Hi Mike
I have not considered the temperature coefficient of the core. That is a great
suggestion.
I did find some generic plots of permeability vs temperature and there can be a
big change. The data sheet from Fair Rite does not quote any temperature
coefficient values. I’m not sure how to to test that easily other than heating
the core in the field on a cold day. Or perhaps cooling it with an air can
upside down in the summer.
The other option is to sweep the raw wire in the field, but at -38C that would
be rather difficult.
I do plan to experiment with changing the termination resistor value this
summer to see if I can replicate the cold weather sweep.
73, de steve ve6wz
> On Jan 21, 2020, at 6:50 PM, Michael Tope <W4EF@dellroy.com> wrote:
>
> Steve,
>
> Have you eliminated the core material in the 9:1 transformer as a possible
> source of the problem?
>
> I don't see how the resistance of the galvanized wire changes the surge
> impedance significantly. When viewing the beverage as a transmission line
> (single round conductor over ground plane), I would expect the distributed
> resistance of the steel wire would to make the transmission line loss/unit
> length higher, but other than adding a slightly reactive component to the
> characteristic impedance, the steel wire resistance change shouldn't make the
> real component of the transmission line characteristic impedance change much.
>
> 73, Mike W4EF...............
>
> On 1/21/2020 3:55 PM, VE6WZ_Steve wrote:
>> 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.
>>
>> YouTube video here: https://youtu.be/3Oft826Q8tA
>> <https://youtu.be/3Oft826Q8tA>
>>
>>
>>
>> 73, de steve ve6wz
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>>
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