10 watt amplifier for MF phasing exciter

Internal view, cover off - amplifier built inside old pc psu box

Bias circuit bottom left, antenna changeover circuit top right

In my last post, ( 2nd November ), I described the phasing exciter I made especially for data mode communications on the 630m 475KHz MF band. I have been using it for beacon transmissions and 2-way contacts. I tweeted news of one such contact using JT9 mode. All this has been achieved with an output power of just 100mW. For consistent results, however, particularly when propagation conditions are not favourable, ( e.g., long periods of fading ), more power would be beneficial. So I have built a single-ended class AB rf amplifier rated for 10W max output which is connected externally between the phasing exciter, low pass filter and antenna.
Once again I have used a metal box from an old pc power supply, complete with cooling fan, for the enclosure. A type 2SC3039 power transistor is fitted to a 5K/W heatsink directly in front of the fan. I have included a variable bias circuit, based on a 2SC2958 transistor; actually it's the same transistor type as the driver in the exciter as I had spares. I have used this to set the current for my chosen no signal operating point for the 2SC3039 for class AB operation.
The amplifier has made a huge improvement to the reception range of my WSPR signal; so far up to 1632kms.

Phasing exciter for the MF 630m band

DDS interface dongle left, antenna c/o top, main board below

antenna change-over circuit removed to reveal main circuit board

So that I can join in the fun of using weak signal data modes such as JT9, JT65, FT8 and WSPR on the 475KHz 630m band I have built a phasing exciter for a low power single-sideband transmitter. Audio input signal is generated by pc data mode software and sound card. The rf signal source is a cheap AD9851 DDS module bought at an online auction site. Its frequency of operation is changed using the usb dongle and interface software obtained from www.spectecs.com. The rf phase shifter is a dual J-K flip-flop chip, af phase shifter comprises two quad opamps, and the mixer is a dual 1to4 mux/demux fet bus switch.
To give the project a smart appearance I chose a Hammond type 1455N1201BK box for it. Even before the prototype was finished, I couldn't resist the temptation of connecting it to my inverted 'L' antenna and trying it out. The exciter on its own produces only 1mW ( 1 milli watt, 0dBm ); but this was sufficient for my WSPR beacon signal on 474.2KHz to be received with SNR -23dB at a distance of 11kms by a monitoring station.
The prototype was completed with the addition of a class A rf driver amplifier to increase the output power to 100mW, and an automatic rf sensed antenna change-over switching circuit. Since then my signal has been received in Norway, distance 1071kms, Estonia, distance 816kms and Germany, distance 700kms, which surprised me as the antenna's directionality favours the east. Sadly there don't seem to be any foreign receiving stations in the east at the moment.

An UN-UN transformer for my end-fed wire antenna

Prior to installation; coax cable ( left ), insulated wire ( right )

Construction detail - eye bolt is for installation purposes

Raised into position above house roof ridge

Method of installation and connection

For many years, my general purpose antenna for short wave has been a horizontal 46m long end-fed wire; see post dated 12 August 2011. However it had always been difficult to match it's feed-point impedance to my transceiver's 50 Ohm antenna connector on some bands using my station antenna tuning unit, ( ATU ); the impedance being too high. For those particular bands I would sometimes temporarily change the length of the wire; but that wasn't very convenient. So instead I have now made an UNbalanced to UNbalanced ( UN-UN ) impedance step-down transformer, placed it in a small plastic food storage box, connected it to the feed-point of the antenna and to the ATU with coax cable.
The transformer windings consist of 9 turns, trifilar wound, on a T200-2 iron powder toroid core, using insulated wire. The core is held in place with a nylon bolt and a small piece of acrylic. The slots shown in the picture are not required; it had been used for something else. The coax cable is secured inside the box with a 'P' clip. The windings were interconnected to produce a turns ratio secondary:primary of 3:1; hence impedance ratio 9:1. This had the desired effect of transforming the impedance to lower values within the tuning range of the ATU on all bands.
With this modification I converted my simple end-fed wire antenna from a multi-band antenna to an all-band antenna !

New mosfets for the dual band amplifier

Four IRFP360 mosfets mounted on heatsinks inside the dual band amplifier

In March I bought new mosfets for the dual band amplifier ( see post 6 December 2015), but only recently had the time to fit them. I always knew that the original IRF640 types were underrated when I started running the amplifier from a 54V power supply, and there were reliability issues with several of them failing with a loud bang. The new type I've now fitted is the IRFP360 which is a 400V mosfet. During the last couple of evenings I've been transmitting with the amplifier for long periods without any further mishaps occurring. I had to fit these mosfets with a different orientation from the IRF640 ( see previous amplifier images ) as the mounting hole is insulated ( no insulating collar required for the bolt ) and the drain connection was made to the centre pin, not the case. A mica insulator, however, was still necessary under each mosfet between it and the heatsink. The insulator required is slightly larger than the standard TO-220 size. To begin with I didn't have any suitable until I found that Farnell stock them, ( item code 520-214 ). The mosfet and data sheet can also be found at Farnell, ( item code 864-9359 ).