What changes when changing the applied frequency range in radio communications is not always correctly formulated even by experienced radio amateurs. On the one hand, the transfer formula of Friis is extremely simple, and it would seem there is nothing to discuss. On the other hand, in this formula, besides the explicit mention of the wavelength λ, it is implicitly hidden in other coefficients. There are many statements, notes and articles that with higher frequencies the energetics of radio links are worse, there are no less articles of “revelations of myth” - they say that high frequencies are not worse, learn the materiel.
Both statements are true, and the third is also true - with increasing frequency, link energetics can significantly improve. It all depends on the application scenario (restrictions imposed).
Any transmission of information, not only using radio waves, but also any other waves (sound, EM waves of higher frequencies - that is, light, gravitational waves) can occur in 3 scenarios:
- Omnidirectional radiation and omnidirectional energy reception.
- Directional (sector, narrow-radiation) radiation and omnidirectional reception
- Directional radiation and directional reception
In the first case, neither side knows the location in space of the second side, or has no means of pointing its antennas at the correspondent.
This scenario includes almost all types of radios (military, civilian, aviation), home appliances (WiFi, Bluetooth, radio telephones, IoT, wireless sensors, telematics, key fobs), the connection between the descending probe and its space station. The antennas of both mobile correspondents must be omnidirectional (isotropic) or close to them.
In the second case , if one of the sides is the stationary and probable location of the mobile correspondent is limited to some sector of space - on the stationary side it is possible to use a directional antenna, which concentrates energy in the chosen direction, forming a beam. The subscriber is mobile, he either does not know his location or the position of the base station (or has no antenna pointing facilities).
This type of scenario includes all types of services when a fixed base station serves mobile subscribers (cellular communications, repeaters for military or civilian walkie-talkies, broadcasting to mobile subscribers, satellite communications with mobile subscribers, ground-based space communications stations serving high-mobility space probes). The antenna of the base station has a moderate directivity and forms a beam to serve the desired zone of space. Ideally, at any point in the service area at the same distance R from the base there would be the same energy flux density W / m2. The antenna of the mobile correspondent must be omnidirectional (isotropic).
In the third case , if both parties are aware of the location of the other side and have the opportunity to send their antennas there - you can significantly save energy or increase the speed of communication with the same cost of energy, due to the concentration of the beam in space.
This scenario includes all fixed point-to-point lines: radio-relay, WiFi point-to-point, amateur radio connection between 2 subscribers using directional antennas; sedentary subscribers with accurate positioning of antennas to the correspondent (ground station for space communications and space station with servo drives of directional antennas or positioning engines of the entire station with a fixedly fixed directional antenna; advanced 5G mmWave or StarLink modems Ilona Mask with automatic beam tuning of the phased AFAR array; promising massive-MIMO modems and 4G / 5G base stations using a large number of antennas like AFAR)
Here r (receiver) and t (transmitter) refer to the receiving and transmitting antennas, Pr / Pt is the ratio of the power at the terminals of the receiving antenna to the power at the transmitting (more is better), d is the distance in the same units of measurement as λ (for example in meters)
An antenna aperture A (the same as the “Effective / effective area”) is associated with the radiation pattern (NF) of the antenna and its antenna ID (D = Directivity):
For an antenna in reception mode, the effective antenna area (the term effective antenna surface is also used) characterizes the antenna's ability to collect (intercept) the electromagnetic radiation power flux that falls on it and convert this power flux into power at the load.
Regardless of the type and design of the antenna, its aperture A and directivity D are mathematically connected through a wavelength.
In the omnidirectional (isotropic) antenna, D = 1 (0 dBi). There is no ideal isotropic radiator in practice, the closest analogue is a usual half-wave dipole, in which D ~ 1.64 (2.15 dBi)
Let's compare the aperture of a half-wave dipole (or its analogue - a quarter-wave pin with a counterweight), in which KND = 2.15 dBi
The transmitting antenna in all bands forms the same, close to spherical, radiation pattern. The power flux density W / m 2 from all sources at the same distance R will be the same.
But since the aperture of the receiving (also omnidirectional) antenna differs by orders of magnitude, the amount of energy collected from the same flux density will be very different.
Take an abstract communication channel, in which the transmitter power is TX = 1W and the receiver sensitivity is -101 dBm (2 µV at 50 Ohm load). In open space (obstacles, absorption, reflections, interference are not considered here), the communication range will be:
In open space (as long as the range is not limited to visibility), a 2-fold increase in frequency increases the transmitter's power requirements by 4 times. With the same transmitter power, increasing the frequency by 2 times reduces the range by 2 times.
It is this effect that is dominant to explain why:
- CDMA / LTE-450 is long-range for GSM-900, which in turn is long-range for GSM-1800.
- Long-range WiFi-2400 for WiFi-5400
- The radio sets are 27-40 MHz long-range for 144-174, which in turn are long-range for 433-470
In scenario # 2 , if it is allowed to use a low-directional (sector) antenna on one side, the situation is exactly the same as in scenario # 1, only the transmitter power can be reduced by the gain of the base station antenna. Since the required service sector does not depend on frequency, the BS antenna directivity is the same (the aperture of the BS antenna will of course be different for different ranges). With a BS directivity of 12 dBi (10 dB or 10 times more than a 2 dBi dipole) - the power gain will be 10 dB (10 times), the communication range per mobile subscriber may be the same as in the previous table, but already with TX = 0.1W. For 5400 MHz, it will again be 25.7 km, and for 27 MHz - 5142 km.
