Fly swatter ham radio transmitter

[joeqsmith]

Simple spark gap transmitter tuned to the 80M ham band.  Pic 1 showing no secondary and antenna attached to the primary.   Pic 3 showing with secondary attached. 

A bit flipped than what is normally shown with the spark gap across the fly swatter rather than in series with it. 

[RoGeorge]

I've read that spark-gap transmitters were eventually banned, mostly because of their broad spurious spectrum, though until now never seen a plot of that, thanks!

[joeqsmith]

I did not see any spectral data as well which I also thought would be interesting to demonstrate.   Following video showing it operating after wound for the 160 Meter band.   

Not having any sort of radio license, I'm sure a few hams will be upset and turn me into the FCC for transmitting in their bands.   

[joeqsmith]

Using my old Drake TR-4 radio to listen to the fly swatter spark gap transmitter.   

[joeqsmith]

Using a signal generator and car ignition coil as a source to simulate the sound of a rotary spark system.  I use the both CW and AM modes of the Drake to compare the sound.   

[joeqsmith]

A viewer had asked to see what the signal looks like with a scope.   The voltage across the output capacitor isn't something I would want to probe directly, but here it is with the fly swatter attached and using a standard 10X probe with the ground strap attached to the tip for a pickup.

36 is a zoomed out view of a single breakdown of the sparkgap showing the decay of the tank.

37 is zoomed into the decaying area. 

[joeqsmith]

This is with it tuned to the high side (4.8MHz).   The goal wasn't to have a real high frequency but rather something I could pickup with my old Drake radio and something I could make from junk I had laying around.    That radio will work up to the 15 meter band so it's certainly possible to build something that would work at a higher frequency. 

[joeqsmith]

A higher frequency spark gap setup for the 15 - 10meter bands.    Tuned for roughly 28.5MHz.  As a sanity check, cursors are 4 cycles apart or 4X6.85 or 27.4MHz.   

Using a new set of AAAs in the fly swatter, the pulse period is roughly 1.37ms or about 730Hz.   I was able to increase this to 4.52kHz by running the fly swatter from the bench supply and increasing the voltage, until the transistor fried.   

No doubt it could be cycled much faster with a better supply.     

[joeqsmith]

This transmitter can be tuned down to about 15MHz or so.  Shown tuned for the 15 meter band.  Now running from my ESD gun's DC supply with a 10Meg source.  The ESD gun's supply with this combination gives up with a pulse frequency of about 9.43kHz.   

This comes from a viewer asking about rotary spark gaps.  Consider a motor driving say 8 spark gaps.  9.43kHz / 8 = 1178 rotations per second.  Or 71,000 RPM to achieve  what is shown using minimal effort and no mechanical switching.   

The rotary spark would certainly look a lot more impressive but loud sounds and lots of big sparks has not been a goal. 

[joeqsmith]

14.05MHz is the lower limit.   It's a pretty wide span for that trimmer cap.  Output is reasonably clean.   

[joeqsmith]

Phase noise is about as bad as the first transmitter.  Showing the 1-4 harmonics as well.   

[joeqsmith]

Rather than using the toroid,  added a winding.  No real attempt to optimize it.  Note use of the cheap 40dB attenuator to the SA (which has another 10dB attenuator).  Shown tuned for 22MHz.  Harmonics are shown out to 10.  The first harmonic is high and it could use an additional filter.   The spark occurs at a bit over 13kHz running from DC with the 10M source.   

[joeqsmith]

The following quotes were taken from this site on spark gap transmitters:
http://w2pa.net/HRH/spark-radio/

Data was taken using a 10x scope probe attached to one of my 50 ohm thru terminators without any attenuation.   Note that a MHV connector was added to supply the power.

The radio regulations at the time of spark’s heyday, specified in the 1912 law, dictated that no transmission must have a decrement larger than 0.2, which corresponds to a decrease of 18% per RF cycle.2 With 0.2 decrement, each pulse or wave train lasts for 24 cycles. Anything shorter (that is, any decrement larger than 0.2) would exhibit “undesirable tuning qualities,” and 0.2 decrement was defined as the boundary between broad and sharp tuning3—zero decrement (CW) being the sharpest possible.

LeCroy61:  Transmitter was tuned for 20.0MHz, or a period of 50ns.  While the signal is still decaying beyond to 10us captured with the scope, ignore that.   10us / 50ns or 200 cycles. 

