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 Post subject: New Project: Audio/Visual Part 15 Antenna Tuning Indicator
PostPosted: Aug Fri 20, 2021 11:40 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Texas
Here's a YouTube video of my new project. https://youtu.be/6GBt_cXDGgg

In developing a new Part 15 transmitter, I get tired of hauling out my oscilloscope just to optimize tuning. I started first with an LED that would light up at resonance. I found that bright was bright and it was hard to distinguish exactly when the LED showed perfect peak resonance. Knowing that the human ear is more sensitive to minute changes in pitch than the eye is to bright light, I connected a voltage controlled audio oscillator to the LED.

In the YouTube video, with the five foot wire antenna tuned to resonance, the oscilloscope is monitoring the 1680 KHz wave form. In that version of the transmitter, the tuning is adjusted by sliding a ferrite core in and out of a coil taped to the counter in front of the transmitter. This coil is a do it yourself loopstick made of 16 feet of #36 magnet wire scramble wound on a syringe. It's a dirt cheap version that performs better than the legendary Miller 6300 loopstick, probably because the wire is so much thicker.

In front of the scope is my new antenna tuning monitor that offers both visual and audible indication of resonance. On that board, visible under the scope's black Vertical Position knob is a red LED that starts to light up as the antenna approaches resonance. It's brightest at resonance.

As you can see and hear, the audible monitor provides greater tuning sensitivity. With tiny changes in position of the ferrite core, we hear very distinct differences in tone while the LED doesn't seem to change at all.

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The circuit is very simple, consisting of a transistor wired as an emitter follower that amplifies the RF and feeds it into the voltage control pin of a 555 timer wired as a Voltage Controlled Audio Oscillator. It's capacitively connected to the antenna by wrapping a few turns of wire around the antenna. A "gimmick" cap works as well.

The 555 was not primarily designed to be a Voltage Controlled Oscillator, and one quirk is that the pitch decreases as the control voltage increases. Although this is opposite the function of typical voltage controlled oscillators, it makes no difference in the functionality of this circuit. There are dozens of ways to modify this circuit to turn it around, but as you will see, the goal of perfect antenna tuning is met with this simple circuit as it is.

Here's an animated gif of the LED in action. Note that the LED lights fully before and after peak resonance. If you want to keep things even simpler and forego the audio, you can just use the five visual indicator components (transistor, LED and three resistors.) That's ok, but the audio really nails it. Below that is the PCB layout.


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 Post subject: Re: New Project: Audio/Visual Part 15 Antenna Tuning Indicat
PostPosted: Aug Sat 21, 2021 2:48 pm 
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Location: Monroe GA 30656 (from Coventry, UK)
Nice work Tom


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 Post subject: Re: New Project: Audio/Visual Part 15 Antenna Tuning Indicat
PostPosted: Aug Sat 21, 2021 8:28 pm 
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Location: Wayside, NJ Monmouth
I agree nice work. Are any of the P.C.B available ?


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 Post subject: Re: New Project: Audio/Visual Part 15 Antenna Tuning Indicat
PostPosted: Aug Sat 21, 2021 9:14 pm 
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Location: Texas
N2LXM wrote:
Are any of the P.C.B available ?


I made the prototype board using the laser print iron transfer method and swimming pool acid (HCL) with hydrogen peroxide for etching. But even that simple method could be more trouble than it needs to be, as the circuit layout is simple enough to be drawn directly onto a blank PCB with a sharpie. The only critical spacing is for the 8-pin DIP.

I'm certainly open to buying the boards and giving them away if there's enough interest. Two of those schematics will fit on a mini board from ExpressPCB. Miniboard is their special discount board, and its dimensions must be 3.8 in by 2.5 in. It's bare bones and that means there is no silkscreen available with the mini board. I believe three boards cost $51 which would mean six boards for $51 if you have two layouts on each mini board

I think the circuit can be improved with no additional parts. I was thinking how loud that little piezo speaker is, and it can be quieted by lowering the frequency by changing a resistor or capacitor or both, or possibly putting a resistor in series with it.

It's so raucous that I found myself occasionally wanting to turn the speaker off and just watch the LED, and I think that putting a second LED next to the original that's there but with a much higher resistor, you might get more fine-tuning with the LED if you have two with different brighrnesses. It would be good to get this thing into a final form before making boards to share.


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