Re Tank coil components in LF Transmitters:
Use the equation V = 4.44 . F.N.A.B to determine the number of turns needed
on a ferrite or iron dust core. The T200 core Mal mentioned, if it is the
normal Amidon iron dust type has a torroidal diameter of 51mm so the core cross
section will be around about 12 * 15mm, so a core cross sectional area of
180mm^2 or 180E-6 m^2
For a 400W PA module operating from a 50V supply in full bridge, RMS voltage
into the primary load will need to be 50V pk * 4/pi / SQR(2) = 45V. With a
tank Q of 6 the voltage across the tank is only 270V, so lets take a value of
300V for safety.
For an iron dust core, Bmax can be taken to around 0.4 Tesla to keep below
saturation (note, only 0.15 to 0.2 T can be allowed for ferrites) so :
Minimum turns N = 300V / (4.44 * 137000 * 180E-6 * 0.3) = 9 turns.
Since I recall 43 turns were quoted in the earlier Email, it looks as if that
is more than enough.
A T200 core does intuitively seem a bit small, but the theory says it should
work, so provided the 40 odd turns of wire needed to get the correct inductance
can be wound on in practice, and this wire can be thick enough to give minimal
copper loss - probably the difficult bit - it ought to do the job. I still
much prefer an air wound coil in this position though - it feels better.
In my 700 W switch mode Tx running direct from mains supply, the voltage on the
tank is a lot higher with a voltage supplied to the primary load of 150V RMS,
so a ferrite core would be even more dodgy than the one in the Decca units,
and an air wound coil (surrounded by aluminium) wound with 2.5mm Litz wire
does an excellent job.
The design for this 700W Tx is outlined on the G3YMC LF web site, and has been
submitted for publication in an Amateur Radio tecnical magazine, probably
around November time.
Andy G4JNT
-----Original Message-----
From: James Moritz [SMTP:j.r.moritz@herts.ac.uk]
Sent: 2002/06/11 10:40
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: DJ
Incidentally, an inductor that works OK for a low pass filter won't
necessarily be suitable in this type of tank circuit - in the low pass, the
loaded Q of the circuit is usually 1 or less, depending on design, while in
the tank circuit it is about 6. This means the tank circuit inductor has
to cope with reactive power several times greater than the low-pass
inductor for the same power throughput - so beware!
Cheers, Jim Moritz
73 de M0BMU
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