Building and measuring the 2el 160m Array

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!

Radials added to the 160m vertical

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

  1. Reflector connected to Ground (red)
  2. 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.

Conclusions

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

About Tobias (DH1TW)

Self-confessed Starbucks addict. Loves to travel around the globe. Enjoys the technical preparations of Amateur-Radio contests as much as the contests themselves. Engineer by nature. Entrepreneur. For more, follow him @DH1TW

Comments

  1. Jorge EA2LU says:

    Why not detuned the 23m tower to try his effect?
    GL and QRV for any help from my little experience.
    EA2LU

    • Hi Jorge,
      detuning a tower is in my experience not that easy. Basically these are the next steps how I plan to proceed:
      1. Measure the resonance frequency of the tower (N6LF has done great experiments)
      2. When I know the resonance frequency I can generate a NEC2 Model
      3. Add slooping wires from the top of the tower in order to tune the tower.

  2. Hi there!

    Have you considered that you may perhaps measure the F/B ratio too close to your TX site? You should be about 10 lambda away which is 1,6km km on 160m.

    Have you measured the currents in the two antennas ? (You may have to increase drive level and use a current transformer(s) to do that with a current transformer). It is the relative ratio of currents and the phase of the currents relative to each other that should give an indication whether this antenna actually operates as intended.

    To check if other structures radiate, I would suggest you would do current measurements on your separate tower legs with a split core current transformer. If the current is near the current in your verticals you may have a clue to what is going on.

    73
    LB3HC

    • Thanks for your comments Marius!
      A distance of almost 2 Lambda isn’t that bad. With this technique my friend Hannes, DK1NO has successfully measured all of his vertical lowband arrays. Also from a mathematical point of view I don’t see the necessity to go as far as 10 Lambda. Bear in mind that the electrical nearfield component decrease by the power of three (1/r^3) with the distance.
      Measuring the current in the wires is a good idea and I have already 4 current probes in the box. Unfortunately I had no proper Oscilloscope available on site.
      Rudy, N6LF has dedicated quite some time on measuring the influence of towers. In one of his papers he measures the resonance frequency of the tower with an attached loop. I already bought the components. When I’m next time back at the contest station I’ll give it a try.
      73 Toby

  3. Mozilla just for you but I can’t even give you tumbs down :-) Let see if this goes thru. Any progress with 2 verticals?
    73 de Mario, S56A, N1YU

    • Hey Mario,
      I’m glad to see that you finally moved on to a proper browser ;-)

      Yes, there are news. This weekend I have planed to measure the resonance frequency of the two towers. N6LF has described a way to measure it with a coupling loop and observing phase changes over frequency.

      In the meantime I did some additional tests. By connecting and disconnecting the koax to ground I could see a clear change in the f/b (peaking 10dB between 1900-2000kHz). So I suppose this confirms the theory that the tower(s) is/are influencing the 160m antenna.

      73 Tobias

    • I moved temporarely just for you. Cheaper than Paypal :-) GL with 3 verticals, Herr Amigo!

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  1. [...] Finalmente el pasado fin de semana 21 y 22 de enero, DH1TW, EC1KR y EC4DX nos pusimos a trabajar en la antena y pudimos comprobar in situ el funcionamiento de la misma del cual Tobi ya saco los resultados  – Click aquí para leer articulo. [...]

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