Broadcast FM band attracts ham radio. Free frequencies occupy the area of 145 - 433 MHz, this is where we will demonstrate the skills of designing equipment. They take to do VHF antennas do-it-yourself residents of remote villages, who hesitate to receive the signal. There are many reasons for the manifestation of independence, it is important to ignore the forbidden areas of transmission - you will not endure problems.
Conceived engineer friends broadcast, the law is not prohibited when done without violating government regulations. Assembling a transmitter, adjusting the process is a separate problem, each subscriber will need an antenna for VHF.
Amateur range prices bite. Non-standard products are not very popular, to produce unprofitable, because the cost is high.
Amateur band 145 MHz
VHF fixed-band stationary antennas are made relatively simply. The basis is the quarter-wave vibrator circuit. Range products are equipped with a relatively wide bandwidth, fine tuning for frequency is not needed. Consider the examples of structures:
- For the simplest method of manufacturing a receiving antenna - in nature, at home, anywhere - you will need a T-tee. The perpendicular tap is supplied with a coaxial, the other two - with the spire of the radio station twisted out, with counterweights( analogue of the VHF range).
- A straight corner with square sides of 4 cm is attached to the outer wall, 5 cm long counterweights are bolted to the edges of the horizontal platform, and an antenna connector is fitted in the middle. Since the removal of the coaxial cable going to the structure is the main cause of signal loss, the length of the segment should be minimal. It will be quite difficult to make gold connectors on your own, to clean out the existing steel ones, to wipe them with alcohol, increasing sensitivity, it is necessary without fail. Since the standard antenna of an amateur radio station, inserted into the jack of the site, has a resistance of about 40 Ohms, the connection is made with a 50 Ohm coaxial. Finally, the wave impedance of the remote antenna is adjusted by turning the counterweights. To replace the factory antenna, you can use a piece of copper wire with a diameter of 1-2 mm, a length of 48 cm.
- If the branded antenna for the VHF receiver is broken, replace it with a 50-ohm coaxial cable with a length of 48 cm with the screen removed. Expose the bare core. You can replace the product with a piece of wire in the past way.
- A more complicated version will be obtained by winding half a meter of copper wire on a coaxial internal dielectric. The difficulty lies in reconciling the resistance of the resulting home-made design with the wave resistance of the radio station. After debugging, secure the coils with electrical tape.
145 MHz Half-Wave Antenna
The above-mentioned quarter-wave homemade VHF antennas are not the only way out. The advantage of low impedance, half-wave options are allowed to exist. A piece of wire with a diameter of 1 mm, a length of 103 cm has a resistance of 1 kΩ, 20 times the standard coaxial( 50 Ohms).
U-shaped contour is used to coordinate the difference of values. Cutting a future wire antenna should be several centimeters shorter / longer than 103 cm. Slightly increased losses due to an increase in the reactive component of the impedance, significantly reducing the real part of the impedance, the matching device will be easier to adjust.
The inductance of the filter is switched on in series to the antenna, formed by 5 turns of wire with a diameter of 1 mm, wound in 2 mm increments on a mandrel with a diameter of 6 mm. KPVM-1 trim capacitors( 5-14 pF) are switched on with one lining on the ground on both sides of the coil.
Antenna tuned for VHF radio by measuring the CWS, field strength. The minimum of the first parameter coincides with the maximum of the second. Otherwise, the length of the antenna is shortened, measurements are taken again. It is recommended to initially choose the length of the wire 102 cm, gradually cut from the top end, selecting the optimal value.
Wideband Antenna To manufacture a VHF stationary antenna with a height of over one and a half meters, tunable to two amateur frequencies, 145 MHz, 433 MHz, you will need dielectric rods with a diameter of 7-17.5 mm. Wound coils are fixed with glue, compound. They need to be reeled up precisely, what was said will not be a simple matter.
Work is performed with 2 mm solid wire. The direct indent from the top is 38.7 cm, then the dielectric rod with a diameter of 7.5 mm is wound strictly 12.5 turns with a step so that the total inductance height is 63 mm. Having retreated 42.2 cm of the straight section, wind 64 turns onto a 7-mm rod so that the total inductance height is 28 cm. Then, a straight-line section of 36.7 mm, again turns of 7 pieces( height 32 mm) on a 10-mm rod. Finally, the last wire segment with a length of 56.4 cm ends with an inductance formed by 4 turns( height 20 mm) over a rod with a diameter of 17.5 mm.
One and a half turn from the top of the last inductance is diverted to the main core of a coaxial cable with a resistance of 50 Ohms. A trimmer capacitor of 1-10 pF is included in the circuit in series. The ground of the VHF antenna connects to the screen. Parallel to the last inductance, the 1 pF correction capacity is switched on at 70 cm.
The antenna bottom is equipped with eight counterweights:
- four 145 MHz bands;
- four frequencies 433 MHz.
After assembly, the product is adjusted according to the standing-wave coefficient by a resistance meter. In both ranges, select acceptable values. Such an antenna, assembled by yourself, will last for a long time, if you place it in a strong protective case made of dielectric material and protect it against moisture ingress from the compound.
The honor of developing a VHF antenna version belongs to Alexander RV9CX.The author advises the capacity of 1 pF to perform a length of cable SAT-50( 2 cm).One winding will serve the screen, the second - lived. The center wire can be extended-inserted backward by changing the capacitance of the capacitor.
FM
range For radio amateurs, it is a usual thing to dig into the element base, collecting complex devices. But a homemade antenna for a VHF receiver is useful to the average Beacon amateur.
First you need a square board with a side of 20 cm, or an equivalent piece of plexiglass. A square with a side of 15.5 cm is cut out of the foil, a square hole with a side of 11.9 cm is cut through in the center. At the intersection of the lower extension of the right inner side of the square, the middle line of the lower wall of the concentric figure solder the wire of the central core of the coaxial cable. Four centimeters to the left is soldered to the wire connection to the screen.
The resulting design confidently receives FM broadcast stations.
Applicability of self-made antennas
Self-made HF-VHF antennas enjoy considerable popularity. In contrast to the complex receiving and transmitting equipment, where the indisputable leadership goes to factory products, the wire structure, when properly tuned, gives excellent results.
Instruments required for parameter estimation are rarely available. For proper operation of the homemade VHF antenna, a SWR meter, a field strength meter, is required. Ultimately, the problem lies not in the geometrical dimensions of the parts, in the relative position, but in matching the impedances of the submarine coaxial cable and the antenna itself.
. It is allowed to use any methods of troubleshooting, it was shown above how to accomplish this, with the help of resonant circuits. A slight adjustment is performed by changing the position of the counterweights, it is difficult to find the necessary parameters by practical means. Resonators are rarely the best solution due to the associated difficulties of use.
