do not drill in the windings.
Select a ignition coil that already have a outer metal shield, as then is shielding already is prepared. Tune secondary HV-coil with a capacitor to resonance at 22 Hz and filter/amplify signal taken from primary low voltage coil.
It will be a very ineffective antenna, but probably good enough for your need as a magnetic pickup coil at distance of a few meters from a transmitting unit.
Filtering and impedance matching is critical for total performance. Needed formulas for this, as well if you want to calculate your own loop antenna can also be found at www.vlf.it.