[TenTec] OT: Selection of ferrite material (was SWR and Coax)

[Dr. Gerald N. Johnson]

On Mon, 2008-07-07 at 18:18 +0800, Marinus Loewensteijn wrote:
> 
> First of all thanks to all about rectifying my misconception on SWR and 
> radiating coax.
> 
> My rig is working again but not much to work tonight on 7 MHz so am 
> up to some more internet investigations. (mischief?)
> 
> Need to make a 1:9 broadband impedance transformer and had found
> (as reported earlier) a design with the Amidon FT114-61 material.
> Someone else muttered that #61 is no good and I need to use #43. 

If you want your copy to work like the original, you need to use the
same material as the original.
> 
> However one item drew my attention and that is ferrites have a 
> temperature which should not be exceeded because then the magnetic
> properties of the material is destroyed and hence the toroid becomes
> useless. I discovered that the #61 has a Curie temperature of 300 C
> and #43 a Curie temperature of 130 C. 

So far as I know the effect of exceeding the Curie temperature is
reversible. Its not permanently destructive, just that the magnetic
properties change drastically. My Weller temperature controlled
soldering irons use a permanent magnet and its Curie temperature for
control of the tip temperature and it cycles for years and still is
magnetic when cooler than the trip temperature.

You just can't plan on having the magnetic properties above the Curie
temperature that you had at room temperature. Besides 300C is hard on
most wire insulations. 130C on common wire insulation except for teflon.
> 
> This big variation means to me that in addition to the magnetic
> properties a close eye needs to be kept on the Curie temperature when
> selecting a toroid for use in a balun or broadband transformer. I am 
> sticking with the #61 design.

Core cooling can be a big problem with high power. I saw a 50 KW ferrite
cored balun at Collins once, it had heat sink plate between the stacked
thin toroids and was operated immersed in oil. And was redesigned at
least once after the first time 50 KW of RF was applied.
> 
> 73, Marinus, ZL2ML

73, Jerry, K0CQ