Magnetic field strength indicator. 20170715 / 20171221

(C) The use, copy and modification of all info on this whole site is only permitted for non-commercial purposes, and
thereby explicitly mentioning my radio amateur call sign "PA0NHC" as the original writer / designer / photographer / publisher.

A set of two PCBs + 1x AD8361ARTZ + free worldwide shipment,
can be ordered at : . Price : 20 Euro.

I do not supply other components, nor completely build units.

Later on a sensitivity switch was added.

This broad band system gives an indication of the magnetic near field strength of a "Magnetic loop" antenna.
The little sensor unit of it is installed outside, near a loop antenna. Its input is protected against over voltage.
The relative field strength is read at the transmitter site with a meter unit, connected to the sensor through a twisted 3-wire cable.
The sensitivity of the system is switch able between "Normal" and 20dB more sensitive, enabling non-annoying antenna tuning with low power.
Power source is a 4.5V from a three AA alkaline battery pack. Total supply current is 2 - 8 mA. As long as the unit is only switched on during antenna tuning, very long battery life can be expected.

Why Schema Description Materials
Partslist Assembling
antenna unit
meter unit

Hold the coil of a dip meter in line with the sensor ferrite rod, close to the grounded end of the antenna coil. Set the dip meter signal strength to maximum.

At the prototype a good meter deflection was observed between 1.5MHz and 70MHz using a Heathkit HD-1250. Suggesting that this system is useable over the whole SW amateur band range.

See the drawing at the right for a possible orientation of the ferrite sensor coil in respect to the orientation of the loop antenna.

In order to minimize RF induction onto the signal cable, this cable should run at 90 degr. angles away from both  :
a. the ferrite rod in the sensor unit
b. the lowest part of the Magnetic loop antenna.

With S2 in position "QRO", and with full transmitter power applied to a correctly tuned loop antenna, you should get nearly full meter scale deflection.
You have to experiment with the location of the sensor unit (closer / further / higher / lower). 

        See drawing.
Start with the sensor unit near the center of the lowest conductor of the loop, and change the distance in respect to this conductor.
Keep the sensors ferrite rod at 90 degr. in respect to the loop surface.