Monday, 10 February 2014

All together, accurately

One of the problems I have with all this construction, and in particular with the QRP side of things, is measurement.

I have for instance just built a little QRP power meter that should read about 100mW up to 2W, BUT, I have no way to calibrate it that I know is accurate! I tested it on my HF rig, at what I think was a couple of watts, but I only have the meter in the ATU to go by, and Im not sure thats accurate! So for even lower powers, making a meter is tricky as I dont know the power out of anything is accurate enough to calibrate it against. Swings and roundabouts here. I cant check the levels out of oscillators and small amps etc without some calibrated measuring kit, and I cant calibrate it without a known power signal source.

The 28MHz 3rd overtone oscillator I built however should be giving 7dBm (5mW), so its theoretically possible that I could use that as a known source, IF I can prove its output level. Im hoping I can measure it on the spectrum analyser. From that I should be able to build a QRPp power meter and calibrate it.

Ive learnt a fair bit about the pullability of crystal oscillators recently, in an attempt to find the best way of putting TennaLady onto the PSK and WSPR 10m frequencies. It looks like the best course of action will be to rubber a 14.060MHz crystal up a notch, and make the oscillator self doubling.

Not much I can do until the parts arrive now.

Ive also ordered a bag of snap-on ferrites to put onto the LED lighting circuits in the bathroom and kitchen to try and kill the DAB RFI. I dont think I have enough cable available to wrap around the cores, so will just have to hope I can fit enough on to kill the RFI as single turns.

Ive also bit the bullet and ordered a copy of 'Experimental Methods in RF Design', spending the very last of my hobby funds. Oh, well, another call-out payment due end of the month! Ive already, from a preview online, selected one of its designs to build! So long as I have a 1mA panel meter amongst my junk, I should have a 50mW RF power meter ready in a day or two, that should read down to at least the 5mW expected from the oscillator mentioned above.

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