Wednesday, 13 January 2016

A Dual Band Ten Watt CW/AM Transmitter.

There is no greater feeling of accomplishment than to get on the air using your first home brew rig. But emergences of complex technologies like SSB, DSP and DDS etc. combined with limited resourcefulness of an average newbie proved to be the biggest hindrance in the way of home brewing. As I already discussed that involvement of these technological advancements, though good and beneficial in many ways but they have significantly aggravated the situation and thus the home brewing part of this hobby which was once considered joyous and was the backbone of amateur radio, has started to diminish. It is not uncommon to see that an average newbie starts his dream project exuberantly and very enthusiastically, spending a good wad of money and considerable amount of time but only to end up having that last hard to get component unavailable. Consequently, if we opt for exclusion of advance technologies in regular home brewing, we can still develop simple and effective designs. Simply put, if we choose to leave out SSB for example, for its complex phasing methods or all those expensive filters, we can still evolve simple to go transmitters similar with designs of the yore, using CW and AM.

Here is a simple CW/AM transmitter capable of putting 10 watts of output on both 80 and 40 meter bands. The circuit is simple and quite straight farward and needs little explanation. I used a VXO based on digital gates to drive Q1. You can copy this from my "Nano Transceiver" project described in my last post. Such a VXO provides frequency agility with lesser cost and components. It will put you on right segment of band without any initial alignment and will offer good stability. If you wish to use your existing VFO you can include Q6 as shown. D1 is included to retain proper base bias in low battery condition. L1,2,3,4,7 and L8 are wound on pig nose balun cores.

I used an IRF510 for its low gate capacitance but almost any MOSFET used in power invertors or computer UPS will work on these frequencies. In case of non availability of 2N4427, 2N3866 or BD 139 can be substituted or you can even use two BC547 in parallel. VC1 provides fine tuning of the output matching network.

The final adjustment is simple. Just connect a multimeter in series with the drain of final MOSFET and adjust gate bias of it with VR1,  just to keep the mosfet on the edge of conduction, and that's all. You are ready to go.....! Feed in your VFO/VXO , adjust VC1 for maximum output power and fire your transmissions...

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