Two antennas on one cable. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Television antennas
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Often in practice there is a problem of combining two antennas of different channels on one feeder. To do this, you can apply the typical scheme shown in Fig.1.
Pic. 1
Coils L1 ... L5 can be wound with enameled wire with a diameter of 0,5 ... 0,6 mm on one frame, turn to turn. In this case, it should be taken into account that the distance between the ends of adjacent coils must be at least 8 mm. Frameless winding can also be used.
If there are no capacitors of 12 and 20 pF at your disposal, you can get by with foil getinax or textolite. According to fig. 2a, contact pads are cut out. The slots are 1 mm wide (according to the thickness of the cutter). According to Fig. 26, c, plates are cut out of aluminum, duralumin or brass with a thickness of 1,5 ... 2,0 mm.
Pic. 2
These plates are applied from the side of the foil to the foil material, having previously placed polyethylene gaskets 0,1 mm thick. Suitable polyethylene from a conventional plastic bag. On fig. 2a, dotted lines indicate the location of the plates on the foil. The plate according to Fig. 26 forms capacitors C3, C4, C5, the plate (Fig. 2c) forms capacitors C1 and C2. It remains to solder the coils L1 ... L5 in accordance with the diagram. Under the M4 screws for this purpose, you need to put washers made of tinned sheet, to which it is convenient to solder the coils. To protect against atmospheric moisture, it is necessary to cover the entire structure with varnish or place it in a sealed enclosure. The holes for the M4 screws on the side of the foil should be slightly countersunk so that the capacitors do not close through the plate fixing screws.
If a radio amateur needs to make capacitors for other purposes in the same way, then you need to be guided by the formula:
C=(8,85*10-3*ES)/d
where C is the capacitance of the capacitor in pF;
S is the area of the capacitor plate in mm2;
d is the thickness of the dielectric layer in mm;
E is the permittivity of the dielectric.
Pic. 3
There is another option for connecting receiving TV antennas to one feeder (Fig. 3). A1 - the place of connection of the active vibrator of the antenna tuned to the X1 wave. The vibrator of the second antenna tuned to the L2 wave is connected to the A2 points. l2, l4 - balancing quarter-wave cable segments, the length of which is determined by the formula:
l=K(L/4),
where l is the length of the balancing segment;
L is the wavelength;
K - coefficient of cable shortening.
For cables with polyethylene insulation K=0,67.
Both antennas are connected to a common feeder at distances from the vibrators, which are determined from the system of equations:
where n is any integer.
It is based on the following principles:
- a quarter-wave segment of the line, loaded with resistance less than the wave resistance, transforms it into a resistance greater than the wave resistance;
- if a resistance much greater than the wave resistance is connected to the line, it practically does not load the line.
Author: A. Zherdev, Gomel; Publication: cxem.net
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