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Re: [Amps] Broadband transformers, and ferrite plate chokes

Subject: Re: [Amps] Broadband transformers, and ferrite plate chokes
From: Manfred Mornhinweg <>
Date: Sun, 09 Mar 2008 16:22:14 +0000
List-post: <">>

that's interesting information! It's tempting to run the calculator on 
these chokes:

> Those mentioned R&S transmitting  chokes (90?H) use 25 windings on a 2 1/4"
> ceramic former containing a 1.5" diameter ferrit rod 3.5" long.

If the turns were quite close together, that would result in a flux 
density of 23mT at 1MHz, which is slightly high. But with a space wound 
coil, the flux density should drop into the safe range.

 >  The wire is #18 spacewound at 5 tpi.

Which means that the coil is longer than the core! Interesting. I have 
never seen that. It does make sense, though. The ferrite is used only 
where most flux lines can converge. That saves losses and cost.

> Original use in 1-30MHz amps running at 3KV anode voltage, 1.3A at full
> output.
> Two chokes are used at right angles and one is switched out above 10MHz.

Oh, should read all of the message before replying! :-) This halves flux 
density at 1MHz, leaving it at about 11.5mT before considering the 
spacing of the turns. That's already in the green zone. So, with the 
turns spacing, there is ample headroom built in, in terms of ferrite losses.

The DC flux density turns out as only 4mT, which is completely 
insignificant. This choke could take close to 100A before the ferrite 
would saturate! Of course, the wire would vaporise well under that.

> I am going to use the same pair of chokes for a 2x8877 project I am busy
> with.

They should work very well. Good luck with the project!

It's interesting to relate these chokes with what I wrote:

>> In order to keep a ferrite core of 2cm diameter working at low enough
>> losses, about 100 turns of wire would be needed for a typical amp running at
>> 2.5kV RMS RF voltage. 

So the R&S have a total of 50 turns, on 3.8cm diameter ferrite, for 3kV 
at 1MHz, resulting in 11.5mT or a bit less due to turns spacing. At 
1.8MHz, it would be 6.4mT.  And my quick math above , with 100 turns on 
2cm diameter ferrite, 2.5kV and 1.8MHz, results in 10.4mT. While my 
10.4mT are a bit at the upper limit, but workable, R&S opted for a bit 
more safety margin, not surprising for a company selling very high 
quality equipment at high cost.

I love it when I see things coming together!


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