Here is a simple modification to extend the coverage of the Tecsun loop antenna AN-100 to longwaves.
If you chosed the fancy look of the AN-200 i am afraid it will not be as easy to open and work on it (and it’s not the only downside of that base).
The modification consist in inserting a 3.5mm female audio jack connected in parallel to the loop and its variable capacitor and then connect an external variable capacitor to it.
When two capacitors are connected in parallel the resulting capacitance is the sum of the two and increasing the capacitance will decrease the tuning frequency.
The tuning frequency can also be decreased inserting a inductance in series with the loop; i tried that but the result was not as good.
Let’s start!
- Look at the bottom cover, remove the 4 adhesive pads that are hiding 4 screws; remove the 4 screws and then remove the cover
- Drill a hole on the side to fit a 3.5mm female audio jack (i use this to make holes in plastic)
- Connect the female jack to the two terminals of the variable capacitor soldering two short wires
I am not showing a photo of the inside because my loop has also a switch added; you do not need it in this case and the photo would be confusing. Here is where i positioned the 3.5mm socket
This was the easy part…. now you need a capacitors box to connect to it.
The capacitors box
The capacitors box is composed from one variable capacitor and a switch to select some fixed capacitors in parallel.
More selectable capacitors are needed because the capacitance to cover all the LW band has a range of about 2000pF and this range cannot be covered by a variable capacitor alone.
The number and value of the fixed capacitors depends from the value of your variable capacitor.
These are the variables needed for the calculation:
The internal variable capacitor of the Tecsun loop is max. 250pF
The capacitance needed to tune at 153 kHz is about 2700pF
The capacitance needed to tune at 270 kHz is about 1030pF
Using a variable capacitor CBM-443BF-1DF4, it’s range is from about 10pF to 530pF;
the fixed capacitors will be C1=680pF, C2=1200pF, C3=1800pF, C4=2200pF; position 1 of the rotary switch is left not connected.
With these values can be covered all the NDB band and the LW broadcasting band, from 150 kHz up to the MW broadcasting band without gaps.
Be aware that this modification does not make miracles on LW because the loop is designed for MW and its low inductance is compensated adding high capacitance in parallel. The values of inductance and capacitance are highly unbalanced.
But it will improve reception if using radios with small ferrite antennas (or without antenna) or, like in my case, a Xhdata D-808 that is completly deaf on LW
For the NDB reception it works better because closer to the MW band.
Here below some videos. My capacitors box has two variable capacitors but you do not need two unless you want to double the range and use only a 3 positions switch.