Anyone serious about LF/MF activities= should=20 consider building this amplifier, those dabbling on these frequencies wi= th QRP=20 are making it very difficult for the Receive operator, causing both= EAR=20 and VISUAL discomfort, and anxiety when they receive nothing.

G3KEV

----- Original Message -----

From:=20 Stefan Sch=E4fer=20

To: rsgb= _lf_group@blacksheep.org=20

Sent: Monday, January 03, 2011= 5:07=20 PM

Subject: VLF: DK7FC's "big" VLF= =20 PA

Dear group,

Since a long time i wanted to publis= h the=20 circuit of my "big" VLF PA. It is titled as a 600W PA since it has nev= er=20 handled more power so far, due to the power limit of my /p generator.= In=20 fact i think it will handle at least 1 kW, probably even 2 kW. As anno= unced=20 i will try a 2 kW generator in my next VLF experiment and i am not afr= aid to=20 use this PA for the first test.

The circuit is nothing new or= very=20 special. The mains voltage is rectified and stabilized by a 2.2 mF=20 capacitor. A H bridge using 4x IRFP 460 FETs is switching at th= e=20 desired frequency. To achieve an accurate 50% duty cicle, a CMOS401= 3=20 is used in front of the MOSFET driver. It is switched as a frequency= divider=20 (/2), thus the input frequency has to be 2x the actually wanted freque= ncy=20 (17.94 kHz > 8.97 kHz). The driver(s) are 2x IR2110. The ope= rating=20 frequency is about 0.1...20 kHz (320 VDC) or 0.1...150 kHz (100= VDC).=20 It is designed to work properly below 9 kHz. The input waveform has to= be a=20 12Vp rectangular waveform, ideally. The waveform is not very critical,= e.g.=20 a sinusoidal or triangular waveform is no problem as well.
A small= =20 transformer is used to generate a local 15V supply for the drivers, IC= s and=20 LEDs.
There are several methods how the input signal can be applie= d.=20 Either directly (please use an audio transformer to decouple the PC fr= om the=20 circuits ground which has mains potential!), as shown in the circuit= or via=20 a fiber optic cable, as i do it. The fiber optic solution is a special= and=20 (for some) strange method so i prefer to show the usual way to do=20 it...

If the mains is used to supply the PA (much lower output= =20 impedance), a resistor has to be used to charge the 2.2 mF capacitor.= A 100=20 W/230V lamp might be used. If the capacitor is charged, this resistor= should=20 be bypassed by a relay.

One has to take care about the= =20 output: There is no isolating transformer used so the output has ma= in=20 potential! Thus, a galvanic isolated coupling winding has to be us= ed for=20 applying the RF to the loading coil. No ferrite output transformer is= =20 needed!

This PA is still using the first pair of FETs, there= was=20 no fault so far. It was received in several countries that it is= no=20 problem to transmit a one hour carrier at P > 500 W. Here, the heat= sink=20 gets not even hand warm! So the efficiency can be assumed to be almost= =20 100%.
The design can handle pretty much reactive power. It was no= problem=20 to drive an antenna that is totally out of resonancy (kite lying on th= e=20 ground or wire suddenly disconnected due to strong pull).

The= actual=20 (active) output power can be regulated continuously by adjusting the= =20 resonance.

So, if one of the new VLF TX stations want to chang= e from=20 an audio PA to a high power switching mode PA, this well proven design= may=20 give some ideas. Just ask if something is not understood.

See= once=20 again one of my very amateur like ugly hand drawings ;-) at http://www.iup.uni-heidelbe= rg.de/schaefer_vlf/pic/DK7FC_600W_VLF_PA.pdf

Vy=20 73, Stefan/DK7FC

PS: Once again i explicit want to say that one= has=20 to take care about the parts that are on mains potential! If you are= not=20 sure about the handling, add an isolating transformer at the 230V AC= =20 input!