# Rewinding a Tecsun AN-100 MW antenna for LW application

The antenna has a diameter of 23cm. It is 0.8mm diameter wire, with a total of 30 turns.

The primary winding is 360 µH with a variable capacitor for tuning with a capacitance between 5.3 pF and 280 pF.

The secondary winding is 6.4 µH and connects to the output.

Information online varies wildly, but by my calculations, that should be a tunable range of 500 kHz to 3.5 MHz.

I want to rewind the antenna to be resonant in the LW band, in particular 60-200 kHz, for SDR applications. My option for a different variable capacitor is up to around 500pF. Just from the numbers, that would get resonant for 60 kHz with 500pF and 14mH, so that would put me in the order of 200 turns. I could add a series capacitor to the variable capacitor to lower Q and the required L, as I don't need the ability to tune up to 600 kHz (14 mH @ 5 pF).

But I wonder how the relationship between primary and secondary is supposed to be, and whether I should even bother with having separate windings.

Well, any help or ideas are appreciated.

## 1 Answer

At low frequency, it's hard to tune with a variable capacitor alone. Distributed capacitance in the large coil would set the minimum capacitance, and the ratio with a 500 pF cap would give only a narrow tuning range.

Why not test adding one (or more) switched capacitor in parallel with the existing variable cap, keeping the coil as is? With the present 360 µH coil, a 0.0068 µF capacitor would give a resonant frequency of ~100 kHz.

If that works with your receiver, then add a 6 pole switch for no cap, 0.001 µF, 0.0022 µF, 0.0047 µF, 0.01 µF and 0.022 µF caps to cover the 60 - 200 kHz band, and leaving the BCB still available.

Note that small loop antennas become less effective at low frequencies, so you might consider putting four map-pins or thumbtacks into a door and winding a new antenna on that "coil" -- and the door provides a means for aiming the antenna. You'd still need switched capacitors to cover 60 - 200 kHz, though.

• Thanks for the answer. I played around with some values and the physical dimensions. Basically, just from a physical standpoint, I have the following options (diameter of wire, number of turns in single layer, length of wire, calculated resistance and inductance): Commented Mar 8 at 15:45
• 0,15mm -> 160x72cm = 115,2m = 112 Ω 9,651 mH - 0,20mm -> 120x72cm = 86,4m = 47 Ω 5,429 mH - 0,30mm -> 80x72cm = 57,6m = 14 Ω 2,413 mH Commented Mar 8 at 15:45
• Maybe you have some recommendation. I already ordered a variable capacitor with 700pf, to get a wider tuning ability right out of the gate, and have looked at some options for even bigger variable capacitance. Really hard to come by these days. Adding a static capacitance would only allow me to move it to a certain center frequency, but it would be hard to cover the full bandwidth. But you are right, I could just add a rotary switch. Commented Mar 8 at 15:48
• Oh, and I already unwound the antenna, because I am sure that the Q factor for the amount of turns and inductance is just too low for any meaningful reception at LW. This antenna really was optimized for around 1 MHz, and I basically need an order of magnitude lower center frequency. Commented Mar 8 at 15:50
• Basically, I cheat. E.G., for inductance of a round coil, see 66pacific.com/calculators/coil-inductance-calculator.aspx and for L-C resonance, see goodcalculators.com/resonant-frequency-calculator . BTW, for a rectangular coil on a door, not too far out of square, I'd guess "dia"≈0.7 x (length+width). When done, not only measure the inductance, but use a "grid dip oscillator", en.wikipedia.org/wiki/Grid_dip_oscillator, to get resonant frequency. Still, you'll likely need to add at least one switched cap to cover the whole band. Commented Mar 8 at 16:53