Shielded Loop Antenna / Yagi-Uda array of loops

It is plenty of article on home builds antenna onto the internet. Most of them are for antenna that can work for both emission and reception.

We will see here two different designs for reception into the FM and TV band. Those 2 designs are using the same base type of antenna: the loop antenna.

Shielded loop

A crude representation of a shielded loop:

The principle is the same than for a loop. From wikipedia: a continuous conducting path leading from one conductor of a two-wire transmission line to the other conductor. All planar loops are directional antennas with a sharp null, and have a radiation pattern similar to the dipole antenna with E and H fields interchanged.

The main advantages of loops antenna are tree:

1) An outstanding signal quality. It is a simple design that perform very well.

2) For large loops, higher gain (about 10%) than the other forms.

3) Large loop antennas are less susceptible to localized noise, partly due to their lack of a need for a groundplane.

The advantage of a shielded loop over a non shielded one:

4) The electric interference from the big city (streetlights, television's , cars etc...) have no influence on the received signal.

Practical realization

Like we will not perform emission, any good quality coaxial cable may be used to realize such an antenna. Critical to correctly design this antenna is the wavelength of the program you want to receive best.

Less critical but important is the capacitance of the coaxial cable. The circumference of the loop will be equal to the wavelenght, and that capacitance is not negligible. In practice, any good coaxial cable for TV installation will do the job.

My first concern with this antenna was the FM band, because I was completely unable to receive my favorite radio program on 93.8 MHz where I am living.

Wavelength [meter] = 300 / frequency [MMHz]

For 93.8 MHz, we have 300 / 93.8 = 3.198 meters.

According to the Balanis (Antenna Theory: Analysis and Design, by Constantine A. Balanis, it is better to use a little bigger circle. (Read the book for the details.)

Antenna circumference = 1.1 * wavelength = 1.1 * 3.198 = 3.518 meters.

So, we need 3.52 meters of coaxial cable.

For the realization, it is up to you. Metal will not work because it will interfere with the antenna. Plastic or wood are best. The best design is one that will approach a perfect circle, like a bicycle wheel.

I made mine with one vertical stick of wood and 6 smaller sticks of the same wood. I made small holes at the extremity of each stick in order to be able to attach the coaxial there. I used 2 more little pieces of wood at the center of this wheel in order to fix the sticks.

We also need to cut 1 cm of the shield at the top of the antenna. The center wire must be left intact. This is very important, otherwise the antenna will be a shield, and not an antenna. Use a cutter or a good knife.

Coupling

If you look at the antenna representation, we have 2 signals wires and the ground. Ir is several possibility, The simplest is to use one antenna coupler. They are such antenna coupler into the market with one coaxial socket at one end (for the antenna cable) and 2 symmetrical connection at the other end. They are made to connect an antenna installation to a receiver.

Most modern variant are for radio, but some old models was made fo TV use.

Radio variant: one connection for AM, one connection for FM.

TV variant: One connection for band one to three (VHF low, FM, VHF high), one connection for band four and five (UHF).

The radio variant is the easiest to find, but the TV variant is more versatile.

When you have such a coupler, it is just to wire the 2 signal wires into the socket you want to use, and to wire the antenna ground to the ground of the coaxial cable.

TODO: add some photos

Testing

I was finally able to listen to 93.8 FM. It was necessary to put it outdoor. I even can get the stereo, but with a lot of noise. But mono is fine. And all the other stations are working fine.

I also can get the digital TV on the UHF band with this antenna, and that without changing anything to the antenna or the coupling.