Ferrite Rod Antenna Coil

Designing an AM radio ferrite rod antenna coil (also known as tank circuit) for a crystal radio project or any radio project is something GCSE students and hobbyists love rushing into with guesswork. This article takes the guesswork out of building a coil, and shows how you could calculate the values to design and build an accurate tank circuit. Let us assume that we wish to design a coil with an approximate operating range of 526.5 kHz to 1606.5 kHz in the medium wave band. Radio engineers usually like to build a coil that is approximately 300 µH in value, because it requires a variable capacitor that ranges in value from 32 pF to 304 pF. Therefore, as a general rule of thumb, radio engineers tend to design their tank circuits around 300 µH coil and 300 pF variable capacitor. If the capacitor value is slightly less, then they usually increase the number of turns in the coil. With the use of the calculators in the following pages, you should be able to build an accurate coil for almost any variable capacitor value.

The values shown in the table below (for MW coverage) are approximate because there is bound to be variance in CAT5e cable formula from one manufacturer to another. In addition, there are different manufacturers making ferrite rods using different formulas. Therefore, please consider these values a rough rule of thumb for anyone who does not have an LCR meter.

Here is a table that shows some commonly available variable capacitors with their values in pico-farads together with the required coil value for the reception of the medium wave band. The third column shows the number of turns required of solid core (CAT5e) wire on a 10 mm diameter ferrite rod. As the variable capacitor gets bigger in value, the required coil gets smaller; hence, that is where the trade-off is. If your variable capacitor is insufficient, then you will need a larger coil, which equates to more coil, more turns, longer ferrite rod, and ultimately more money. If you have a variable capacitor not listed in the table, then feel free to contact me and I shall try to add it onto the list.

Litz wire

The best type of wire for an AM radio project is Litz wire, which is usually found in commercially manufactured radios. It consists of individual strands of wire bundled together. Consequently, a ferrite coil made of this type of wire tends to be more sensitive and efficient for radio reception. When buying, the manufacturer usually states the number of individual strands in the bundle, and the more strands it has, the more expensive it is.

Litz wire is also known as Polyurethane Single Tetron (USTC) Wire. A single strand is usually 0.07 mm in thickness and they bundle five of these strands by twisting them together to make one wire. Approximately 73-turns of this type of wire is required around a ferrite rod of diameter 10 mm to produce an inductance of approximately 300 µH. Since the wire is so thin, it occupies very little space, and is therefore ideal for long wave (LW) coverage, because it requires a lot of wire.

2 m of White Solid Copper Core CAT5e UTP Wire Please!

For medium wave coverage, I usually use the cheapest and most readily available wire that has become a standard these days. Since we live in a technological era, almost everyone must have, or at least have heard of CAT5e networking wire. You use it in your homes and businesses when you wish to connect a computer to another computer or the Internet router. This type of wire is fairly standard and made of high-grade copper core. It is cheap at less than one pound per meter and the great thing is that if you strip the outer covering, then you should find four twisted pairs inside, hence you actually get eight single wires! You can get it from thousands of hardware and electrical shops on-line and therefore this is why I tend to use it in my projects. The exact printing on the cable is CAT5e; EN 50575; 2014+A1; 2016 Euro Class D-s2, d2, a1.

I was able to buy a cheap ferrite rod with 10 mm diameter from an online site! For the variable capacitor, the cheapest I could find was one with a value of 224 pF. These tiny variable capacitors come all the way from China, and are great if you were working on making a pocket spy radio!

LCR Meter

An LCR meter is a special measuring instrument that measures inductance (L), capacitance (C), and resistance (R). If you have such a meter, then you can make precise coils by winding your favourite wire and making sure it is the correct value in Henrys. This way, you can tune into the complete band with somewhat accuracy. A Fluke meter would be great if you have "Daddy Warbucks", but if you are an orphan Annie like me, then there are always cheap alternatives such as the UA66243L on auction sites.

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