This antenna was designed to meet the requirements of a light body worn small magnetic loop covering all the frequencies continuously from 7 MHz to 29.4 MHz

The antenna diameter is 97centimeters, and it is made with 1.2 mms section aluminum mounted on “balsa”, the light wood used to construct model airplanes.

This small magnetic loop project required the design of a new conception of a remote control to adjust the variable capacitor to the resonating point.

To carry the antenna on my back I decided to construct a new device to fix this small magnetic loop instead of using a heavy metal structure backpack that is the usual solution to attach the antenna pole.  

Tuning box front view
Coaxial and control cable
Inner loop front view
Inner loop back view

Inside view of the reduction geared motor coupled to the variable capacitor

As a new project I had to overpass some classical problems, and one of them was the need to be advised when the variable capacitor reached out the end of it course. The solution was a micro switch and a micro push button, actuated by a metal blade soldered on the variable capacitor main shaft.

On the right top of the black box we can see the low voltage buzzer with one of its leads soldered to the “+” contact of a 1.5 volts button battery. The blue and yellow wires are connected to the end of course switches.

Variable capacitor remote control

The variable capacitor remote control is mounted inside a section of a plastic water pipe.

The adjusting knob was installed on the top to ensure free movement for both sides using the thumb when the control is handheld.

Two switches and a pair of micro touch buttons was mounted on the control unit. One of the switches turns on and off the control. When the switch is on the power off position there is no power applied to the circuit and it locks the controls preventing the capacitor detuning.

The other switch selects the control on the black tuning knob or the fine tune micro touch buttons.

The circuit tested and ready o go into the plastic tube

The circuit is powered by a pack made with two AAA metal hydrate batteries salvaged from a Motorola cell phone 6 volts pack.

The circuit is charged by a Nokia cell phone charger adapted with a 100 ohms series resistor supplying 100 milliamps hour to full charge the battery pack in 8 hours.

In case of being out of energy during a field operation, I have designed a simple emergency battery charger using a low cost dynamo flashlight.

I carefully disassembled the flashlight and install a switch to reverse the dynamo output power from the flashlight inner rechargeable batteries to a piece of cable and a connector to plug into the antenna control charger input.

Remote control with the emergency flashlight dynamo charger

The control accommodates perfectly on the hand permitting perform all the commands with the thumb.

A microphone headset is the remote control “best friend” for a pedestrian mobile operation; we can select the VOX on the radio and tune the radio and the antenna while walking and talking.

I have designed a very simple magnetic bracket to firmly fix the remote control on the FT817 metal cabinet .The strong magnet was salvaged from an old HD drive, fixed on the plastic bracket by plastic ties and covered with thin rubber. The rubber avoids scratching and helps keep the bracket in place.

Magnetic bracket
the magnetic bracket fixed, by pressing to the control
Remote control magnetically fixed to the FT817 metal cabinet


Light magnetic  loop on the pedestrian mobile antenna back support

PY1 AHD ALEX (Alexandre Grimberg).
e-mail: py1ahd@ig.com.br