Hello Tom, hello Topbanders!
Today I measured a number of 9/1 transmission line-type transfomers made
with Amidon cores with 75 and 43 material. For the tests I used a Rhode &
Schwarz Network Analyser, and connected two transformers back to back. The
attenuation listed, is half of the measured attenuation. I also measured
the return loss for the trasnformes terminated ina 450 ohm resistor. The
SWR ws is all cases less than 1.1/1.
I could not find toroidal cores with 73 material, unfortunately. I bought 3
sizes, FT50, FT82 and FT114
All the transformers were wound as a transmission line style transformer
with 8 trifilar turns.
1/ 75 material.
---------------
To my surpise I needed about 8 turns for 50 ohm, in order to achieve low
losses. I had expected less as W8JI said. I was very spurprised to see an
enormous differnce in insertion loss as a function of the core diameter:
FT50 FT82 FT114
1.8 MHz 0.7 dB 0.47 dB 0.17 dB
3.6 MHz 0.65 0.42 dB 0.21 dB
7.0 MHz 0.65 0.38 dB 0.28 dB
The choice here is clear: Take the 114-size core The smaller cores are
substantially worse.
2/ 43 Material
--------------
Here too, 8 turns were required for 50 ohm. Lower turns gave worse
attenuation on 160.
FT50 FT82 FT114
1.8 MHz 0.26dB 0.33 dB 0.27 dB
3.6 MHz 0.26 0.26 dB 0.20 dB
7.0 MHz 0.25 0.20 dB 0.19 dB
Here too, the largest cores gave the best results. The difference between
the FT114 and the smaller corse is much less outspoken than with the 75
material.
3/ MN8-CX cores (stack of 2):
-----------------------------
Only 4 turns are used (3 x4 )
1.8 MHz 0.27dB
3.6 MHz 0.25
7.0 MHz 0.23
CONCLUSION:
-----------
Contrary to what you stated, in order to obtain the lowest attenuation,
both the 43 and the 75 material cores needed twice as many turns as the
MN8-CX cores.
I can recommend three types of cores:
1/ the MN8-CX cores (which I recommended all along in my book)
2/ the FT114-75 core
3/ the FT114-43 core, the FT82-43 and the FT50-43.
My personal conclusion is that I will keep using the MN8-CX cores, as I
really see no need to replace them.
73
John, ON4UN
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