The Gain Master antenna from Sirio is a very popular antenna for CB and the 10m amateur band. In my article I will explain how the “Gain Master” can be designed and built for different frequencies. Moreover, I will show how to further increase the gain.
I will start by describing the parts of the antenna. Generally speaking, the Gain Master is a simple dipole antenna. When we look at the construction of the Gain Master, we can see that it consists of a coax cable and a wire. The wire forms one pole of the dipole and the second pole is formed by the coax cable which goes from the middle of the antenna to the coil at the bottom of the antenna. The bottom inductor forms a parallel resonant circuit between its inductance and the winding capacitance. As we know the impedance of a parallel resonant circuit is at its maximum at the resonant frequency. Therefore, the inductor (in reality a parallel resonant circuit) prevents the wave from traveling down the whole feeding line of the antenna. This form of inductor is also known as common mode rejection choke. I found a graph from G3TXQ which helps building such coils for different frequencies.
In addition, the upper part of the coaxial cable forms a series capacitor where the outer conductor is cut. Strictly speaking the “coax capacitor” is part of the upper dipole pole since its screen is connected to the wire (upper pole). This means the dipole centre is the point where the outer coax conductor is cut. The stub (shorted parallel coax line) on the bottom pole and the coax line between the dipole centre and the stub in combination with the capacitor forms an impedance matching network. Such networks can be calculated with the help of various Smith Chart programmes.
You may ask yourself how you can increase the gain of a dipole. According to Wikipedia the gain of a dipole antenna increases when the total length is increased. The antenna length can be increased until lambda * 5/4 where a gain of about 5.2dBi is achieved. If the length is further increased unwanted extra lopes begin to develop in the radiation pattern. Below lambda * 5/4 the radiation pattern consists of a single “donut”. If the dipole is placed high enough above the ground, the main slope has an angle of 0°. That is a reason why this antenna works wonders for local communication. For DX, a horizontal dipole would be ideal, since then a bigger part of energy is directly radiated towards the ionosphere. Or one could simply place the Gain Master directly on the ground which would rise the angle of the main slope.
Designing the matching network
For designing the matching network it is necessary to know the impedance of the dipole. There are some calculators on the internet for calculating the impedance, but the results are always a little bit off the real value (as expected). So, one solution is to build a dipole of the desired length from two wires and measuring the impedance. For this procedure it is very important to hang the dipole as high as possible from the ground. Otherwise the effect of the earth on the impedance will be to big. Once the impedance is known a Smith Chart program should be used to match the antenna to 50 Ohms. I used this online calculator.
The series capacitor is used to remove the inductive part of the antenna impedance. If the imaginary part of the antenna impedance is capacitive a series inductor could be used (for very long antennas) or you simply totally remove this matching element. The series capacitor helps to move the impedance more towards the centre of the Smith Chart (it therefore reduces the length of the transmission lines). Even the series capacitor could be removed. Then the impedance is transformed to the “50 Ohm circle” on the conductance Smith Chart by using a series transmission line. It is very important to enter the correct velocity factor and characteristic impedance of the cable into the program (if these values are wrong you will get the wrong cable length). The velocity factor is found in the cable datasheet and can be converted to the permittivity using the following formula:
Er = 1v^2
The third matching element is a shorted, parallel transmission line. It is used to remove the imaginary part of the “new impedance” so we get a 50 Ohm load with no capacitive or inductive part.
In this screenshot you can see each matching element I described and its effect in the Smith Chart. The value of the “Black Box” is the dipole impedance.
Now coaxial cable is added to the input of the matching network until the full desired length of the dipole is reached. Then the common mode rejection choke is added.
A more practical way to design the matching network is adding matching element by matching element to the antenna and rechecking the impedance after each element with a VNA. So, each element can be exactly cut to length (calculations are of course not perfect). When doing this it is important to place all transmission lines at their final positions (transmission lines must be parallel to the part of the dipole that they later replace). Note: Sirio tried to patent many principles used for this antenna, but it seems that the patent is currently withdrawn. The patent can be found here .
I must admit I that have not found a cheap revolutionary way to mount this antenna vertically yet. A common way is to use a fishing rod. There are some cheap models from China available on Amazon, but these will even cost about 50€ for one that is about 8 meters or longer. Another solution would be buying fiberglass rods and recreating the original Gain Master in some way. If you have a high tree you could also use a branch to mount the antenna when the distance to the trunk is big enough.
Update: The antenna can be mounted on a fishing rod. There are models with a length up to 9m available on Askari . The best about this is that the longest pole only costs about 30€ with shipping.
Instead of a coaxial cable a twin lead cable can be used for matching. This has one main advantage: a twin lead can be taped easier compared to a coaxial line. Unfortunately, the antenna gets heavily detuned when the twin lead gets wet. That means it must be protected from rain. The reason for this problem is, that most of the electromagnetic field lays outside of the twin lead. In other words, the close area next to the cable is part of the cable and defines its impedance. If you want to build the twin lead version anyway, you can find some building instructions here (I must admit I never build the coaxial version because I found this site). One could hide the antenna inside of the fishing rod to protect it, but I do not know if this solves the problem (space between the wet pole and the twin line might be to small).