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
resisance, 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, moving
axay from the impedance it is designed for will result in addidional losses.
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 matching may
be not needed.
Disadvantage of this filter is the far less attenuation of the harmonics
compared to a Pi ot T lowpass filter with the same number of inductors. I
estimate a 3rd harmonic attenuation of 25-35dB (depending on the load
impedance) while a 7th order LPF easily reaches 60dB.
73, Rik ON7YD - OR7T
________________________________________
Van: [email protected] [[email protected]]
namens mal hamilton [[email protected]]
Verzonden: zondag 18 december 2011 12:10
Aan: [email protected]
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" <[email protected]>
To: <[email protected]>
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 but
> > 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 pitch
> 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 particular
> 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
>
>
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