VHF loop antenna made of coaxial cable. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Antennas. Measurements, adjustment, coordination

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Loop antennas are sometimes made from the braid of a coaxial cable. One of the options for such an antenna is in the second part of my book "HF and VHF Antennas". It has not only many advantages (cheapness, wide bandwidth, speed of manufacture), but also a minus.

The input impedance of a round or square frame is about 120 ohms, and the feeder usually has a characteristic impedance of 50 ohms. There are only two options in this case. You can stretch the frame into a narrow rectangle with an aspect ratio of 1:2. Only in this form does it have an input impedance of 50 ohms. However, this solution is inconvenient from a constructive point of view. And with a more familiar and convenient frame shape (circle, square), a matching device must be used for matching. This also does not decorate the design due to the need to introduce additional elements.

The article describes a convenient constructive version of the frame and its matching device (for an input impedance of 50 ohms) from one single piece of coaxial cable with a characteristic impedance of 75 ohms.

The idea is to use a piece of coaxial cable with a characteristic impedance of 4 ohms as a λ/75 matching device, which transforms 120 ohms into 50 ohms. And from the same cable to make the antenna frame itself.

 
Fig. 1

The resulting design is shown in Fig. 1. The antenna is made from a piece of 75 ohm cable (for example, RG-59, as in this figure). Its length is chosen from the following considerations.

The electrical perimeter of the frame itself should be 1,03 ... 1,05λ - on VHF, increased values ​​​​of the elongation coefficient are required due to the large (relative to the wavelength) diameter of the frame conductor. But in our case, it is covered on top with a rather thick layer of plastic (external insulation), which has a noticeable shortening effect and compensates for the elongation factor. Therefore, the physical perimeter of the frame from the cable is about 1λ.

Small inaccuracies (for example, due to the spread of the dielectric constant of the cable insulation) are not terrible. The bandwidth of the antenna will be large, and this forgives small errors in its manufacture.

The electrical length of the matching segment should be R/4. And the physical one - in the shortening coefficient Cook (internal, from the cable passport) times less. Total length of the cable section

equal to the sum of the above lengths. For example, for RG-59, which has a velocity factor of 0,66, the total length will be 1λ+0,66λ/4=1,165λ.

This antenna is made like this. A piece of coaxial cable with a characteristic impedance of 75 ohms is cut off - its length should be slightly larger than the value calculated above. At its upper end, the braid is cut to a length of several millimeters, and the central conductor is exposed.

Exactly one wavelength is retreated from the split upper end, and in this place the outer insulation of the cable is carefully opened without damaging the braid, so that its fragment is available for soldering. At this point, the center conductor of the upper end is soldered to the braid, and this connection is waterproofed (for example, with hot glue). The resulting loop is shaped into a circle or square.

The lower end of the cable is connected either directly to the antenna socket of the transceiver with an output impedance of 50 ohms, or to the main feeder with a characteristic impedance of 50 ohms.

According to the recipe described above, a measuring antenna for a frequency of 290 MHz (λ = 1,03 m) was made from a piece of RG-59 coaxial cable 1,2 m long (1,165λ). The dependence of the SWR of this antenna on frequency is shown in fig. 2.

Fig. 2

The band in terms of SWR <1,5 turned out to be 30 MHz. This means that similar antennas with a large margin and low SWR will cover the entire amateur bands 144 or 430 MHz.

The manufacture of the described VHF loop antennas (amateur VHF bands, Wi-Fi, GPS, PMR, etc.) will require only half an hour of time and small costs for the purchase of a cable, connector and hot melt adhesive.

Author: Igor Goncharenko

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