Question about this spark gap transmitter - Page 1
Forum section: electronics / beginners

author: VooDust
author_rank: Regular Contributor
timestamp: September 20, 2021, 07:42:28 pm
content: See this video of a simple spark gap transmitter:Question: Is the LED in fact excited by the energy of the radio waves or, as some commenters stated, it's a phenomenon of either induction or capacitive coupling. I am talking about the FIRST experiment without the battery amplification.There seems to be a lot ofin the comments. I know it's not a "radio" per se, I'm just interested in the experiment, I don't want to tell a student that this is radio waves (of mixed frequency) when it isn't.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 20, 2021, 08:28:59 pm
content: For a short distance the antenna is still in the near field. So the description as capacitve coupling is about as good as an EM wave.

author: Manul
author_rank: Super Contributor
timestamp: September 20, 2021, 10:57:47 pm
content: In my opinion, to call that experiment a radio transmittion is a bit of a strech. It produces some broadband RF, thats true, that dipole likely resonates somewhere in the UHF band. But that LED is not lighting up because 300 Mhz radio wave just traveled through space and passing through it. Most LEDs will likely have trouble to light up from such high frequency AC anyway. It lights up because there is a huge voltage swing and capacitance between the "antennas".On the other hand it should not be very difficult to modify this device so it actually transmitts and receives EM waves. It should have a separate spark gap, both ends tuned (with reasonably high Q to prolong the transmittion pulse), and some proper rectification stage on the receiver side (fast diode). With more power or some amplification it could work up to meters and more.

author: VooDust
author_rank: Regular Contributor
timestamp: September 21, 2021, 06:27:27 am
content: Thank you. To verify capacitance is the energy source, we could apply a voltage from a lab power supply instead of creating a spark, to see if the LED still lights up, correct?I have another question: I thought current needs to flow in a circuit. Where's the circuit on the receiver side? (where the LED is simply bridging the dipole).

author: iMo
author_rank: Super Contributor
timestamp: September 21, 2021, 07:13:01 am
content: As Kleinstein wrote the coupling between the High Voltage source and the LED is mainly capacitive.Below the circuit - a schematics - you may see a 3 kiloVolt pulses generated by the transmitter (V1) and even with pretty small 0.05 picoFarad capacitive coupling (the C1 and C2 capacitors representing the copper wires) the pulsing current through the diode is 160 microAmperes peak to peak which is enough to lit the LED.PS: mind the operating a sparkgap transmitter is not allowed in most countries.

author: Manul
author_rank: Super Contributor
timestamp: September 21, 2021, 09:04:42 am
content: Quote from: VooDust on September 21, 2021, 06:27:27 amTo transffer energy through very small capacitance we need really fast change of voltage (google capacitor impedance). So connecting lab power supply will not work.To verify that it is capacitance effect (basically electric field effect) we could attach a high bandwidth scope on the LED and analyze the pulse content.Or we could take a dipole which is made from flat strips, not round wire. Say 1mm thick and 5mm wide. Then we can face strips by their edges or flat sides. The flat sides facing should increase distance quite noticeably. This would not happen if it would be purely EM wave transmittion.We could also try to use loop antenna on the receiving side. It will mostly not react to electric field, just magnetic. See if that can receive the pulse and light a LED.

author: AnalogueLove1867
author_rank: Regular Contributor
timestamp: July 05, 2022, 03:55:42 am
content: I can guarantee you now that this isn't capacitive or magnetic coupling.The near-field electrostatic and magnetic effects are no where near strong enough in that setup.A guy before Hertz used a similar setup ( dimensions and distance-wise ) to transmit radio signals. It was written off by others as a case of induction.It is only now ( With a recreation of his experiments ) that we know he was yet another early radio pioneer.Just look at the LED light up in 4:34 of the video. That is well and truly due to radio transmission.You can detect the radio waves emitted from turning on an electric light when you are outside the house, using nothing more than a Darlington triplet ( or quadruplet )connected to a three volt power supply, a small loop of wire and an LED.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 05, 2022, 04:14:52 am
content: If you want to watch a good video about (among other things) spark gap transmitters, then go watch the Radio episode of "Secret life of Machines".

author: AnalogueLove1867
author_rank: Regular Contributor
timestamp: July 05, 2022, 04:31:51 am
content: Quote from: imo on September 21, 2021, 07:13:01 amThat is because a simple spark gap transmitter is extremely broadband RF noise. For the earliest spark systemsyou would always have a whole spectrum from the 100's of meter to less than a millimeter.For some reason people are now claiming that jagadis chandra bose was the first person to create mm waves and use semiconductor rectifiers as recievers.That is all absolute bullocks. He literally just copied everything from others after reading their books when he arrived in Britain (even admitting this in his diary).He never added anything new what so ever.One thing is just copying. Another thing is copying and also adding nothing new.

author: AnalogueLove1867
author_rank: Regular Contributor
timestamp: July 05, 2022, 04:34:32 am
content: Quote from: Doctorandus_P on July 05, 2022, 04:14:52 amHadn't heard of that doco. It is an old one too. Love old docos

author: Kleinstein
author_rank: Super Contributor
timestamp: July 05, 2022, 09:38:40 am
content: A dipol antenna is somewhat directional. If really just the wave, it should not work with antennas at a right angle. With some overlap capacitive coupling can still work.An other point is the length of the antennas: the wave action would be more sensitive to the lenght / resonance frequency of the 2.