Magnetic field strength indicator idea 20170518

Mounted near a magnetic loop antenna, this simple instrument gives a indication of the generated magnetic field strength. 

During tuning of my "magloop", sometimes frequency differences seem to occur between the "best-VSWR-frequency" and the "max-received-signals-strength-frequency". On 80m the latter seems to be higher, possibly caused by an unwanted capacitive coupling to the loop, and influenced by the physical length of the coax feeder.

In stead of pre-tuning my loop to maximal received noise while listening at a given frequency on 40m or 80m, i want to tune the loop for maximal magnetic radiation while transmitting on that frequency.
And once tuned, let the ATU do the definitive matching. 


PCB 40mm x 51mm

This little unit is meant to be placed on the roof near a transmitting loop, while meter indications are read near the transmitter. Both units will be inter connected through a thin twisted 3-wire cable (mains cable or CAT5). Power source could be 4.5V from three AA alkaline batteries connected in series. IC1 still operates at 3V supply. Total supply current is minimal, due to the use of a sensitive moving coil meter. It is between 2mA and 8mA, depending on the smeter deviation. Expected battery life is a few years, as long as the unit is only switched on during antenna tuning.

Antenna coil L1 converts the magnetic field strength into RF-voltage, and could have an inductance of abt. 50uH. Together with the total circuit capacitance, L1 then  resonates around 7MHz. The antenna circuit has a very wide bandwidth, due to the heavy loading by the input resistance of detector  IC1 (225 Ohms).The ferrite rod for L1 must best be a Amidon material #61 rod, with Ui=125. The cheapest still use able ferrite rod is a small 50mm x 8mm rod Ui=300 (Conrad 535575).

Do not use a MW or LW radio rod.

D1 and D2 prevent damage to IC1, which converts the received RF-voltage into a DC voltage. IC1 is sensitive, and can detect RF voltages as low as 10mVrms. With the max. 1.4Vpp antenna voltage, the max. output is abt. 3.9Vdc. In this circuit, the output current is set to between 0 and 1.37mAdc. At the max. output, trim potmeter "P" can be used to adjust the 100uA/210Ohms moving coil meter for full scale deflection. L2 to L7 block possible RF on the connecting lines.