Diamond VX-4000 Antenna
- Length:1.3m / Weight:0.7kg
- Gain:2.6dBi (144MHz), 5.8dBi (430MHz), 9.2dBi (1200MHz)
- Max.power rating:100W FM (Total) / Impedance:50 Ω/ VSWR:Less than 1.5:1
- Rated wind velocity: 60m/sec. / Mast diameter accepted: 30mm to 62mm
- Connector:NJ / Type:1/2wave C-Load radialless (144MHz), 2×5/8 wave C-Load
- radialless (430MHz), 5×5/8wave C-Load radialless (1200MHz), FRP outershell
above. It shows part of the specifications of the VX4000 3-Band antenna.
Impedance is described as 50 Ω. If you look closely at the picture posted, there is no radial installed. This means that it is neither a ground plane antenna nor a whip antenna that operates in the same way as a quarter-wave vertical antenna. As far as I know, the antenna is a voltage-fed type when the antenna does not have radials installed.
Current feed type and Voltage feed type
As I mentioned earlier, this VX4000 has no radial elements like the ground plane antenna. It is usually called a non-radial antenna. It is devised by an internal matching circuit so that matching can be achieved even without radial elements.
When thinking about antennas, there are methods to separate them by type of power feed. One is “current fed” and the other is “voltage fed.” From an amateur’s perspective, there are radials for current fed antennas. There are times when we get a little disturbed because of the radial.
The element length is basically λ/2 (half-waves) of that of the voltage fed type. It’s also relatively easy to make your own. The non-radial type antenna is mostly a voltage fed type, and its installation is convenient because there is no need to consider the grounding of the mounting part. The radiator element length is basically λ/2, which is longer than the one for λ/4 antenna. Both have advantages and disadvantages.
Current fed type
When installing a λ/4 vertical whip antenna on a car, the shield wire part of the coaxial cable must be securely connected to the car body. This body ground is very important because it acts as the λ/4 vertical ground antenna, which is the same as a grounded antenna.
The current fed distribution curve of the λ/4 vertical ground antenna is shown in Figure. The current fed distribution is maximized at the base of the antenna. It is called a “current fed” type.
Next figure shows standing waves of the current and voltage distribution on the element of the λ/2 Zepp antenna.
The black broken line shows the current distribution that is standing on the antenna. The current distribution at the feed point is minimum. With this, the signal received by the element cannot be extracted as power. Contrast with a λ/4 vertical antenna. The red broken line shows the voltage distribution, which is maximum at the feed point. Voltage matching can be achieved by matching the antenna and the feed line.
Disassemble of the VX4000
Now we will start disassembling the VX4000. When you loosen the antenna base part screw and pull the N type connector, the element shown in the picture will come out. Sponges are attached to the element as cushioning materials.
Shows each part as an illustration. This illustration was provided by Diamond Antenna. Due to the VHF and UHF bands used, the structure is more complex than expected. If you wanted to make this type of 3-band antenna, it may be impossible to get 50 ohm impedance for all three bands because the structure is very complicated. Honestly, it may be easier and faster to buy an original VX4000 antenna instead of trying to make it by myself. The key point seems to be in the matching part at the base, which is also very complicated and it seems to be like an art.
In addition, Figure 5. shows a picture of the matching part with the black plastic cover removed. I mentioned at the beginning that this antenna has a non-radial structure because it is voltage-fed. We received information from Diamond Antenna that the shape of the sleeve antenna underneath it covered with a black cover works as a filter, and prevents the antenna from being voltage-fed. Let’s dig slightly deeper into the characteristics of this Sperrtopf and below.
|In general, a Sperrtopf is a metal pipe concentrically placed on the outside of a coaxial tube, and its length is 1/4λ of 1200MHz.|
The upper end is open with the coaxial tube * 1, and the lower end is shorted * 2. This is a kind of filter that uses the characteristic where the impedance at the upper end becomes infinite.
On the other hand, a sleeve antenna is a 1/2λ antenna where the pipe length becomes 1/4λ by shorting the upper end * 3 and opening the lower end * 4.
The VX4000 uses the Sperrtopf to prevent the matching from becoming unstable when radio waves in the 1200MHz band flow outside the coaxial tube or the jacket of the coaxial cable. A normal ground plane antenna has a Sperrtopf radial.
Standing waves on the VX4000
Below shows the status of the radio waves standing in the element for each band.
Antenna vertical directional pattern of the VX-4000
Shows the vertical direction characteristics.