Visual Tuning Indicators for Part 15 AM Transmitters,.

I've seen schematics for 'build your own' AM transmitters in science books that had a 'grain of wheat' bulb in the antenna circuit as a tuning indicator. I guess the wheat bulb, having a filament and being low impedance, would have to be in series, and before, the loading coil. It would also get brighter in time to the modulation, and be an indicator for how much you were pushing the sound.

The problem I see is it takes power to light the bulb, so that's lost signal, unless you need to lose it if the circuit has excess power to begin with.

With a neon bulb, that could work at the antenna end where the voltage is high, I think it would show something when it conducts, but it could be a problem, because at the moment of conduction, the impedance drops because current has started flowing. That would upset the antenna system tuning.

A neon bulb that's not conducting has a very high resistance. I read an article or report on it, you might think the resistance of it is infinite, non-conducting, but it's not, it's 100, maybe 1000 megohms, very high.

I like the idea of putting a neon bulb across a transmitter's output at the low impedance end in a transistor unit, to try and discharge static electricity from the antenna. If the voltage would build up harmfully, the neon would conduct and short it to ground. Zener diodes can also be used too, and sometimes are in these circuits. Transmitters like the AMT and Procaster have gas discharge tubes in them to do conduct dangerous voltages to ground.

One interesting circuit I saw in a science experimenter's book one time was a 'Lightning Detector', which was a neon bulb and resistor in series, connected to a longwire antenna and ground. The idea is that storms would build up static charges in the air, making the neon buld flash. An almost continuous glow would mean that the air was charged up and the storm close by!

I imagine a nearby strike would damage the bulb.

Boomer

I used to do lots of playing around with neon bulbs, and some of the earliest electronics experimentation in junior high was with building neon bulb relaxation oscillators, using a resistor, capacitor, diode and NE-2 bulb. Those were fun to make a light flasher from parts costing around 50 cents at Radio Shack prices. I even made one green neon bulbs, I think they were regular neon with a phosphor coating on the inside, they were white and you couldn't see the electrodes inside of the bulb.

The key to the relaxation oscillator was the character of the neon bulb itself. Voltage has to raise up high enough to make it conduct, then it suddenly does, producing a flash of light. In conduction, the voltage across the neon is fixed pretty much, and as you lower the voltage the brightness drops some, but not much and it extinguishes pretty suddenly. Neon might be low current, but I don't feel it's good for an RF field strength indicator.

A better field strength indicator would be one that uses a LED, with a transistor to amplify the RF, and show the result as LED brightness. It's better to have something that's away from the antenna a bit, so the amplifier is good. You build it in a little box with batteries and a switch, and maybe 5 components, and a pickup wire.

The idea is sensitivity, RF will make the transistor conduct, and the battery will power the LED instead of the RF signal itself lighting anything.

Boomer

That's a dune buggy.