What band is this antenna for?
I don't know, check.
— WHAT?!?!
How to determine what kind of antenna you have in your hands if there is no marking on it? How to understand which antenna is better or worse? This problem has plagued me for a long time.
The article describes in simple terms a method for measuring the characteristics of antennas, and a method for determining the frequency range of an antenna.
For experienced radio engineers, this information may seem banal, and the measurement technique may not be accurate enough. The article is designed for those who do not understand anything at all in radio electronics, like me.
TL; DR We will measure the SWR of antennas at various frequencies using the OSA 103 Mini instrument and a directional coupler, plot SWR versus frequency.
Theory
When a transmitter sends a signal to an antenna, some of the energy is radiated into the air, and some is reflected and returned back. The ratio between radiated and reflected energy is characterized by the standing wave ratio (SWR or SWR). The lower the SWR, the more of the transmitter's energy is radiated as radio waves. At SWR = 1 there is no reflection (all energy is radiated). The SWR of a real antenna is always greater than 1.
If you send a signal of different frequencies to the antenna and simultaneously measure the SWR, you can find at what frequency the reflection will be minimal. This will be the operating range of the antenna. You can also compare different antennas for the same range with each other and find which one is better.
Part of the transmitter signal is reflected from the antenna
An antenna rated for a certain frequency should, in theory, have the lowest SWR at its operating frequencies. This means that it is enough to radiate into the antenna at different frequencies and find at what frequency the reflection is the smallest, that is, the maximum amount of energy that has flown away in the form of radio waves.
By being able to generate a signal at different frequencies and measure the reflection, we can plot the x-axis with the frequency and the y-axis with the reflectance of the signal. As a result, where there is a dip on the graph (that is, the smallest signal reflection), there will be an antenna operating range.
An imaginary plot of reflection versus frequency. The reflection is 100% over the entire range, except for the operating frequency of the antenna.
Device Osa103 Mini
For measurements we will use . It is a versatile measuring instrument that integrates an oscilloscope, signal generator, spectrum analyzer, frequency response/phase response meter, vector antenna analyzer, LC meter, and even an SDR transceiver. Operating range OSA103 Mini is limited to 100 MHz, the OSA-6G module extends the frequency range in the mode of the frequency response up to 6 GHz. The native program with all functions weighs 3 MB, works under Windows and through wine in Linux.
Osa103 Mini is a universal measuring device for radio amateurs and engineers
Directional coupler
A directional coupler is a device that diverts a small portion of an RF signal traveling in a specific direction. In our case, it must branch off part of the reflected signal (coming from the antenna back to the generator) in order to measure it.
Visual explanation of the operation of a directional coupler:
The main characteristics of the directional coupler:
- Operating frequencies - the frequency range at which the main indicators do not go beyond the norm. My coupler is designed for frequencies from 1 to 1000 MHz
- Branch (Coupling) - what part of the signal (in decibels) will be diverted when the wave is directed from IN to OUT
- Directivity - how much less signal will be diverted when the signal moves in the opposite direction from OUT to IN
At first glance, this looks rather confusing. For clarity, let's imagine the tap as a water pipe, with a small outlet inside. The diversion is made in such a way that when the water moves in the forward direction (from IN to OUT), a significant part of the water is diverted. The amount of water that is diverted in this direction is determined by the Coupling parameter in the datasheet of the coupler.
When the water moves in the opposite direction, much less water is discharged. It should be taken as a side effect. The amount of water that is removed during this movement is determined by the Directivity parameter in the datasheet. The smaller this parameter (the greater the dB value), the better for our task.
Schematic diagram
Since we want to measure the level of the signal reflected from the antenna, we connect it to the IN of the coupler, and the generator to the OUT. Thus, a part of the signal reflected from the antenna will get to the receiver for measurement.
Tap connection diagram. The reflected signal is sent to the receiver
Measuring setup
Let's assemble the installation for measuring SWR in accordance with the circuit diagram. At the generator output of the device, we additionally install an attenuator with attenuation of 15 dB. This will improve the matching of the coupler with the output of the generator and increase the accuracy of the measurement. The attenuator can be taken with attenuation of 5..15 dB. The attenuation value is automatically taken into account during the subsequent calibration.
The attenuator attenuates the signal by a fixed number of decibels. The main characteristic of the attenuator is the attenuation coefficient (attenuation) of the signal and the operating frequency range. At frequencies outside the operating range, the characteristics of the attenuator may change unpredictably.
This is what the final setup looks like. You also need to remember to apply an intermediate frequency (IF) signal from the OSA-6G module to the main board of the device. To do this, we connect the IF OUTPUT port on the main board with INPUT on the OSA-6G module.
To reduce the level of interference from the switching power supply of the laptop, I carry out all measurements when the laptop is powered from the battery.
Calibration
Before starting measurements, it is necessary to make sure that all components of the device are in good condition and the quality of the cables, for this we connect the generator and receiver with a cable directly, turn on the generator and measure the frequency response. We get an almost flat graph at 0dB. This means that over the entire frequency range, the entire radiated power of the generator reached the receiver.
