Message-ID: <4A666044.6090405@charter.net>
Date: Tue, 21 Jul 2009 20:41:40 -0400
From: John Andrews <w1tag@charter.net>
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Subject: Re: LF: Loop (was Re: IGBT in 136 KHz TX?)
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Ha-Jo,

That's one way to look at it, and a valid approach. But consider another 
example:

Suppose you have a single-turn loop that has an (arbitrary) impedance of 
0.1 +j10 ohms at a frequency of interest, and you desire to make it look 
like a 10-turn tuned loop. A 1:10 transformer will provide the same 
voltage step-up as a 10-turn loop, and will ideally step the impedance 
up by 10^2 = 100, giving you 10 +j1000 ohms. You would then have an 
option of series or parallel tuning the loop on either side of the 
transformer. The secondary side is much nicer for our use than the 
primary side. Parallel tuning would be a good choice into a hi-z input 
preamp. Series tuning would leave you with the 10 ohm resistance, which 
you could then step up to the characteristic impedance of your line, or 
leave at a lower value if BW were an issue.

In the above case, I'd suggest a primary inductance on the transformer 
of 5X the magnitude of the loop impedance, but your 4X would probably 
do. Leakage reactance is pretty much inevitable, and will throw things 
off a bit.

There's all sorts of ways to approach this, depending on your desire for 
tuning, etc. But I just wanted to point out that the much simpler single 
turn construction can work just as well as multi-turn.

John, W1TAG


hajo.brandt.dj1zb@t-online.de wrote:
> Dear John, LF,
> 
> I would like especially to comment on using a one winding loop combined
> with a step-up transformer: