Help: how to design air-core transformer?

[budgie]

I agree about the ferrite, but he needs a 100:1 impedance change so a
10:1 turns ratio.

Awww ferk - misread that bit didn't I ....

 

[Winfield Hill]

Jim wrote...

If I simply need a narrow-band transformer, e.g. 300MHz with
bandwidth>1MHz, what is the best method? I don't need continuous
tuning, so I can make a set of such transformers to cover the
range of 100-500MHz.

Ah, that's easy. You want a tuned impedance matching network,
or perhaps a tuned transformer, or a tapped resonator, etc...
http://en.wikipedia.org/wiki/Impedance_match
http://beradio.com/departments/radio_impedance_matching/
http://emclab.concordia.ca/~trueman/elec453/MW_Lecture_13_2004.pdf

And I did find an amazing report of a modestly wideband (15%)
50-ohm to 1-ohm transformer, working from 1200 to 1400 MHz.
http://ece-www.colorado.edu/~drc/eds/previous/mccalpin_4_03.pdf

One thing, keep in mind that 0.2nH of inductance is more than
0.5 ohms at 500MHz. Watch out. Do you know what you're doing?

 

[Jim]

It isn't clear (to me, anyway) why you are specifically seeking an air-cored
solution. What is the problem with a ferrite core? You can get a very good
match with a 3:1 turns ratio, or if you really want to get precise then increase
the turns so you get M:N ratio closer to SQRT(10).

I switch to ferrite core now.
Could you please tell me what is the upper frequency limit of ferrite
core? I heard from someone that the limit is 200MHz.

 

[legg]

Jim wrote...

Ah, that's easy. You want a tuned impedance matching network,
or perhaps a tuned transformer, or a tapped resonator, etc...
http://en.wikipedia.org/wiki/Impedance_match
http://beradio.com/departments/radio_impedance_matching/
http://emclab.concordia.ca/~trueman/elec453/MW_Lecture_13_2004.pdf

And I did find an amazing report of a modestly wideband (15%)
50-ohm to 1-ohm transformer, working from 1200 to 1400 MHz.
http://ece-www.colorado.edu/~drc/eds/previous/mccalpin_4_03.pdf

One thing, keep in mind that 0.2nH of inductance is more than
0.5 ohms at 500MHz. Watch out. Do you know what you're doing?

Note also the links to course notes, transmission line and Z-matching
SW by Trueman at Concordia and Pozar at Amherst;

http://emclab.concordia.ca/~trueman/

http://emclab.concordia.ca/~trueman/trline/index.htm

http://emclab.concordia.ca/~trueman/bounce/index.htm

RL

 

[Rich Grise]

I switch to ferrite core now.
Could you please tell me what is the upper frequency limit of ferrite
core? I heard from someone that the limit is 200MHz.

The guy who made the core will tell you.

Good Luck!
Rich