The above picture is drawn by Maxwell&39;s equations. The black one below is the grounded metal support bar, and the black small square above is the small metal plate on the MiniWhip. The red line indicates the equipotential plane, and the different lines have different potentials with respect to "ground". If the "ground" is 0V, then the first red line at the bottom is 1 μV, the previous one is 2 μV, and so on. (Annotation: In other words, the electric field lines are not absorbed by the grounding metal rod, but exist in a twisted compression manner, which is derived from Maxwell&39;s equation theory and is theoretically based.)
3. When the support rod is an insulator
What happens when the support rod is an insulator? At this time, since the support rod is not a metal body, the tip potential is no longer zero. The MiniWhip&39;s amplification circuit measures the potential difference between a small metal plate and the "amplifier circuit&39;s own ground." If the amplifier circuit itself is reliably connected to the ground through the shield of the coaxial cable, then the amplifier circuit still measures the potential difference between the small metal plate and the earth, which is consistent with the case where the support rod is a grounded metal. If the shield of the coaxial cable is not reliably connected to the real earth, or is simply connected to a "disorganized ground" (such as a strong ground), the noise on such ground will be amplified by the MiniWhip&39;s amplifier circuit. Send to the receiver. It can be seen that grounding is extremely important.
If the fiber is used instead of the coaxial cable to transmit the amplified signal of the MiniWhip, will the signal transmitted to the receiver be ideal because the fiber eliminates all electrical noise? actually not. Because there is no "ground" for the conductor to connect to the amplifying circuit, the potential on the small metal plate is the same as the potential at all places on the amplifying circuit. Without the potential difference, no signal can be amplified, and the entire MiniWhip antenna loses its meaning. PA0RDT Great God has recently experimented with this idea. It is true that no signal can be received.
4. Polarization of radio waves
PA0RDT did another interesting study. Instead of placing the MiniWhip antenna vertically, it was extended out of the window and placed horizontally. It was also introduced into the indoor receiver with a coaxial cable. He found that the reception was the same as when it was placed vertically. At first glance, this antenna does not seem to care about the polarization method. It can be placed horizontally and vertically.
But it is wrong to think that it does not care about the way of polarization. Regardless of whether the support rod is metal or not, the source signal amplified by the amplifier is the potential of the small metal plate relative to the "earth", but only the direct "ground" of the metal support rod, or the shielding through the coaxial cable shield. The difference between the "ground" that is bent over, for the amplifier, the effect is the same in both cases; but this "around a large curved ground" is much shorter than the wavelength of the received signal. (Annotation: That is, if the coaxial cable is too long, its length is close to the wavelength, the receiving effect will be worse. In other words, when the coaxial cable is too long, the antenna receives almost no effect in the low band, and the high band effect becomes difference.)
5, the vibrator is good with a small metal plate or a whip wire
Many active antennas use a whip-shaped wire about one meter long to receive the vibrator, while the MiniWhip uses a small metal plate, which is the core technology that distinguishes the MiniWhip antenna from other antennas. If we use a whip wire as the vibrator, then the vibrator is also measuring the electric field potential in its spatial position. As long as the vibrator length is not as long as the wavelength, it does not improve much.
The shape of the receiving vibrator has an important influence on the other parameter, namely the distributed capacitance of the vibrator. The whip vibrator has a capacitance of about 10 pF per meter, and its thickness also slightly affects the capacitance. Small metal plates have a capacitance of approximately 0.35 pF per cm, and their capacitance is proportional to the diameter, not the area. I still can&39;t find a suitable equation to fit the capacitance of a small rectangular metal plate, but I can estimate that a typical MiniWhip small metal plate has a capacitance of about 2 pF. The size of the vibrator capacitor is very important, because the input voltage of the amplifier and the vibrator form a capacitor divider. If the vibrator capacitance is too small, the signal voltage of the input amplifier will also become smaller. (Note: the amplifier input capacitor should refer to the junction capacitance of the transistor input) .
6, directionality
Before discussing the directionality of the antenna, what is the electromagnetic field of a typical antenna? As shown above, a radio wave transmitted by a vertically polarized antenna is located far away from the antenna (ie, far field), and the electric field line is vertical (red line). The magnetic field lines surround the antenna in a horizontal direction (blue line).
The above figure also shows the Poynting vector (green line), and the Poynting vector indicates the direction of propagation of the electric wave, which can be determined by the left-hand rule.
How to make a receiving antenna more sensitive in some direction than other directions? In theory, if the antenna is exactly the same as the Poynting vector, then the antenna can achieve the best sensitivity. But unfortunately, the antenna does not actually catch the Poynting vector, but reacts to the electric field lines and the magnetic field lines.
