10-11-2017, 11:07 PM
Another busy couple of days. I had been thinking about what sort of aerial would work best with the transmitter. I was wondering about using a frame aerial. I do have a large one I constructed (80cm square) which I use sometimes for reception. I visualised a much smaller example in the lid of my box.
While out last weekend I called into our local secondhand shop. They had a pack containing three embroidery hoops with silks for fiftty pence. I do embroidery at times (when I have time
) so thought I could use one for an aerial frame, and the others for their intended purpose.
I took things a bit as they come, and wound 9 turns of 24SWG enamelled copper wire on an 180mm diameter inner wooden ring. This was about all that would easily fit (see photograph). With a bit of work using my dip meter I determined what I thought was about right and put together a tuned circuit comprising this ring, a 47uH inductor in series and 440pF plus what I think is an 100pF trimmer in parallel with the combined inductance. Later work leads me to believe that the original coil was about 50uH (checked from a coil calculator Coil32 – probably around 40uH). This gives around 100uH in parallel with around 550pF – around the right frequency) .
I coupled the constructed assembly to the anode output loosly via a 6.8pF capacitor. This effectively gives two tuned circuits loosly coupled.
My initial tests have shown that tuning is fairly sharp and output appears to be somewhat higher than with my short wire previously used. I still have quite a bit of work to do in checking and optomising (maybe even changing things), but it seems more logical to me to have a specific load (and indeed the intended one) for checks. As the transmitter is intended to be used on one frequency only I am hoping only minor adjustments will be required in use. The tuning is faily centralised on the trimmers and initial tests seem to show things are faily stable. I have got quite a bit to learn here so do not intend to pull things about too much unless I understand why I should.
Next checks – harmonic output and then try other than sinewave input. For theory, as I am not hot on aerial therory I will look at the coupled double tuned tuned circuits theory as well for issues with coupling. One thing I am learning is that when testing it is tricky not to detune things with probes etc.
Not much on the rod tubes this time, but I will descibe more fully my modules at some point.
While out last weekend I called into our local secondhand shop. They had a pack containing three embroidery hoops with silks for fiftty pence. I do embroidery at times (when I have time
) so thought I could use one for an aerial frame, and the others for their intended purpose.I took things a bit as they come, and wound 9 turns of 24SWG enamelled copper wire on an 180mm diameter inner wooden ring. This was about all that would easily fit (see photograph). With a bit of work using my dip meter I determined what I thought was about right and put together a tuned circuit comprising this ring, a 47uH inductor in series and 440pF plus what I think is an 100pF trimmer in parallel with the combined inductance. Later work leads me to believe that the original coil was about 50uH (checked from a coil calculator Coil32 – probably around 40uH). This gives around 100uH in parallel with around 550pF – around the right frequency) .
I coupled the constructed assembly to the anode output loosly via a 6.8pF capacitor. This effectively gives two tuned circuits loosly coupled.
My initial tests have shown that tuning is fairly sharp and output appears to be somewhat higher than with my short wire previously used. I still have quite a bit of work to do in checking and optomising (maybe even changing things), but it seems more logical to me to have a specific load (and indeed the intended one) for checks. As the transmitter is intended to be used on one frequency only I am hoping only minor adjustments will be required in use. The tuning is faily centralised on the trimmers and initial tests seem to show things are faily stable. I have got quite a bit to learn here so do not intend to pull things about too much unless I understand why I should.
Next checks – harmonic output and then try other than sinewave input. For theory, as I am not hot on aerial therory I will look at the coupled double tuned tuned circuits theory as well for issues with coupling. One thing I am learning is that when testing it is tricky not to detune things with probes etc.
Not much on the rod tubes this time, but I will descibe more fully my modules at some point.
