Critically about U-knee
Many television and radio Amateur antennas as the active element is used, the loop-vibrator Pistohlkors. It has an input impedance of about 300 Ohms, and therefore, when connecting to coaxial cable 75 Ohm balun is often used-matching U - knee. If you have used this connection is so pervasive that even do not have the idea that it may be a problem with the radio. On Fig. 1. shows the connection single loop-vibrator. Pistohlkors isolated from other elements of the antenna. In this case, crosstalk and noise on the small vibrator and no problems.
The situation is completely different when this node is part of a complex multi-element antenna on a common yoke and the mast (Fig. 2). In this embodiment, all interference and crosstalk on the mast, boom and guy wire are directly connected to the Central conductor of the coaxial cable and freely reach the receptor. A particularly bad case of the use at the entrance broadband antenna amplifier channel without inlet filters. It's also possible to damage the input circuits at lightning discharges nearby.
Maybe you have noticed that a passing motorcyclist bothers you, and the neighbor, with its simple vibrator directly connected to a coaxial cable (... without balancing - how disgusting!..), interference not?! It's possible that that's the fault of the above-described channel penetration interference.
To solve this problem in different ways:
- Active vibrator of your antenna must be isolated from the mast and beam.
Point adhesions of the ends of the cables you need in the shortest way to connect with midpoint of the vibrator (see Fig. 3). So you can do with an already installed antenna.
- To abolish U-knee, applying antenna amplifier with balanced input and connecting it directly to the terminals of the vibrator (see Fig. 4).
To the last point we may add, what generally loop-vibrator Pistohlkors consisting megale antenna element has a resistance of about 240 Ohms (for example in the antenna Spindler) because of the influence of passive elements. Therefore, the cable reduction transforms 240/4=60 Ohms and fit to connect a 50 Ohm cable! The use of the same antenna amplifier with balanced input will make 100% agreed tion.
Radio Amateurs of ultramarathons Kam applying U-knee-deep in their antennas, it is useful to imagine that all mast with braces (and bad contacts) is connected to the output of the transmitter. Maybe this is the reason for TVI?
P. S. Only by connecting the U-knee according to the scheme of Fig. 5 managed to get an acceptable quality of reception of TV programs, when the location of the receiving wave channel 20 metres from paintings shortwave transmitting antenna of the SRS-8/8-RA worked for 2 000 000 watt transmitter!
Author: E. shoustikov, UO5OHX ex RO5OWG; Publication: www.shustikov.by.ru, www.cxem.net