An antenna is sensitive to the direction. The best way is to sense the radio waves at different locations and compare the phase differences of the radio waves. For example, the Yagi antenna works like this: the electric wave first arrives at the induced vibrator and then reaches the feed vibrator. (Annotation: Yagi antennas generally have reflective vibrators, which lead the vibrator and the reflected vibrator to induce different phases of the electric wave, respectively, so that the signal received by the feeding vibrator doubles.) However, it is not necessary for a small-sized antenna. When the size of the antenna is much smaller than the wavelength of the electric wave, the electric wave in one wavelength reaches almost any part of the antenna at the same time, and the antenna cannot induce the difference in the phase of the electric wave.
7, the directionality of the small size antenna
For a small-sized antenna, the only possibility to consider its directionality is to follow the direction of the electric field line or the magnetic field line. But again, unfortunately, when the electromagnetic field lines propagated to the antenna, the direction was already messed up. (Annotation: Long-to-medium-short-wave signals are either wavelength-diffused or the ionosphere is reflected multiple times, and its polarization direction becomes unpredictable.)
Conversely, the direction of the magnetic field is much more useful. If the signal is on the west side of us, then the magnetic field line is north-south. If the transmitted signal is on our north side, then the magnetic field line is east-west, and when the transmitted signal is on our south side, the magnetic field line is also east-west, but the direction is 180 degrees apart. That is to say, for a vertically polarized magnetic field signal, we can use the received signal to determine which direction it comes from, despite 180 degrees of uncertainty. Just like the medium wave magnetic rod antenna in the radio, it is directional sensitive, but after 180 degrees of rotation, the receiving effect is the same.
The 80 m band direction finder also uses a magnetic bar to determine the direction of the magnetic field of the direction finding signal. To resolve the 180 degree uncertainty, the direction finding receiver usually adds a whip antenna to sense the direction of the electric field. As mentioned earlier, the electric field line does not tell us the direction of the signal, but it can help us solve the 180-degree uncertainty problem after relying on the magnetic rod antenna to determine the general direction: to determine whether the electric field direction and the magnetic field line are in phase. . A direction finding machine designed by the DF6NM has two mutually perpendicular magnetic bars, plus a whip antenna for measuring the electric field, which can directly indicate the direction of the signal on the graph.
All the above conclusions assume that the radio wave is a vertically polarized signal. When the radio wave is horizontally polarized, all the conclusions are opposite: the magnetic field line is vertical and cannot be used to judge the signal direction; the electric field line is horizontal and can be judged. Signal direction, but with 180 degrees of uncertainty.
8, MiniWhip&39;s directionality
The above talks about the directionality of small-sized antennas, so what about the conclusions for the MiniWhip antenna? We already know that the MiniWhip antenna only receives the vertically polarized electric field signal, and its size is much smaller than the wavelength. The only conclusion is that it is also directional-free.
However, the MiniWhip did not respond to signals from the top of the head. Since the electric field lines and magnetic field lines of the radio waves in this direction are both horizontal, it is not possible to generate a potential difference at the antenna position. From the WebSDR receiver I set up at the University of Twente in the Netherlands, it can be seen that the Dutch Ham friends often complain that the WebSDR is very poor at receiving the 80-meter band signal in the Netherlands. The reason is that the 80-meter band signal around this distance passes. When the reflection reaches the WebSDR antenna, it is almost vertical.
Many friends suggested that in order to receive the signal in the direction of the head, a horizontal metal plate was attached to one end of the small metal plate. But this is not useful, the MiniWhip still only measures the potential difference between its location and the earth. For the radio wave signal from the top of the head, the potential difference that can be generated is zero.
9. Conclusion
1. The MiniWhip is vertically polarized.
2. Grounding is very important. If the antenna is only connected via coaxial cable
The ground of the station will introduce noise. But "land" does not have to be able to
Circulating ground, virtual ground not connected to the earth is also possible, as long as gold
The genus is large enough to meet the input of the amplifier input
The need for capacitors.
3. The received signal strength is proportional to the height of the antenna from the ground, but the height is smaller than the wavelength.
4. Whether the support rod is metal or not. But if the support rod is metal, then the MiniWhip antenna should be placed above the top of the support rod.
When in position, it cannot be placed next to the shaft.
5. The MiniWhip antenna reception is omnidirectional except for the overhead direction.
6. The shape and orientation of the small metal plates are irrelevant, that is, the use of whip vibrators.