By Harry Lythall SM0VPO

A few years ago I built one of those projects that never worked but it did do something unusual. The PSU side of a portable receiver was very unusual as it GENERATED RF in the HF spectrum. The fault was traced to a couple of errors on the PCB. After a little investigation I used the error to build myself an HF CW (QRP) transmitter, based upon an LM317T voltage regulator chip!!!

The circuit needs very little explanation. It operates from a 10 volt supply and regulates to about 4.5 volts. This I used to excite the tuned circuit VC1/L1. The tapping is about 3% of the total number of turns of L1. A 6% L1 tap I coupled to the control input if the LM317T regulator chip and BINGO! It "hoots". You can draw WATTS of power from a 12% tapping of L1.

As a STARTING point for VC1/L1, L1 is the usual formula: the number of turns is the wavelength in meters and the maximum value of VC1 is the same in pf (multiply by 2 because it is a variable capacitor).

CoilWavelength (in meters)=number of turns
CapacitorWavelength (in meters) =Capacitance (pf) X2

I see no reason why you cannot use a regulator chip rated at a few amps to get even more power out. I used this circuit on the air, on 1.8 MHz, & actually had a few contacts. It will certainly work on the new 70KHz band, but I do not know how far you can push it into the HF spectrum. I have just presented notes from my lab-book, & now it is up to those who wish to experiment a little further.

Have fun, and if you have any major success, such as exceeding 10 watts, then drop me a line and tell me what you did.

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