**EMRFD Message Archive 1571**

MessageDateFromSubject1571 2008-04-21 03:26:32 Loren Moline RF Transformers Hello,

I have a hypothetical situation in which one has to transform the

collector impedance of 200 ohms down to 50 ohms using a FT37 core.

The impedance ratio is 4 to 1 so the turns ratio is 2 to 1.

If this transformation takes place at 14MHz how do I determine the

minimum number of turns I want to use for the primary of this

transformer which is the collector of the amplifier transistor?

Thanks!

Loren WA7SKT1572 2008-04-21 05:25:29 Nick Kennedy Re: RF Transformers The impedance of the winding is called the magnetizing impedance. Since it

shunts the actual load, you want it high with respect to the load value, a

multiple of maybe 5 or 10. I usually go with 10. So on the 200 ohm side,

choose 2000 ohms reactance. Convert that to uH at 14 MHz. Then use your

"a" value for the specific core to calculate the specific number of turns.

73 - Nick, WA5BDU

[Non-text portions of this message have been removed]1573 2008-04-21 05:45:35 DSNman@comcast.ne... Re: RF Transformers Thanks Nick!

--

Loren Moline

WA7SKT

-------------- Original message --------------

From: "Nick Kennedy" <kennnick@gmail.com>

The impedance of the winding is called the magnetizing impedance. Since it

shunts the actual load, you want it high with respect to the load value, a

multiple of maybe 5 or 10. I usually go with 10. So on the 200 ohm side,

choose 2000 ohms reactance. Convert that to uH at 14 MHz. Then use your

"a" value for the specific core to calculate the specific number of turns.

73 - Nick, WA5BDU

[Non-text portions of this message have been removed]

[Non-text portions of this message have been removed]1575 2008-04-21 06:55:33 wimmie262000 Re: RF Transformers 1577 2008-04-21 20:21:48 Johan H. Bodin Re: RF Transformers Yes, 5 to 10 times the impedance is a good rule of thumb. In narrowband

circuits you can "parallel resonate" that shunting inductance away by

connecting a capacitor across.

73

Johan SM6LKM

-1578 2008-04-22 05:46:28 Nick Kennedy Re: RF Transformers That's a good point, Johan. In such cases (narrow band), I'd normally be

using an iron powder core for higher Q and greater stability, plus the fact

that I just don't need as much inductance as in the broadband / ferrite

case.

On 4/21/08, Johan H. Bodin <jh.bodin@telia.com> wrote:

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[Non-text portions of this message have been removed]1581 2008-04-22 18:44:56 ehydra Re: RF Transformers Hi Nick -

Can you outline when it is better to use iron powder and not ferrite? Is

iron powder more temperature stable?

regards -

Henry

Nick Kennedy schrieb:

> That's a good point, Johan. In such cases (narrow band), I'd normally be

> using an iron powder core for higher Q and greater stability, plus the fact

> that I just don't need as much inductance as in the broadband / ferrite

> case.

>

> On 4/21/08, Johan H. Bodin <jh.bodin@telia.com> wrote:

>> .

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> [Non-text portions of this message have been removed]

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> Yahoo! Groups Links

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>1582 2008-04-22 19:39:20 Nick Kennedy Re: RF Transformers Well, to me a rough guide would be that when you need a lot of

inductance but you aren't particular about the exact value (or ohmic

losses), you use ferrites. That means 1) broadband transformers and

2) RF chokes. Typically, it doesn't mean tuned circuits, but surely

there are exceptions.

Iron powder cores are generally used when lower values of L are

needed, obviously when you compare the 'a' value of type to to type

43, for example, but you need a specific value, higher Q and greater

stability. So now we're talking about tank circuits for VFOs and

inductors in filters.

Take a look at some published designs and you'll probably see ferrites

in 4:1 transformers as well as conventional untuned RF transformers,

and as RF chokes. But in VFOs and tuned circuits you'll almost always

see iron powder.

73--Nick, WA5BDU

1596 2008-04-25 18:37:54 ehydra Re: RF Transformers OK Nick. Thanks!

- Henry

Nick Kennedy schrieb:

> Well, to me a rough guide would be that when you need a lot of

> inductance but you aren't particular about the exact value (or ohmic

> losses), you use ferrites. That means 1) broadband transformers and

> 2) RF chokes. Typically, it doesn't mean tuned circuits, but surely

> there are exceptions.

>

> Iron powder cores are generally used when lower values of L are

> needed, obviously when you compare the 'a' value of type to to type

> 43, for example, but you need a specific value, higher Q and greater

> stability. So now we're talking about tank circuits for VFOs and

> inductors in filters.

>

> Take a look at some published designs and you'll probably see ferrites

> in 4:1 transformers as well as conventional untuned RF transformers,

> and as RF chokes. But in VFOs and tuned circuits you'll almost always

> see iron powder.

>

> 73--Nick, WA5BDU

>

>

>