“…It’s a neat trick !…” -Fred Harris-
In the last article, we’ve seen that under typical ‘newcomer’ installation conditions, vertical antenna potentially perform better than inverted-Vee antenna. But, there is two weaknesses of the vertical antenna ; 1. Vertical antenna needs a very good ground system installation (not trivial) and 2. Vertical antenna that is ¼ lambda long with a very good ground system yields an input impedance of 30 Ohm or there about (resistance component). This impedance actually easy enough to match to 50 Ohm cable, but under various circumstances creating a proper matching network is not a trivial job and can lead to an excessive power loss.
Maybe you asking “…Is there any way to increase the input impedance to (let say) 50 Ohm without increasing ground losses so that it will match perfectly to the coax cable ?…” Glad you asked, the answer is yes, there is. Like I already said in the last article, improper ground system for vertical antenna can gives you an excessive ground losses so that the input impedance when resonance, measured at 50 Ohm when it should be about 30 Ohm (indicates of 20 Ohm of ground losses). To attack this problem, we should do the technique (I will show you) under the practical absolute-minimum ground loss environment, that is sea water. On this environment, we can guarantee that the increase of input impedance is caused by the applied technique, not caused by ground losses.
The Process of Increasing Input Impedance
Firstly, we recalculate the dimension and the input impedance of ¼ lambda vertical antenna above sea water. In this calculation, we will using frequency of 21,05 MHz (15m band CW segment).
3D view (the green mat bellow the antenna represents salt water) :
4NEC2 numerical results :
In this situation, we simulate a 15 m band elevated vertical antenna with 2 elevated radials above sea water. When situated above sea water, the ground loss is at absolute-minimum, hence we don’t need much radial wire to install. We just need the minimum amount of radial number, which is two. It is important that the whole antenna still needs to be elevated above sea water (1 m of elevation is enough), because, while the sea water has the absolute-minimum ground loss, it is still non-zero. (Please read the story of K2KW).
The 4NEC2 simulation results that the vertical antenna above has impedance of 31,7 Ohm at resonance and peak gain of 4,6 dBi (!). The peak gain figure of 4,6 dBi is expected due to low loss ground beneath it. Simulated impedance figure is match to the prediction, which is still about 30 Ohm. We will use this results as the baseline for our technique to increase the input impedance. Figure bellow shows the dimension of the antenna used in simulation.
The vertical radiator and radials all made of stranded copper wire with 1,6 mm copper dia. and 3,6 mm PVC insulation dia. (2,5 mm sq. insulated cable).
From the ON4UN’s Low-Band DXing 4 th Ed. Book, Chapter 9, Fig 9-7, Fig 9-8, and Fig 9-9, we can expect that at certain vertical radiator length, it will yields radiation resistance of 50 Ohm. The radiator length to be expected is somewhat longer than ¼ lambda. With that vertical radiator length, it will produce 50 Ohm radiation resistance, but at the price of large inductance component. The impedance will be 50+jX Ohm, where X will be some large positive value, which is far from resonance. To combat this situation, the simplest solution is to bring back to resonance by decreasing radials length until X equals zero. While the resistance component is largely determined by the vertical radiator length, decreasing the radials length virtually only effects the reactance component on the feed point (the jX part). The technique which we increase the vertical radiator length to achieve 50 Ohm is known as ‘overloading’. Luis, HC1PF (ex. IV3PRK) once did this to his 160 m band vertical antenna and produce very good signal all over Europe. (The term ‘overloading’ itself credited to Remco, PA3FYM). Figure bellow shows the method of ‘overloading’ to achieve good 50 Ohm match of vertical antenna.
Besides of the experiment Luis performed on his vertical, to confirm this technique, we can simulate it on 4NEC2. The first step is to increase the vertical radiator length to achieve 50 Ohm radiation resistance (very easy using optimizer tool), shown on the figure bellow.
From the 4NEC2 simulation above, the required vertical radiator length to produce 50 Ohm radiation resistance is 3,87 m or 15% increase from 3,37 m (dimension isn’t shown on the picture). With the radials length is still 3,37 m long, it will produce input impedance of 50+j122 Ohm. The second step is to decrease the radials length to bring the antenna back to resonance. Again, we can use the optimizer tool to do this. Figure bellow shows the results.
From the simulation above we can already see that the technique we use to achieve good 50 Ohm match of vertical antenna works. Please note that as we decrease the radials length to achieve resonance (from +j122 Ohm back to +j0 Ohm), the radiation resistance slightly decrease to 48,8 Ohm. In this case VSWR value is 1:1,02 and already gives a very good match to 50 Ohm coax line. With several iteration of lengthening the vertical radiator and shortening the radials, eventually it will produce perfect match. In my case, the last iteration that gave me best 50 Ohm match is 3,90 m of vertical radiators length and 2,34 m of radials length (about 15,5 % increase for vertical radiator and 30 % decrease for radials). Finally, the last iteration results and the dimensions is given on the figures bellow.
If you are a person who obsessed with VSWR figure, here is a graph to please you.
Simple vertical antenna which has good gain (4,7 dBi) at very low radiation angle (5 degrees from horizon), very good VSWR across entire low portion of 15 m band (great for CW contest), only needs two elevated radials (above sea water), and of course relatively easy to erect, what else do you ask ?
The overloading method to achieve 50 Ohm impedance of vertical antenna is presented. Only with increasing the vertical radiator length by 15,5% and decreasing radials length by 30% both with respect to ¼ lambda long, 50 Ohm perfect match can be achieved.
Discussions are welcome.