Last weekend we spend quite some time to prepare our contest station ED1R for the upcoming CQWW 160m contests. The biggest task was the installation of a 2el parasitic Array of two verticals. During the installation I took the time to document the measurement results. Find them with my (preliminary) conculsion in this blog post.
The effect of adding radials
The first tests were conducted with a single vertical and the 25 radials which we have always used on 160m so far. Find below the impedance chart:
Now, after adding another 50 35m long radials the impedance on the same vertical looks like this:
The S11 / VSWR curve is sharper (as a proper vertical should look like) and the impedance dropped from 20 Ohm down to 15 Ohm. This means that we reduced the losses by 5 Ohms!
80 radials in place
Below is the simulated (NEC2) impedance curve of our vertical. (17m tall, a 35m long, slooping T-hat and MININEC ground (13mS and epsilon 26)).
The picture above shows the radiator of our 160m array
Above the simulated SWR curve is shown
And finally the simulated impedance is shown as well
Even if the real antenna impedance is likely to be a little bit lower as simulated (14.2 Ohm), I’m happy with the measured 15 Ohms.
Making the Reflector work (or not…)
After having set up correctly the radiator it was time to install the reflector. The reflector was another vertical, 18m tall with an almost horizontal T-hat. At the feedpoint an additional roll inductor was connected between the reflector and ground. The roll inductor (0.5… 15,8µHenry) was used to adjust the reflector for a maximum front / back ratio.
Macbook + DG8SAQ Vector Network Analyzer + Reflectorbox
In order to measure the front / back ratio, we installed a probe (4m tall vertical, terminated with 75Ohm) in a distance of approx. 250m behind the reflector in the field. While transmitting with low power on the vertical (array) I used the DG8SAQ as a Spectrum Analyzer in order to optimize the reflector length for optimum f/b.
Unfortunately for still unkown reasons, the F/B did not exceed 3dB. This is of course very poor.
I’m still struggeling for reasons… Probably the most likely reason is mutual coupling with the 22m tall tower (with a 2el 40m yagi) just behind the reflector.
Coupling between radiator and reflector
Next I measured the coupling between the radiator and the reflector. The chart above shows the feed impedance for the two cases
- Reflector connected to Ground (red)
- Reflector not connected (black) -> invisible to radiator
This measurement confirmed that at least some coupling is happening between the two elements. The simulation shows the following SWR and antenna impedance for the array:
The chart above is the SWR curve of the 2el vertical array simulated at the feedpoint of the radiator
And this chart above shows the simulated antenna impedance
Impedance of the reflector
I also measured the impedance of the reflector (while have the radiator disconnected).
The chart above shows the SWR and impedance measured without the roll-inductor. The self resonance of the reflector without roll inductor is located at 1.87 MHz.Unfortunately, the antenna impedance is 25 Ohm, which means that the earth screen of the reflector is still quite lossy.
In the chart above, the roll inductor (2,6µHenry) make the reflector resonant at 1.81 MHz, approximately 20kHz below the radiator.
Unfortunately we couldn’t verify yet the desired antenna performance. It is not clear what is the reason for the low front / back. Two possible reasons could be:
- Too little coupling between the two antennas
- Too much looses on the reflectors ground network. More radials must be installed in order to reduce the losses
- The 23m tall tower behind the reflector is resonant on approx 1.8MHz and distorting the antenna pattern.
… investigations to be continued