Magnetic field strength indicator. 20170715

Sets PCBs are available.

        Why i developed this instrument.
Tuning to resonance of a 1/4 lambda (or shorter) capacively tuned loop antenna, is more critical than tuning low Q antenna systems, for instance 1/2 lambda dipoles. The resistive, inductive and capacitive parts of the loop impedances, vary sharply beside its resonance frequency. A not perfectly matched coax feed line then can act as a frequency dependant "matching" system.

The unwanted result can be, that at the transmitter site suggested "frequency of optimal match", is NOT coinciding with the loop resonance frequency. 

So the loop antenna is matched at a frequency where it is NOT resonating. Resulting in noticeable lower than optimal radiation efficiency. 

My conclusion :

1 : A capacively tuned loop antenna should only be tuned to resonance indicating max. radiated magnetic energy.
2 : After that, a not ideal VSWR could be corrected with a tuner, NOT by detuning the loop antenna.

As a consequence, the produced magnetic field strength at the loop site should be observed.

        General info.
This broad band system gives an indication of the magnetic "near field" strength of a loop antenna. The antenna unit of it is installed outside, near a loop antenna. The relative field strength is read at the transmitter site, and the meter unit is connected through a twisted 3-wire cable. Power source is a 4.5V three AA alkaline battery pack. Total supply current is 2 - 18 mA. As long as the unit is only switched on during antenna tuning, long battery life can be expected.

here to download the schematic diagram.

Antenna PCB.

Meter PCB.

Wide band antenna coil L1 (self inductance 50-100uH) is severally damped by the low input impedance of IC1 (225 Ohms). D1 and D2 limit to strong signals to 1.4Vpp, to prevent overloading of the IC. This IC is capable of detecting voltages as low as 10mVrms, and operates with a supply voltage as low as 3Vdc. It converts the antenna RF voltage into a proportional DC voltage, with a conversion factor of 7.5Vdc/Vrms. The max. output voltage in this circuit is 3.9V. With S2 in position "QRO", and transmitting with full power, the meter should indicate full scale. With S2 in position "QRP", the sensitivity of the system becomes 5 times higher. Then the antenna could be tuned using only 1/25 of full power (5W). Thereby causing no hinder to other stations nor the own transceiver. D3 protects the meter for over voltage. The max. output current is abt. 10mA, and max. supply current abt. 18mA, and dissipation is abt. 80 mW. Safe values. Pi-filters L2-7 / C5-12 prevent influence of possible induced RF voltages onto the connecting cable. 

Click here to download the parts list. C55 is a tantal elco. All other capacitors should be multi layer types with low self inductance. Resistors are 0.25W metal film. Chokes are miniature ferrite types. Bend to 7.5mm or 10mm.

The AD8361ARTZ  6lead SOT-23 IC can be ordered here (cheapest at verical ?). Also available at mouser.comBe sure to order the 6-lead ARTZ type. 

        Ferrite rod.  
A small and cheap 8 x 50 mm rod Ui=300 (Conrad 535575) with 42 turns 1mm thick wire wound on it has a self inductance is abt. 50uH.

However, the best ferrite material for wide band SW use is Amidon / Fairite #61 Ui=125. It is suitable for frequencies up to 30MHz. Available among others at amidon.de.

More turns means higher sensitivity and bandwidth.

For 100 uH self inductance : use 1.4   times the number of turns.
For   25 uH self inductance : use 0.71 times the number of turns.

Ferrite antenna orientation.


Winding examples for abt 50uH 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

Do not use a MW or LW radio receiver rod, as that ferrite has wrong properties for SW use..

The double sided antenna PCB contains three holes :
1. Two holes for screwing the PCB into a Hammond 1554BGY or weather resist 1554B2GY enclosure.
2. One for fixing the ferrite antenna rod using a ty wrap.

The moving coil meter (Conrad 103538) and the double sided meter PCB fit nicely into a Hammond 1554EGY enclosure. The M+ and M- connections fit the moving coil meter. There is ample room for drilling holes to fit other moving coil meters. 

The three wires of the twisted signal cable are connected to PCB terminal blocks. A (DIN) plug connection on the meter unit is sturdy and handy. 

    Antenna unit.
Bend RM5 capacitor legs to straight RM2.5mm. All capacitors should be soldered directly down onto the PCB surface (with the shortest possible wire lengths).

- Solder all components except the IC.

- Wind 42 turns 1mm thick insulated wire at the 8 x 50mm rod. 
- On a longer rod, L1 should fill nearly the whole length of the rod, by equally spacing all turns. 
- Fix the coil ends to the rod by means of some thermal glue.
- Fix the completed antenna rod to the antenna PCB by use of a ty wrap and solder its connections.

Now take anti-static measures to prevent damage to the IC. Do not overheat the IC.

Small marks on the IC and the PCB bottom locate pin1. Be careful not to pull-off the tinned surfaces of the PCB. I made the IC solder islands longer, to enable you to pre-heath those islands for easier flow of tin solder between PCB and IC legs. Try to keep the IC as cool as possible.

I found that the the easiest way of soldering the IC is :
    - Fist flow a minimal amount of solder tin onto all PCBs IC solder pads. 
    - Position the IC.
    - Solder ICpin2. Let cool down.
    - Check the exact IC position.
    - Solder ICpin4. Let cool down.
    - Quickly solder pins 1-3 using minimal solder. Let cool down.
    - Quickly solder pins 4-6 using minimal solder. Let cool down.
    - Quickly remove all existing solder between pins 1-3 using de-solder litze. Let cool down.
    - Quickly remove all existing solder between pins 4-6 using de-solder litze. Let cool down.

Check solder connections and possible short circuits using a digital ohmmeter (NOT in diode test setting).


REM: The orientation of the antenna box, and the location of the cable feed-through, both depend on the orientation of the loop antenna. See drawing.

- Drill a hole for the rubber cable tulle in the lowest side of the antenna box.
Fix the PCB into the box by 4mm screws.

    Meter unit.
- Drill the 38mm and 3mm meter holes, centered in the 1554EGY box. 

    You have to drill away two existing mounting posts. Wear glasses to prevent eye damage.
- Using a 10mm drill, carefully remove two mounting posts, until the bottom of the box is flat. 
- Drill a 6mm holes through both locations for the switches.
- Drill a 5mm hole between them for the LED. 

- Drill a hole in one side for a (DIN) cable connector.
- Install these components in the box.
- Solder wires to them.

- Solder all components onto the meter PCB.
- Solder the wires to it.
- Install the PCB onto the meter connection posts using its 3mm screws.
- Connect the three wires from the DIN connector to the PCB connection block.

- At closing the box, put some plastic foam between the PCB and the battery holder to prevent rattling of the battery holder..

    Cable connection.
Use a thin 3-wire twisted cable.

Hold the coil of a dip meter in line with the 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. Suggesting this system is useable over the whole SW amateur band range.

See the drawing for a possible orientation of the ferrite antenna in respect to the orientation of the loop antenna.

In order to minimize RF induction, the meter cable should run from the antenna unit at 90 degr. angles in respect to the lowest loop part.

With S2 in position "QRO" and with full power applied, you have to experiment with the location of the antenna unit (closer / further / higher / lower) to get a meter full scale deflection. Start with the antenna unit near the center of the lowest conductor of the loop.