Iron Fist transmitter

This is a simple rig that uses a 2N2222A oscillator stage and an RD06HHF1 power amplifier. Note that depending upon construction you may find that you need to insert a resistor of a value of about 10 - 200 Ohms in series with the input gate of the RD06HHF1. The input gate is quite sensitive, and any leakage or other RF "flying around" can result in parasitic oscillations occurring. The series resistor is highly recommended. Some notes on gate bias arrangements. You can ruin a project by having a poorly designed gate bias arrangement. It is essential that RF that is supposed to be fed into the gate does NOT travel up the feed of the gate bias line. My technique is to use something like a 2mH choke (good down to about 1.5Mhz) in series with a 10K Ohm resistor. Fit these physically right up close to the input gate. The choke should have a highly inductive reactance at the lowest operating frequency, aim for 500 - 1000 Ohms. The resistor increases that RF stopping effect, as well lowering the current drawn by the gate should there be a fault condition.

The 4:1 un-un transformer is an FT50-43 with at least 11 turns wound on it. The output impedance of the RD06 is about 12 Ohms, and the 4:1 un-un is used to bring the output impedance up to 50 Ohms before feeding into any low pass filters. Some useful data about making Baluns and un-un's can be found here.

Here's one version of my Iron Fist TX. It's a bit unusual because it is built into a box that has no knobs and can only be tuned with a trimmer tool. The idea was to eliminate any knobs that could get bashed bout in my rucksack when travelling. I also included a variable voltage regulator to adjust oscillator voltage and included two switched quartz crystals for some frequency agility. Also included was a microswitch Morse key. Also included is a simple ATU and LED-O-METER (a simple RF tuning indicator). For use with an external receiver, a simple antenna manual antenna changeover was provided if required. You could also use the external receiver as the CW sidetone. The antenna output is through the red and black speaker connector, since this rig is intended as simple in the field QRP rig and a wire antenna thrown up the nearest tree or pole. The RD06 is mounted on the rear of the Veroboard / stripboard and has a small piece of aluminium attached to it for heatsinking which is necessary.

RF Chokes. Please don;t use cheap and crappy RFC's in RF circuits. The ferrite very easily saturates and cause the reactance to change badly. When used in things like oscillators and PA's you will notice things like bad drift or power drift. In most cases, use a small toroidal RFC.

Here's my later appraisal of the design. The interstage coupling between the oscillator and power amp shows a 470nF coupling capacitor. At 10-14Mhz this really ought to be more like 47pf, or maybe 100pf with some series resistance, possibly variable. I found that depending upon antenna loading, the oscillator would fail to oscillate otherwise. Some experimentation on values should get you to a point where oscillation remains reliable as well as optimising power output. Less coupling makes oscillation more reliable. An alternative could also be a 3db pad, as this would maintain a constant impedance to the oscillator.

The tiny ATU is really only suitable for a few watts, and really needs a bigger core otherwise they soon heat up. Stacking two cores of the same type held together with some PVA woodglue would be a good idea. (That's the one that you can see between the two variable capacitors that form a simple T match ATU.

The PA can run up to 30 volts and provide as much as 20w, but I recommend no more that 10w to be safe. A combination of supply volts to the PA and the oscillator can be used to optimise the system. 6 watts is bullet proof. That's why it's called and RD06. 6 watts refers to the maximum level that the device will work under any load condition, assuming that you have bothered to have a half decent heatsink of course. It's quite adequate to bolt it to the chassis of a rig. But no heatsink will destroy the device quickly ! You can tell if you have inadequate heatsinking by looking at the device tab. Does it look a bit brassy / gold colour ? If it does then it's probably let failed and you used it without a heatsink or a bad load.