In scenario 3 , very different combinations of solutions are possible.
If we discard the design constraints and difficulties, then with an equal area (aperture) of both antennas, the directivity of both antennas D r and D t is proportional to the square of the frequency. Therefore, the efficiency of the receiving antenna will remain unchanged (the same terminal power will be extracted from the same density flux W / m2, regardless of frequency), and the directivity of the transmitting antenna will increase in proportion to the square of the frequency. By increasing the frequency by 2 times, the beam will become 4 times thinner, the flux density W / m2 in the direction to the subscriber will increase 4 times.
With equal restrictions on the size / weight of antennas, higher frequencies are more energetically advantageous.
In practice, to realize such a fundamental advantage is not so simple.
For antennas with a fixed frequency-independent aperture, only mirror parabolic antennas are used. The amount of energy that such a mirror collects does not depend on the frequency, and the beam of the radiation pattern becomes thinner with increasing frequency.
But the complexity in the production of a parabolic antenna of a given diameter depends not only on the diameter. The higher the frequency, the higher the requirements for the accuracy of the mirror surface and the higher the requirements for the accuracy of positioning and, in general, the rigidity of the whole structure.
With other, non-specular antennas, the situation is much more complicated. All designs of such antennas can be described in frequency-independent sizes (in lambda) and have a fixed radiation pattern inherent in this type of antenna, which does not depend on the chosen design frequency. In other words, for example, a 7-element antenna wave channel (Uda-Yagi) will have the same radiation pattern and gain of ~ 10 dBi regardless of which frequency to calculate: 30 MHz or 3000 MHz. In the second case, its aperture will be 10,000 times smaller. Just so, to take and increase the size of some type of antenna to increase the aperture - it is impossible. The addition of any passive (parasitic) structures adds directivity very little (compared with an increase in size) and only to small values of the order of 16 dBi (40 times).
Further enhancement of the aperture, which corresponds to the directivity of more than 16 dBi in practice, is possible only by connecting many antennas to the HEADLIGHTS (phased antenna array). Theoretically, doubling the number of elements in the lattice can increase the aperture by 2 times, i.e. form a 2x thinner beam with a +3 dB gain. But in practice, the construction of such phased arrays involves great difficulties: a signal from a single source must be matched (by impedance) waveguides in-phase to each of the N lattice elements.
For a small number of elements, for example 2x2, 2x4, 3x3, such a task is solvable, and for a large number of elements it is so complex that it always loses to mirror parabolic antennas, with the help of which 20-40 dBi directivity is easily created, and in large projects (like ground stations space communication) reaches 70 dBi (gain of a parabolic antenna with a diameter of 70 meters at a frequency of 5885 MHz).
For example, we calculate the communication distance of the point-to-point line with TX = 1W, sensitivity -101 dBm with a pair of parabolic antennas with diameter D = 1 meter and aperture efficiency k = 60% (typical value for modern mirror irradiators)
To calculate the KND of a parabolic mirror, we use the formula:
Increasing the frequency by 2 times increases the range by 2 times or allows you to use an antenna with an aperture diameter less than 2 times on one side, or reduce the antenna diameter in SQRT (2) ~ 1.4 times on each side.
The requirements for beam pointing accuracy (antenna alignment per subscriber) also increase in proportion to the square of the frequency.
In this article, we do NOT deal with other issues at all, such as reflection, diffraction, refraction, absorption in gases, obstacles, the atmosphere, the ionosphere, noise and interference conditions.
findings
Increasing the radio frequency can provide both advantages and disadvantages depending on the application scenario (technical task).
In the conditions of a mobile, non-tuning connection, low frequencies are more advantageous, since The omnidirectional aperture is proportional to the square of the wavelength. Increasing the wavelength by 2 times increases the aperture of the antenna by 4 times. This makes it possible either to increase the range by 2 times (in terms of visibility and limited range of communication in the energy budget) or to reduce the transmitter power by 4 times, all other things being equal.
For this reason, military backpack, car and tank radios continue to be projected to the very bottom of the VHF band - from 27 to 50 MHz, while civilian and commercial communications inexorably master ever higher frequencies.
A half-wave dipole (or a quarter-wave pin with a counterweight) is larger at low frequencies, which is a disadvantage on the one hand. On the other hand, it is this disadvantage that allows you to collect more energy from space.
In the conditions of point-to-point lines, low frequencies are also more beneficial in all cases, except for the use of fixed-aperture parabolic antennas. For antennas with the same directivity, the aperture decreases in proportion to the square of the increase in frequency. With a frequency increase of 2 times, the dimensions of the antenna of the same type are reduced by 2 times (in each dimension, i.e., the volume decreases 8 times), but the price paid for this is a 4-fold reduction in the aperture of such an antenna.
But in point-to-point lines with parabolic antennas - on the contrary, switching to higher frequencies allows, with the same mirror diameters, to improve the energy budget by 4 times while the frequency increases by 2 times. Increasing the frequency by 2 times allows you to:
- other things being equal, increase the range in visibility conditions by 2 times
- at the same range, reduce the radiation power by 4 times
- other things being equal, increase the line speed by 4 times
The price paid for this increase is increased requirements for precision manufacturing, as the antenna itself, and the mechanism of guidance (alignment) to the subscriber.