Connecting the generator directly to the receiver
Let's add an attenuator to the circuit. You can see an almost even signal attenuation of 15dB over the entire range.
Connecting the generator through a 15dB attenuator to the receiver
Connect the generator to the OUT connector of the coupler, and the receiver to the CPL of the coupler. Since there is no load connected to the IN port, all of the generated signal must be reflected, and part of it must be branched off to the receiver. According to the datasheet for our coupler (), the Coupling parameter is ~15db, which means we should see a horizontal line at about -30 dB (coupling + attenuator attenuation). But since the operating range of the coupler is limited to 1 GHz, all measurements above this frequency can be considered meaningless. This is clearly visible on the graph, after 1 GHz the readings are chaotic and do not make sense. Therefore, we will carry out all further measurements in the operating range of the coupler.
Tap connection without load. The limit of the operating range of the coupler is visible.
Since the measurement data above 1 GHz, in our case, do not make sense, we will limit the maximum frequency of the generator to the operating values of the coupler. When measuring, we get a straight line.
Limiting the range of the generator to the operating range of the coupler
In order to visually measure the SWR of antennas, we need to calibrate to take the current circuit parameters (100% reflection) as a reference point, that is, zero dB. To do this, OSA103 Mini has a built-in calibration function. Calibration is performed without a connected antenna (load), calibration data is written to a file and then automatically taken into account when plotting graphs.
Frequency response calibration function in OSA103 Mini software
Applying the results of the calibration and running the measurements with no load, we get a flat graph at 0dB.
Graph after calibration
We measure antennas
Now you can start measuring the antennas. Through calibration, we will see and measure the reduction in reflection after the antenna is connected.
Antenna from Aliexpress at 433MHz
Antenna marked 443MHz. It can be seen that the antenna works most efficiently on the 446MHz band, at this frequency the SWR is 1.16. At the same time, at the declared frequency, the performance is significantly worse, at 433MHz SWR 4,2.
Unknown antenna 1
Antenna unmarked. Judging by the schedule, it is designed for 800 MHz, presumably for the GSM band. To be fair, this antenna also operates at 1800 MHz, but due to coupler limitations, I cannot make correct measurements at these frequencies.
Unknown antenna 2
Another antenna that has been lying around in my boxes for a long time. Apparently, also for the GSM band, but better than the previous one. At a frequency of 764 MHz, the SWR is close to unity, at 900 MHz, the SWR is 1.4.
Unknown antenna 3
It looks like a Wi-Fi antenna, but for some reason the connector is SMA-Male, and not RP-SMA, like all Wi-Fi antennas. Judging by the measurements, at frequencies up to 1 MHz, this antenna is useless. Again, due to coupler limitations, we won't know what kind of antenna it is.
Telescopic Antenna
Let's try to calculate how much you need to extend the telescopic antenna for the 433MHz band. The formula for calculating the wavelength: λ = C/f, where C is the speed of light, f is the frequency.
299.792.458 / 443.000.000 = 0.69719176279
full wavelength - 69,24 cm
half wavelength - 34,62 cm
quarter wavelength - 17,31 cm
The antenna calculated in this way turned out to be absolutely useless. At a frequency of 433MHz, the SWR value is 11.
By experimentally extending the antenna, I managed to achieve a minimum SWR of 2.8 with an antenna length of about 50 cm. It turned out that the thickness of the sections is of great importance. That is, when only thin end sections were extended, the result was better than when only thick sections were extended to the same length. I don't know how much further one should rely on these calculations with the length of the telescopic antenna, because in practice they do not work. Maybe with other antennas or frequencies it works differently, I don’t know.
Piece of wire at 433MHz
Often in various devices, such as radio switches, you can see a piece of straight wire as an antenna. I cut off a piece of 433 MHz (17,3 cm) quarter wavelength wire and tinned the end so that it fits snugly into the SMA Female connector.
The result turned out to be strange: such a wire works well at 360 MHz, but is useless at 433 MHz.
I started to cut the wire from the end piece by piece and look at the readings. The dip on the graph began to slowly shift to the right, towards 433 MHz. As a result, on a wire length of about 15,5 cm, I managed to get the lowest SWR value of 1.8 at a frequency of 438 MHz. Further shortening of the cable led to an increase in SWR.
Conclusion
Due to coupler limitations, it was not possible to measure antennas on bands above 1 GHz, such as Wi-Fi antennas. This could be done if I had a wider coupler.
A coupler, connecting cables, a device, and even a laptop are parts of the resulting antenna system. Their geometry, position in space and surrounding objects affect the measurement result. After setting to a real radio station or modem, the frequency may shift, because. the body of the radio station, modem, the body of the operator will become part of the antenna.
OSA103 Mini is a very cool multifunctional device. I express my gratitude to its developer for advice during measurements.
Source: habr.com