This is a standard EZNEC 20 meters Quad antenna elevation plot of a typical two element quad. It happens to be the one I have installed on the tower at 35 meters. Measurements used for the plot were taken after construction. The forward gain is as expected around 10 degrees. The development of the second and third lobs are in line with the theoretical text books |
This is the 20 meters azimuth plot of a two element quad antenna and shows a front to back ratio of only 16.2 db's. The measurements used in the plot are those measured after construction and without tuning. To improve the front to back ration requires tuning of the reflector element, usually accomplished my inserting a small tail section. Best F/B is obtained when both loops approach the same electrical size. In order to archive this, it would be necessary to construct the reflector loop initially smaller than the driven loop in order to insert the tuning tail, which would then add physical wire length to the loop. There are several draw backs to this idea in that countless calculations shows a reduced antenna gain as the loops now have the same physical size, the feed point impedance also becomes unstable and requires re matching to the feed line, F/B tuning except under ideal test condition can often be inconclusive. It is a difficult, and time consuming affair then changing the entire loop dimensions when the antenna is installed. It is very tempting to try tuning the reflector as theoretical result under ideal conditions show a F/B ratio in excess of 35 Db's. Personally opting for a stable and reproducibly antenna design without tuning stubs reduces time and effort and in the long run has very little practical benefit. |
This is the 20 meters antenna VSWR frequency sweep showing the antenna resonate matching point. A very good match is obtained at the design frequency of 14.10 Mhz. However please note that this is not a 50 ohm match but 120 ohm match. This is the impedance at the resonate frequency. It is necessary to convert this to 50 ohms. A two element quad has no point of match to 50 ohms. |
These are the 20 meters wire measurements used in the above antenna plots |
The results of the 20 meters antenna plot in which it shows that the antenna is almost resonant by looking at the voltage angle and that the system shows a good VSWR at 120 ohms, very close to the text books. |