Critically about the U-knee. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Critically about the U-knee. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Television antennas

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Many television and amateur radio antennas use a Pistohlkors vibrator as an active element. It has an input impedance of about 300 ohms, and therefore, when connected to a 75 ohm coaxial cable, a balancing-matching U-knee is often used. Such a connection is used so widely that it does not even come to mind that it can be a source of radio reception problems. On fig. 1. connection of a single loop vibrator is shown. Pistolkors isolated from other elements of the antenna. In this case, interference and interference on the vibrator itself are small and there are no problems.

Critically about the U-knee

The situation is quite different when this node is included in a complex multi-element antenna on a common traverse and mast (Fig. 2). In this option, all interference and pickups on the mast, traverse, and guys are directly connected to the central core of the coaxial cable and freely reach the receiving device. Especially unfavorable is the case of using a broadband antenna amplifier at the input without channel filters at the input. Damage to the input circuits is also possible during a lightning discharge nearby.

Or maybe you have noticed that a motorcyclist passing by only interferes with you, while your neighbor, with his simple vibrator directly connected to a coaxial cable (... without balancing - what an abomination! ..), has no interference ?! It is quite possible that the interference penetration channel described above is to blame for this.

You can solve this problem in different ways:

- The active vibrator of your antenna MUST be isolated from the mast and traverse.
- The soldering point of the ends of the cables must be connected in the shortest way to the middle point of the vibrator (see Fig. 3). This can be done with an already installed antenna.
- Abandon the U-knee altogether by applying antenna amplifier with balanced input and connecting it directly to the vibrator terminals (see Fig. 4).

To the last point, we can add that, generally speaking, the Pistohlkors stub-vibrator as part of a multi-element antenna has a resistance of about 240 Ohms (for example, in the Spindler antenna) due to the influence of passive elements. Therefore, 240/4 = 60 Ohm is transformed to the drop cable and it is just right to connect a 50 Ohm cable! The use of an antenna amplifier with a balanced input will make it possible to make 100% matching.
It is useful for ultra-shortwave radio amateurs who use a U-elbow in their antennas to imagine that the entire mast with guys (and bad contacts) is connected to the output of the transmitter. Maybe this is the reason for TVI?

Critically about the U-knee. Interference path illustration

Critically about the U-knee. U-bend alternative

PS Only by connecting the U-bend as shown in Fig. 5, it was possible to obtain an acceptable quality of receiving TV programs, with the location of the receiving wave channel 20 meters from the canvas of the transmitting short-wave antenna SGD-8 / 8-RA, on which a 2 watt transmitter worked!

Author: E.Shustikov, UO5OHX ex RO5OWG; Publication: shustikov.by.ru

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