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Magnetic field strength indicator. 20170607
Preliminary development data.

A set of two PCBs will become available.

Installed outside, near a magnetic loop antenna, this simple instrument can give an indication of the magnetic near field strength. 

After tuning my TRX to a frequency, i want to tune my loop for maximal magnetic radiation.
Then, if the VSWR is not 1:1.0, use the ATU to do the definitive matching.


Radio room display unit.


Roof antenna unit.

Click here to download the schematic diagram.

The AD8361ARTZ  6lead SOT-23 IC can be ordered here (cheapest at verical ?).

Chokes L2-7 are 27uH, Fres 7.2MHz. For instance Conrad Order No 1405948.

C5 is tantalum type, others are low inductance multi layer capacitors. All soldered with minimal short wires !

        General info.
The system consists of an antenna unit and a metering unit. It is meant to be used on 3.5 MHz and 7MHz, placed near my transmitting loop.
Meter indications are read near the transmitter. Both units will be inter connected through a thin twisted 3-wire cable (thin mains cable or CAT5). Power source is 4.5V from three AA alkaline battery pack. Total supply current is between 2mA and 8mA, depending on the input signal voltage. Expected battery life is a few years, as long as the unit is only switched on during antenna tuning.

        Schematic.
Do not use a MW or LW radio rod.
A cheap small ferrite antenna (conrad BNR 535575, Ui=300, lxd = 50x8mm) is useable for 80m and 40m. It fits nicely inside a small and cheap Hammond 1554B2GT polycarbonate weather proof box. The number of turns for L1 must be experimentally found for abt. 50uH to 100 uH self inductance.
Together with the total circuit capacitance, L1 should resonate around 5MHz or 7MHz.

The best ferrite material for wide band MW-SW radio reception is Amidon / Fairite #61 Ui=125. By using a #61 ferrite rod, the frequency range could be changed to between 1 MHz and 30 MHz.
For abt 47 uH coil inductance :
Amidon R61-025-400, 42 turns 2mm insulated wire
Amidon R61-037-300, 38 turns 2mm insulated wire
Amidon R61-050-400, 33 turns 2mm insulated wire
Amidon R61-050-400, 31 turns 2mm insulated wire
For 100 uH self inductance : use 1.4   x the number of turns.
For   25 uH self inductance : use 0.71 x the number of turns.
Use thick lacquered copper wire, thin copper strip, or litze.
All turns should be equally spaced, and spread over the full length of the ferrite rod. Then fixed with thermal glue.


Antenna coil L1 converts the magnetic field strength into RF-voltage. The antenna circuit has a very wide bandwidth, due to the heavy damping by the input resistance of detector IC1 (225 Ohms).

D1 and D2 prevent damage to IC1. It converts the received RF-voltage into a DC voltage. It is sensitive, and can detect RF voltages as low as 10mVrms, and has a conversion factor of 7.5Vdc/Vrms. With the by D1/D2 limited antenna voltage (max. 1.4Vpp), its max. output is abt. 3.9Vdc. In this circuit, the output current is set to max. 1.37mAdc.

L2 to L7 block possible RF on the twisted connecting lines, preventing influence from E-fields. 

At the max. output, trim pot "P" in the meter unit adjusts the moving coil meter for full scale deflection.

        PCBs.
The double sided antenna PCB contains three holes :
- two holes for screwing the PCB into a small cheap Hammond 1554B2GT polycarbonat weather proof box.
- one for fixing the ferrite antenna rod using a tywrap.

Inside the Hammond 1554B2GT box fits the small and cheap ferrite rod (Conrad BNR 103538). If a better (longer #61) rod is used (Amidon / Fairrite) another more expensive weather resist plastic box has to be used.

The single sided metering unit PCB can be screwed directly onto the electric connection points of the 1mA / 210 Ohms meter (for instance Conrad 103538). A cheap Hammond 1599TGY box can contain both moving coil meter and PCB. As on the date of design no accurate data were available, no meter connection holes are drilled into the PCB "+" and "-" soldering pads. You must drill those holes yourselves, fiiting the meter connections.  If + and - on your meter are exchanged, turn the PCB for 180degr.

Bend the legs of trim pot P (a type PV36W or X), and solder it with its legs onto its top surface solder islands.

Cheap useable 4.5V 3xAA battery holders could be : Conrad 651044 or 486047.

In the antenna unit the connecting cable could be soldered directly onto its soldering islands, the cable passing through a rubber gasket on the underside of the box.

On the metering unit a cheap XLR chassis female connector (Conrad 304350) is handier.