From: "Dave Pick" <dave@picks.force9.co.uk>
To: rsgb_lf_group@blacksheep.org
Date: Thu, 17 Feb 2005 12:04:34 -0000
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Subject: RE: LF: RE: Matching Ropex
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The puzzle is why the Ropex has problems with a 50 ohm match at 136kHz if
the coil is grounded? The match is the same on an SWR bridge whether the pi
cct is in circuit or not. The Ropex protection cct seems to be some kind of
overcurrent detector in the 50ohm output but I can't remember which side of
the internal LPF it is placed.
 
Dave G3YXM

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Johan Bodin
Sent: 17 February 2005 12:40
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: RE: Matching Ropex

Dave Pick wrote:
> I put a pi network (LPF) in between and it worked. 

Yes, that's the "impedance inversion" Markus DF6NM wrote about on the topic
of "Wilkinson Combiner" recently.

A Pi filter with all X = Zo behaves like a lumped circuit version of a
quarter wave transmission line.

For 50 ohms and 136.75 kHz that is:

   C1 = 23.3 nF across, L = 58.2 uH along, C2 = 23.3 nF across

Open load => L and C2 in series resonance => input looks shorted Load = 50
ohms => input = 50 ohms Shorted load => L and C1 in parallel resonance =>
input looks open

73
Johan SM6LKM


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