Collinear antenna

The vertical collinear antenna is an omnidirectional high gain antenna. It has been known for a long time and is often called the Marconi-Franklin antenna. A collinear antenna is an antenna array of in-phase half-wave dipoles with serial excitation, elongated in a single line, hence the name — collinear.

There are several ways to form such an antenna array. One of them — using quarter-wave transmission line segments — is a classic version of the Franklin antenna. On this example, you can understand on the fingers how it turns out. In fairness, it should be noted that in the classical version a horizontal symmetrical antenna is viewed, but we will simplify the model to the “lamer” level.

Let's say we have a vertical wire spike with a length that is a multiple of half the wavelength. The high-frequency current is distributed along it in such a way that in adjacent segments of length λ/2 it flows in an opposite phase. The radiation pattern of such a spike consist of many lobes at different angles to the horizon. Then we, leaving the segments with the same phase of the current, fold the segments with the opposite phase in half, obtaining from them quarter-wave segments of a two-wire transmission line. Trust me, the high frequency current will not notice anything. It will continue to run the same length. In other words, the total electrical length of the collinear antenna spike has not changed. However, only linear segments take part in the radiation of such an antenna, since a two-wire line does not radiate, and these segments are all in-phase and their radiation is summed up. In addition, a quarter-wave transmission line segment transforms a short circuit at one of its ends into an infinite impedance at the other, and all linear segments, as it were, “see themselves as” isolated from each other. As a result, such a collinear antenna radiates at a low angle to the horizon and has a gain greater than that of a half-wave dipole. The more segments an antenna has, the higher its gain, but the bandwidth becomes narrower. Do not think that with a doubling of the number of segments, the gain doubles (+ 3dB). Firstly, the radiation power of the collinear antenna is consistent and at the series excitation little energy reaches the last segment, it spends to radiation by the previous dipoles. Secondly, aperture areas of the adjacent dipoles are overlapped.

On the basis of a collinear antenna, a rather successful design of the Amos antenna was created. There is also a good Franklin wire antenna design for Wi-Fi with extended elements, as in the picture at the beginning of this article. Here, dear anonymous, we will view a popular way to make a collinear antenna for Wi-Fi with pieces of coaxial cable, the so-called CoCo antenna. The radiating elements in it are made of pieces of λ/2 coaxial cable and are interconnected by half-wave segments of a transmission line from the same cable, which do not radiate. The method is not new, it was patented back in 1962. To make such an antenna, it is necessary to take a cable with a solid inner insulation, since it has a more or less stable velocity factor. In the practical making of the antenna, one must not forget about matching with the feeder (for example, a quarter-wave sleeve balun) and about compensating the end capacitance. Here is the design of such an antenna:

The last quarter-wave whip “P” is a piece of coaxial cable from which the outer insulation and mesh have been removed. The matching quarter-wave sleeve balun is made of a copper tube with a diameter of at least three diameters of the coaxial cable. One end of such a sleeve is soldered to the outer side of the feeder mesh, the other is insulated. You can do without soldering the sleeve, using the design in the figure. The sleeve in this case consists of two parts. An F-F adapter fits in a copper “thimble” and a copper tube is tightly put on it as a continuation of the sleeve. The feeder is connected to it using F-connectors. White is the main feeder, black is its continuation inside the sleeve. Theoretically, this extension can have an arbitrary length, but in practice, the point of connection of the feeder to the antenna should be made as close as possible to the cut of the sleeve. For additional balancing, three high-frequency ferrite rings are put on the main feeder at a distance of approximately λ/2 from the first antenna radiating element. They serve to introduce additional attenuation into the parasitic current in the outer side of the feeder mesh, which has leaked through the balun. Attenuation is introduced due to losses in the ferrite, so you should not look for some special ferrite here. Suitable the 43 material Amidon rings of a suitable diameter, for example FT50-43.

The appearance of the antenna without a balun:

How to connect the elements to each other (leads sizes on the design drawing):

For mechanical strength, this entire structure must be packed in a radio-transparent plastic tube of a suitable diameter, and then your home-made antenna will be indistinguishable from the branded one.

The calculator below will help you compute the constructive dimensions of the antenna.

Constructive calculation of a collinear antenna from a coaxial cable:

It should be noted that the collinear antenna is very sensitive to “crooked hands”. Moreover, the more elements, the worse the repeatability of the antenna. This is a significant drawback for homemade antennas. Sometimes the good old Ground Plain can work better than a poorly made collinear.

For owners of smartphones on the Android operating system, the calculation of the CoCo antenna is available in the Cantennator mobile app. You can download it by clicking or by scanning the QR code.

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Related links:

1. Build A 9 dB, 70cm, Collinear Antenna From Coax - By N1HFX;
2. High Gain Collinear AIS (162MHz) Receiving Aerial by Neal Arundale - M1CHS;