Re: Help: how to design air-core transformer?
From: legg (legg_at_nospam.magma.ca)
Date: 11/24/04
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Date: Wed, 24 Nov 2004 01:09:27 GMT
On 23 Nov 2004 09:53:58 -0800, jmgong74@tom.com (Jim) wrote:
>legg <legg@nospam.magma.ca> wrote in message news:<n076q0l32ektjtrb9fq93mkvkk2ish00oc@4ax.com>...
>> On 22 Nov 2004 15:08:36 -0800, jmgong74@tom.com (Jim) wrote:
>>
>> >I hope to design an air-core transformer which may operate at 100-500 MHz.
>> >However I found the loss is too high.
>> >Can anybody give me some advices to design air-core transformer?
>> >Thanks a million!
>>
>> What is your loss like WITH a core? Maybe the core is not your
>> problem. What application/environment?
>>
>> Google 'air-core transformer', 'air cored transformer', 'coreless
>> transformer', 'contact-free transmission', 'contactless transmission',
>> 'transcutaneous energy' etc.
>>
>> RL
>I observed two unwanted phenomenon:
>
>1. direct capacitive coupling between primary/secondary coils. It
>interferes with the mutual inductance, and makes the transmissivity of
>the transformer change significantly with frequency.
>
>2. Insertion loss is higher than 10dB.
>
>I googled air core transformer before i posted my question. It is
>weird that the concept is mentioned in many places, but I can never
>find a real one which works in VHF/UFH band except transmission-line
>type.
>
>My purpose is for impediance matching, from 50 ohm cable to a 0.5 ohm
>load.
>
>To my understanding, air-core transmission-line transformers have
>fixed conversion ratio, e.g. 1:1, 4:1 or 1:4, is it right? What i need
>is a ratio of 10:1.
>
>I saw some companies provide RF transformer with ferrite coil inside.
>The insertion loss is only 1-2dB, however, for UHF band, the maximum
>ratio is 4:1 or 6:1 only.
>
>Thank you all again! And hope to hear from you again.
The old Motorola App Note AN1304 discusses the use of coaxial baluns
around 1GHz, to reduce Z mismatches of 100:1 to approximately 6:1.
http://www.freescale.com/files/rf_if/doc/app_note/AN1034.pdf
also
http://www.freescale.com/files/rf_if/doc/app_note/AN1670.pdf
http://www.freescale.com/files/rf_if/doc/app_note/AN1033.pdf
http://www.freescale.com/files/rf_if/doc/eng_bulletin/EB105.pdf
.....don't forget to check references listed in tha articles.
Going to 50ohms in the first place, may be part of the problem, as
your active devices likely exhibit output impedances in the 10ohm
range or less, to begin with. Matching earlier may help.
If galvanic isolation is required, a simpler 1:1 or 2:1 conventional
balun transformer might be introduced additionally, in a convenient
position. because required turns are low, you might consider winding
shapes that are not normally seen in baluns - perhaps a single loop
inside an isolated screen (~output winding).
With the app data aimed at achieving 50 ohms, it may be difficult to
turn it around to make sense for low load values such as 0.5 ohms.
RL
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