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Changing Beverage Impedance in cold weather

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    Changing Beverage Impedance in cold weather

    Over the years I have noticed my Beverage feed impedance will change during the winter.
    My station is configured to take analyzer sweeps in the shack of any of my 12 Beverage wires. All 12 wires show the same change in impedance compared to the summer.

    Two days ago I did some experiments to see if this was because the frozen ground was limiting conductivity. In wet soil with the associated minerals and salts, the ionic transfer contributes to conductivity, just like salt water. When the ground is frozen solid, like a block of ice, it seems possible that this ionic conduction could be diminished.

    To test my theory, I add some radials to both the feed and termination and rescan the Beverage.
    Join me on the adventure to see how it worked out in this video as I brave the -38 C winter cold. I also show some calculations of the effect of cold weather on Beverage wire conductivity and the termination resistor. This is perhaps the most interesting, because the term resistors I use are ceramic units which have a significant negative temperature coefficient. I experiment to confirm the changes in resistance with temperature.

    73 Al 4L5A
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    #2
    A few days ago I posted a video explaining some experiments I did adding short radials to my feed and termination grounds on the Beverage.
    During that week temperatures went as low as -41 C at the station.

    The mystery has been solved.

    I wondered if the frozen ground was limiting conductivity and upsetting the ground resistance part of the impedance. The experiments showed that adding radials to stabilize the frozen ground had no effect.

    In the video I show how the real resistance of the 920 foot galvanized steel wire might drop 10-20 Ohms when the temperature drops 50 deg C from the summertime. This would mean that I need a LOWER termination resistor to match the surge impedance.
    However, I also show that the ceramic termination resistor I use has a very high negative temperature coefficient (-1300 C). This results in an INCREASE of 30 to 40 ohms in my 470 Ohm termination resistor. Therefore, the termination could be 40 to 60 Ohms too low!! I show in the video that the resistor does indeed increase by 40 Ohms when I chucked it out my back door at -30C for an hour.

    Today I was at the remote and it has warmed up to -1C (35 deg C warmer) and the analyzer sweep of the wire has mostly returned to it summer time reading.

    This seems to indicate that the main cause of the Beverage miss match is the temperature coefficient of the wire and the termination resistor. Obviously the ground is still just as frozen today as it was last week at the feed and termination and under the wire length! Perhaps I will look into changing the term resistor with a metal foil unit that has a positive temperature coefficient. Carbon composition units also have a negative TC, but not as severe as the ceramic units. Those ceramic units are almost like thermistors! (well….not really)

    However, as I said in the video, this is like a “solution looking for a problem” because few Hams experience these extreme temperature changes, and even when we do, it's just for a short time. Also, Beverage modelling shows that the termination resistor can effect the F/B, but it has limited effect on the RDF which is what really matters. This exercise was done mostly out of interest and to try and solve the puzzle.

    I made a new video showing the sweeps I made today if interested. The video includes the original content so if you've seem that just jump to the end.

    73 Al 4L5A

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      #3
      I completed my experiment, and it appears that cooling the transformer by 40 deg C has very little change on the matching transformer.

      The 9:1 transformer was terminated with a 560 ohm resistor and swept with the Nano-VNA and showed almost exactly 75 ohms on 160, 80 and 40m at room temperature.
      The transformer was put in the freezer that is at -20 deg C using my temperature thermocouple gauge on the Fluke multimeter.
      The transformer was swept again after an hour of cooling and the sweep was virtually identical. No change.
      73 Al 4L5A

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        #4
        The recent experiments with my Beverages in the cold weather has raised some comments about the termination resistor and the antenna performance. I received a few emails asking about F/B. I made the comment that it really is'nt worth the effort trying to get the termination value exact. Close is good enough.

        Changing the termination resistor value has only a limited effect on the RDF of the antenna. The F/B can change some what, but the zenith and side rejection is mostly preserved independent of the termination value.
        This is well described by John in the the Lowband DXing book. (5th edition, pg 7-64 “conclusions”)

        Today I did some modelling on a 920’ wire. RDF with a 500 Ohm term is 11.7 dB, with 200 Ohm term it is 11.3 dB, and with 800 Ohm term it is 11.4 dB. So only about .4 dB of RDF change with a very significant (+/- 300 Ohm) resistor change. Looking at the azimuth and Zenith plots, the front-to-side remains mostly unchanged while rear lobes develop that diminish the F/B. However, unless there is a specific noise, or QRM problem from the back of the Beverages, this will likely go un-noticed. The important parameter is RDF….the rejection of noise in the full 3D hemisphere compared to the max forward gain.

        In fact, I modelled no termination (by inserting 1 e6 Ohms as a term resistor) and the RDF of the bi-directional Beverage is still 9.7 dB. (almost zero F/B) This is pretty decent, and is better than many other small loop antennas and smaller arrays. The azimuth pattern still shows significant side and zenith rejection which is why the RDF calculates so well. This is perhaps why many ops have had good success with bi-directional Beverages.

        If you are unfamiliar with what the RDF metric, Greg has a good explanation here:

        http://www.seed-solutions.com/gregor.../RDFMetric.htm

        73, de Steve, VE6WZ.
        73 Al 4L5A

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