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Old 24th Oct 2016, 11:23 pm   #121
G0HZU_JMR
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Default Re: 1:1 RF Transformer Design SWR problems

That does look very lively. Much more than I expected to see, especially when you fitted the very long cable. It might not be that bad when you connect it up as in a real setup with a radio and ATU etc but it doesn't look good in your video.

Getting back to the VSWR issue, I suspect that I've failed to convince many people that the bifilar winding in the transformer still acts as a transmission line. When you look at the connections it doesn't look as easy to understand as a current balun for example. But the winding is still behaving as a transmission line. One might as well argue that a BC107 isn't a BC107 unless you connect it up as a common emitter amplifier. But we all know that it is still a BC107 even in common base or common collector. When the windings are connected as in the floating load test it is still possible to explain the performance using a basic tline analysis by following where the energy goes in a time domain analysis. Then do a steady state analysis. You just need to know the characteristic impedance of the line and the frequency where it appears to be a quarterwave long in order to create a useful (and fairly accurate) model. The frequency response won't be the same as a current balun but the frequency response and VSWR of the isolation transformer can still be predicted quite accurately using simple transmission line theory
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Last edited by G0HZU_JMR; 24th Oct 2016 at 11:28 pm.
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Old 25th Oct 2016, 9:34 am   #122
ChristianFletcher
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Default Re: 1:1 RF Transformer Design SWR problems

Hi Jeremy

Many thanks for all your advice. I cannot comment regarding the transmission line theory as I don’t have anywhere near the required level of understanding. I was however pleased that the ferrite clamps did a lot to stop RFI from being radiated I think they actually killed it completely although I forgot to show that in the near field testing. I think it would have been better (worse) if I had the long coax laid out in a straight run as I suspect that the coiled up length probably acted like an air cored choke in itself.

I think there still lots of room for improvements by optimising the windings style and connection details and also be adding something to control the common mode modes such as clamps.

I particular enjoyed the practical experiment and some of the test conducted to probe the performance and problems with the design. It was really interesting to use the spectrum analyser and near field probe to show the common mode on the feeders how dramatic the effect of the clamps were.

So I think I will end it there and thanks for every ones comments and advice.

Regards Chris
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