Representatives of the young wonder why the KV and CB bands are needed. For a long time, television has already switched to the UHF and does not complain. Multiplexes - in frequency above 470 MHz. There is a disadvantage in VHF and higher frequencies. Do not go around the earth's surface. The indicator explains the desire to create digital radio channels in the CB range for further distribution. Let us tell you about VHF a little lower, tell you how a homemade antenna for a TV is connected, what needs to be taken into account.
In the USSR, VHF was understood to mean a range of large span. We heard, of course, that today there is VHF and FM radio, the division is conditional. They simply borrowed from Europe the range of broadcasting in the region of 100 MHz( 87.5 - 108 MHz), called FM, and VHF began to understand the range lying below 87.5 MHz. The real meaning of the term becomes clear if you pay attention to GOST 24375, which states that the VHF range should be correlated, starting from the MV inclusive and ending with the Dm waves adjacent to the microwave. There is a scatter of 30 - 3000 MHz. This includes modern radio broadcasting on FM, any TV channels, satellite and part of Wi-Fi. Considerable range.
They said that the waves propagate linearly, which means they are ideal for transmission over long distances, provided the line of sight is visible. The satellite can be seen from everywhere, there is no difficulty in the absence of houses and trees. The situation is different with VHF on the surface of the planet. The earth is round, the tower is beyond the horizon, and reception is not expected. We have to raise homemade TV antennas higher for better catching waves. If there are no mountains and valleys around, the reception formula looks like this:
Where: H is the height above the ground of the transmitting antenna, h is the height above the ground of the receiving antenna. Obviously, the first parameter is not always possible to find out, therefore, visiting a house in the village, we are forced to touch the desired height of the suspension. The principle basically works: the higher the better. A second moment appears on the horizon. It is called a drop cable. This is a 75-ohm coaxial signal that carries a signal from a homemade antenna to the TV.The longer the cable drops, the greater the loss. As a result, the signal is able to fade out.
Homemade antennas for TV do not provide a large gain with wideband reception, if you adjust the parameters to the selected frequency, for example, the first multiplex( in Moscow - 559.25 MHz), you will achieve remarkable results. If the signal is not caught by a good home-made television antenna, you will need an amplifier. Connects to the mast, is selected for outdoor conditions. It is permissible to buy a device in the store or to collect it yourself. Often it is easy to get chips that are already ready amplifiers. It remains to connect a homemade television antenna.
Homemade amplifier for a television antenna
If we are going to do the thing, it is logical to tune in to our resonant frequency. For example, 559.25 MHz. A positive effect will give even the usual low-noise cascade on the transistor. A sample drawing of the cascade is presented below. This is a cascade scheme, the authors did not bother to tell how long the wavelength was calculated. It was established that the boundary frequency of the bipolar transistor 2N5179 is at least 900 MHz. The television broadcasting band fits into the gap.
The final stage should perform matching functions. Looks like an emitter follower performing these duties. The advantages are obvious: a cascade circuit has a good gain, better than others. At the same time, the scheme is not complicated. We have already chosen the working points for us, it remains to adjust the resonant circuit so that it falls on the frequency of 559.25 MHz. Self-made antennas for TV, equipped with the described device, are much better. Notice the power supply voltage is two. At +10 V you will find an old adapter on the flea market, you will have to get a +20 V from it yourself( without VashTehnik portal).
The signal input goes through a decoupling capacitor with a capacity of 0.01 μF, which protects the equipment from the action of the height current( as is well known, any point elevated above the ground has an electrical potential).This is a galvanic isolation( over current), generally accepted in radio circuits. Resistors 39 and 100 kΩ set the operating point of the field-effect transistor, 0.1 μF capacitors are filters in the power supply circuit. A resistor at the input of 100 kΩ determines the input resistance of the circuit, provides for as much as possible, and 1 kΩ for the output. The central element is an oscillating circuit of capacitance and inductance, tuned to the resonant frequency.
Think and do. The gain is determined by the quality factor of the parallel resonant circuit. The resonant frequency is calculated by the formula:
Where: P = 3.14, Pi number, L - inductance of the coil, C - capacitor capacitance.
If we take the capacitance from the standard 10 pF series, the inductance will be 8 nH.The coil will have to wind independently, in the circuit to include a trimmer capacitor. The result is a similarity to the Taiwanese CTC 038-30RA with limits of 4 - 30 pF.The signal level or picture quality requires the best reception. The review authors did not design the circuit; we do not guarantee that low-noise transistors are selected here. The author of the scheme claims that sending a signal to one gate of the field effect transistor eliminates the Miller effect. Actually, the scheme looks suitable for our purposes.
As a result, we recommend first to try reception without an amplifier, if necessary, to begin to design. Homemade TV antenna can work without an amplifier. To wind the coil correctly, we recommend that you visit the site coli32.narod.ru. There is an online inductance calculator. You can choose a model for taste and color. Please note that you need a wire with a diameter of 0.2 mm, a caliper to measure individual dimensions with great accuracy. Better to take the copper wire.
With amplifiers, we consider the question closed. As a result, we note that before assembling you need to make sure that the types of transistors are low-noise, otherwise the work will be wasted. It's easier to ask on the forum where the amateurs will respond. What if transistors are not low-noise? It is necessary to get the characteristics, calculate the working points according to the resistive dividers in the figure, select low-noise equivalents, where the points are selected in the same way. We do not consider the case of over-amplification, for an approximate calculation, measure the voltage at the output of a home-made television antenna( if there is a professional voltmeter).The resulting effective value is divided by 0.7 for gain and on the input characteristic, see that there is no cut-off mode.
Note. Over-power leads to ghosting of the contours of the picture. Sometimes there is snow. An attenuator is used to compensate for excess power. Adjusting the attenuation coefficient, the master monitors the image on the screen.
This is how it is done. Any transistor has an input characteristic, where the dependence of the current on the voltage. Set the horizontal voltage and remove the scope of the output current. Almost all is transferred to the exit. We take the output characteristic and look, so that the current swing falls on the linear section. Balancing the working section of the input characteristic is carried out by the correct selection of resistors 39 and 100 kΩ.As for the bipolar transistor, the input characteristic is already pulled up to the middle by selecting the supply voltage. Difficulties will not arise.
Matching a home-made television antenna
A home-made antenna for digital television has the impedance. Typically, literature constructions are calculated at 75 ohms. Say more, 75 ohms is the typical resistance of a half-wave vibrator. If a different device is assembled, the specifications may vary. Self-made indoor antenna for the TV does not need to be coordinated, the losses are small on a short cable segment. Do not leave an extra coil, make the feeder the minimum length. The homemade antenna for digital TV is tuned to the middle of the multiplex, and the amplifier has enough bandwidth to skip the channel.
Note. If necessary, use a SWR meter or other device to match the antenna and cable.
Often asked how to make a dish for satellite television. We believe that at home in any way. It is necessary to reproduce with great accuracy the outlines of a paraboloid; the ordinary radio amateur cannot cope with this. Another thing - to change the polarization or irradiators to arrange as necessary. But it will not come homemade satellite dish for the TV.Actions relate to commissioning. If you wish, write in the comments on the amplifier assembly, the scheme is the simplest, the cost is also low.