From: Rik Strobbe <Rik.Strobbe@fys.kuleuven.be>
To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
Date: Sun, 18 Dec 2011 12:25:34 +0100
Thread-Topic: Re: Re: Re: TX system at DK7FC, schematic
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Mal, Jim, all,

maybe the clue is in the filter used by Stefan.
A bridged amplifier as used by Stefan is very flexible towards the load res=
isance, anything between 10 and 100 Ohm will probably give almost the same =
output power.
But a traditional lowpass filter (Pi or T type) is much less flexible, movi=
ng axay from the impedance it is designed for will result in addidional los=
ses.
The parallel / serial resonance filter as Stefan uses is much more flexible=
 and will also do very well in the 10-100 Ohm range. So any impedance match=
ing may be not needed.
Disadvantage of this filter is the far less attenuation of the harmonics co=
mpared to a Pi ot T lowpass filter with the same number of inductors. I est=
imate a 3rd harmonic attenuation of 25-35dB (depending on the load impedanc=
e) while a 7th order LPF easily reaches 60dB.

73, Rik  ON7YD  - OR7T
________________________________________
Van: owner-rsgb_lf_group@blacksheep.org [owner-rsgb_lf_group@blacksheep.org=
] namens mal hamilton [g3kevmal@talktalk.net]
Verzonden: zondag 18 december 2011 12:10
Aan: rsgb_lf_group@blacksheep.org
Onderwerp: LF: Re: Re: Re: TX system at DK7FC, schematic

Tnx Jim
That explains it, by adjusting the number of secondary turns in series with
the main coill a match can be found.
73 Merry Xmas and HNY.
You have not been active for some time expect you are busy at work holding
the nation  together !!




----- Original Message -----
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Sunday, December 18, 2011 10:58 AM
Subject: LF: Re: Re: TX system at DK7FC, schematic


> Dear Mal, LF Group,
>
> > I can see you resonate your 3mH coild with a motor driven variometer bu=
t
> > how
> > do you match this to exactly 50 ohmz for a SWR of 1:1 to the TX
> > I can see your coil and transformer secondary are  in series to earth
but
> > no
> > adjustment for matching.
>
> Essentially the same arrangement is in use here. The transformer ratio is
> adjusted to match the antenna resistance to 50ohms.
>
> A Q of 1000 is typical for a coil of this size wound using Litz wire. You
> might increase that somewhat by optimising length, diameter, winding pitc=
h
> etc. For something big like the Balboa loading coil in Alex's mail, Q can
be
> considerably higher - Watt's "VLF Engineering" has data on this particula=
r
> antenna system - the coil resistance at 25kHz is about 0.06ohms, and the
> reactance 225ohms, making the Q about 3800 - it might be higher at 136k,
> since reactance often increases faster than loss resistance as the
frequency
> goes up. Incidentally, I estimate L of the main loading coil about 1.3mH,
so
> GW0EZY would need the variometer in series as well ;-)
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
>=