|
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!
|
|
