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After
all, you need quite a few V/m to activate Mal's crystal
set..........
Paul G8GJA
Anyone serious about LF/MF activities should
consider building this amplifier, those dabbling on these frequencies with QRP
are making it very difficult for the Receive operator, causing both EAR
and VISUAL discomfort, and anxiety when they receive nothing.
G3KEV
----- Original Message -----
Sent: Monday, January 03, 2011 5:07
PM
Subject: VLF: DK7FC's "big" VLF
PA
Dear group,
Since a long time i wanted to publish the
circuit of my "big" VLF PA. It is titled as a 600W PA since it has never
handled more power so far, due to the power limit of my /p generator. In
fact i think it will handle at least 1 kW, probably even 2 kW. As announced
i will try a 2 kW generator in my next VLF experiment and i am not afraid to
use this PA for the first test.
The circuit is nothing new or very
special. The mains voltage is rectified and stabilized by a 2.2 mF
capacitor. A H bridge using 4x IRFP 460 FETs is switching at the
desired frequency. To achieve an accurate 50% duty cicle, a CMOS4013
is used in front of the MOSFET driver. It is switched as a frequency divider
(/2), thus the input frequency has to be 2x the actually wanted frequency
(17.94 kHz > 8.97 kHz). The driver(s) are 2x IR2110. The operating
frequency is about 0.1...20 kHz (320 VDC) or 0.1...150 kHz (100 VDC).
It is designed to work properly below 9 kHz. The input waveform has to be a
12Vp rectangular waveform, ideally. The waveform is not very critical, e.g.
a sinusoidal or triangular waveform is no problem as well.
A small
transformer is used to generate a local 15V supply for the drivers, ICs and
LEDs.
There are several methods how the input signal can be applied.
Either directly (please use an audio transformer to decouple the PC from the
circuits ground which has mains potential!), as shown in the circuit or via
a fiber optic cable, as i do it. The fiber optic solution is a special and
(for some) strange method so i prefer to show the usual way to do
it...
If the mains is used to supply the PA (much lower output
impedance), a resistor has to be used to charge the 2.2 mF capacitor. A 100
W/230V lamp might be used. If the capacitor is charged, this resistor should
be bypassed by a relay.
One has to take care about the
output: There is no isolating transformer used so the output has main
potential! Thus, a galvanic isolated coupling winding has to be used for
applying the RF to the loading coil. No ferrite output transformer is
needed!
This PA is still using the first pair of FETs, there was
no fault so far. It was received in several countries that it is no
problem to transmit a one hour carrier at P > 500 W. Here, the heat sink
gets not even hand warm! So the efficiency can be assumed to be almost
100%.
The design can handle pretty much reactive power. It was no problem
to drive an antenna that is totally out of resonancy (kite lying on the
ground or wire suddenly disconnected due to strong pull).
The actual
(active) output power can be regulated continuously by adjusting the
resonance.
So, if one of the new VLF TX stations want to change from
an audio PA to a high power switching mode PA, this well proven design may
give some ideas. Just ask if something is not understood.
See once
again one of my very amateur like ugly hand drawings ;-) at http://www.iup.uni-heidelberg.de/schaefer_vlf/pic/DK7FC_600W_VLF_PA.pdf
Vy
73, Stefan/DK7FC
PS: Once again i explicit want to say that one has
to take care about the parts that are on mains potential! If you are not
sure about the handling, add an isolating transformer at the 230V AC
input!
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