An antenna first made by Avanti (The Antenna Specialist Co.) and cloned by many under different names. A Google search of just the antenna name "vector 4000" will yield about 50 million hits. The origin of the antenna is in the hands of a company called "Avanti Research & Development".
The antenna includes a three quarter wavelength vertically disposed elongate radiating element and a plurality of quarter wavelength diverging elements connected at their lower end to the bottom of the vertical radiator. This embodiment of the antenna exhibits a gain, compared to a half-wave dipole, of approx. 2.2 dB. There is a "non-apparent collinear effect" and "the antenna is difficult to model with Eznec".
He replied: "That it will be difficult to model the antenna with Eznec and that accuracy is an issue." If we summarize the possible difficulties we could encounter in modeling the antenna with MoM, it becomes clear. It's easy to make mistakes and the phrase "hard to model" could be applied to the antenna for a novice.
A possible solution to this problem is: not to use different diameters, but to model the antenna with one diameter.
An antenna has several regions
It's parked somewhere and the engine is running, we take a look inside and take a look at the fuel gauge. This finding is based on the interpretation that the graph above shows the current and phase of the antenna and that we can see that the current inside the cone is "limited" and remains in the cone, where the current on the outside of the cone can add to the gain. due to being in phase with the upper 1/2 wave section. We can draw a conclusion from the size of the antenna in combination with the shape of the model.
That because we can see differences between the inside and outside of the cone, it indicates that we are looking closely at the antenna. In the reactive near field, two main fields are active, the electric field and the magnetic field. The confirmation that it is: The circular magnetic H-field in the near-field region comes from the size and distance.
Because the “magnitude” of the magnetic H-field will vary with distance and will lose power with the third power of the distance from the antenna (1/r³), as shown in the graph. If you look at the photo it looks like the RED magnetic H-field is rotating, but you should think of it more or less as a direction indicator. The direction will depend on the period of the RF cycle applied to the antenna.
This is different from the conclusion some have made, that they think they are looking at the antenna current and its phase angle. The magnetic H field extending away from the antenna will lose power, it is round and has magnetic flux direction depending on the phase angle of the RF wave. And the magnetic H-field (near-field region) of a center-fed dipole in FEKO is provided below, including arrows to indicate direction.
And finally a combination of far field and magnetic field H (near field) pattern, including direction arrows. Above, I have placed the far field plot on the bottom of the antenna to provide an indication of "direction" in the cone. Magnetic field in the direction of the fingers (90 degrees from the thumb/guide) - and the direction of this field is in the direction the fingers are pointing.
4 Perhaps we are now able to understand what the CST film (plot) is providing
Conclusion
The reason why the color density is so “bright” (red left / blue right) between the main radiator and the elements of the cone is that all the arrows in that area in the FEKO graph point in one direction. This actually gives us insight into how the antenna works. NOTE: THE DIRECTION OF THE MAGNETIC H-FIELD BETWEEN THE RADIATOR AND THE VERTICAL RADIALS ARE IN THE SAME DIRECTION. And that situation is actually not that strange, we notice that the same thing happens with a transmission line.
In the drawing above we see a transmission line (two black horizontal lines) the current and magnitude are shown by the arrows and the curved red line. Although a transmission line (open line) will have a magnetic and electric field around it, in an ideal situation it will not radiate. We know that the transmission line has an E/M field around it simply because there is current moving.
If we start to bend the last 1/4 wave ends of the transmission line and place them 180 degrees apart, we can see that the currents are in the same direction and no cancellation occurs, it is free to "radiate". The transmission line effect is the same as the Sigma 4 antenna. That's one reason it's not a perfectly balanced cancellation system. Secondly, the top of the cone 'sees' air, where the bottom 1/4 wave radiator.
We know this because the 1/2 wave at the top of the cone will present a high impedance at that point. How the cone behaves alone, without the radiation from the 1/2 wave on top. And the accompanying SWR plot of the cone with the loads inserted in place of the 1/2 wave radiator.
The analysis of the cone with charges inserted proves that there is some radiation from the cone due to the mentioned effects. And this is most likely what Cebik meant by: “non apparent collinear effect” which was one of the questions we hoped to get an answer to (point 4 page). We now have an indication that there is some radiation from the cone, but we have yet to find out if this contributes to the performance of the entire antenna.
5 WHAT GAIN CAN WE EXPECT FROM THE SIGMA IV ?
Rockway at the Naval Ocean Systems Center in San Diego gives us the result provided in the plot below. Above is the free-space headroom provided by a NEC4 engine in Eznec Pro, again indicating 2.27 dBi. If we analyze the gain of the Sigma IV versus frequency in free space, we can provide.
The indication given is that "maximum gain" from the antenna gets higher if we go up in frequency. It is important to realize that these numbers exist in "free space", there is a "catch" that we will discover in Chapter 6. The free space far field gain of the Sigma IV antenna is on the order of 2.2 to 2.5 dBi.
6 REASONS FOR GAINBUT NOT GAIN ITSELF ?
REASONS
It is one of the longest antennas available, and as we will learn: This is an important aspect. What if we measure the signal strength in a real situation at a remote receiving location and compare them. The variable is (the height of the antenna tip) of the broadcast station, where every time the bottom of the antenna was the same ... (the top height of the antennas is not the same).
Most of the CB enthusiasts will have heard of common mode currents, they have heard that they can cause problems with RFI or SWR and some know that they can also affect the antenna pattern. Not only the advantage of the height of the Sigma, but also the expected performance of the end-fed half wave could well not have been what we expected. Many of the "execution" flaws mentioned in the aspect to the closing half wave are true for the 5/8 wave and can also have a negative impact on Sigma IV.
And the 5/8 wave example is certainly an example where the theory is not always fully understood. As soon as we place the antennas at a suitable height, the advantage of the 5/8 wave becomes smaller compared to the dipole. Above say 12..13 meters the dipole will actually start to exceed the vertical of the 5/8 waves (same tip height) and the dipole advantage will increase above that height.
Looking at the above plot and the diagram on page 33, we notice that one of the "claims" mentioned on page 6 (point 6) is very plausible. (p6.6... The amplification will become more evident in the distance horizon... ...). The end result was: That the gain from an antenna based on the principle of Sigma IV was equal to that of the J pole. If we "eliminate" the mast and consider that the Sigma IV is attached to a non-conductive mast.
And if we put a good RF choke at the bottom of the antenna and add 4 radians... to create a "ground" for the antenna. We discovered that many of the claims about the Sigma IV are actually not blown out of proportion. That said, in an average situation, the antenna's performance will most likely outperform others.
The length of the antenna or the performance of the antenna in a real situation in combination with a mast and coax connected to it. And while the Sigma IV is already a solid performer, we also found some grounds to investigate further to optimize the overall performance of the antenna itself.