High power (approximately 500 kW) versions used by commercial stations might typically use a disk with 20 to 30 electrodes contained within an enclosure to muffle the loud screeching noise it made when operating.

I didn't see any photos of what such a system would look like. 

LeCroy55:  The arc cycle time with the latest hardware is about 19kHz or 53us.  The output signal is pretty much dampened out by 10us. 

Using the 30 electrode commercial rotary gap claimed we can calculate the required RPM as 19,000 /30 pulses per rev, or 633 revs per sec.  X 60 = 38,000 RPM.   

Rather than 500kW I doubt I am making 500uW average.   

[joeqsmith]

Attempt to increase the arc cycle time, I went from DC back to AC.  The 10Meg source was removed.  Working with higher voltages, the trimmer cap was replaced with a part that has wider plate spacing.   I tried one of those Chinese HV transformers, hoping I could drive it faster but no luck.  The car ignition coil seems to work up to around 40kHz.  Notice that when running it this fast, there appears to be some secondary arc. 

Showing noise from 1 to 500MHz.  Not sure of the source at 44.5MHz.  Others are TV/radio....   

I was reminded about the craptastic USB bus as during these experiments, my mouse crapped out.  USB is fine for consumer use, but a poor choice if you work with any sort of noise sources.

[joeqsmith]

Another historical post off that site talking about the era when transitioning to CW.

http://w2pa.net/HRH/spark-to-cw/

Found the book he references from Elmer Bucher, 1917, Practical Wireless Telegraphy.  I've been thumbing through it.  Interesting to see some of the original early pre-tube era radios. 

https://ia600201.us.archive.org/33/items/practicalwire00buchrich/practicalwire00buchrich.pdf

[joeqsmith]

Output power is limited by the tuning capacitor. 

[joeqsmith]

Showing the latest transmitter.  The small tuning capacitor was replaced with a part that has wider spacing to handle the higher voltage levels.

[joeqsmith]

baseline_SA_terminated
Showing the baseline noise with my spectrum analyzer terminated to 50 ohms.   

backgroundnoise_12inch
Showing the background noise with a 12" rod attached to the SA.  At this point, nothing with the transmitter is turned on.  I suspect the 10MHz is the GPS clock driver board as it is not shielded.   

Transmit_NoAntenna
Powering up the transmitter with no antenna.  The coils do radiate but the SA is far enough away (5 feet or so) that it doesn't pick it up much of a signal.

Transmit_40mDiapole
Attaching the transmitter to a 40 meter diapole.  The diapole is 40 feet or so away from the SA's antenna.   Maybe my neighbor could pick it up if they had a shortwave radio.  Another perspective on the emissions from this testing, driving to work I pass several power line poles that radiate enough broadband noise, they knock out the local stations.     

It's interesting to see these two signals side-by-side.  It really puts into perspective why these transmitters were banned. 

[joeqsmith]

Hackaday ran an article on my experiments. 

https://hackaday.com/2023/12/11/modern-spark-gap-transmitter-uses-a-rotary-gap/

Comments are pretty typical for the hams.  This one actually has a clue: 

Twisty Plastic says:
December 12, 2023 at 7:45 am
...
OTOH I also think the FCC has failed (probably other country’s organizations too but I don’t know them). They have allowed cheap electronics, especially cheaply made lighting to emit all sorts of interference and be mass marketed anyway.

I have some high intensity LED bulbs that when combined with the house wiring for an antenna structure, splatter from kHz to 10's of MHz.   I ran some tests on them and posted the data here back when I bought them.    As long as the FCC allows importing this crap, I am not going to get too excited about the narrow band, lower power my spark gap experiments produce.   May be fun to see which one will radiate further.   Guessing my light bulbs.

[RoGeorge]

Hackaday ran an article on my experiments. 

Congrats! 

[joeqsmith]

New coil less the hot glue. 

[joeqsmith]

Coil with antenna and flyswatter.   Looking at the signal with the SA, it's fairly broadband compared with other stations as expected.    Also shown is my TECSUN PL-990 receiver.   It's not a bad general purpose radio.   Encoders have been flawless. 

Using this dead spot of the band, I can receive the signal almost on the other side of the house.

[joeqsmith]

ARRL article starts out talking about a car's ignition system.

https://www.arrl.org/files/file/History/History%20of%20QST%20Volume%201%20-%20Technology/Kennedy%20N4GG.pdf

If we grounded the car’s chassis, connected an antenna directly to the ignition coil’s HV output and added a telegraph key in series with the ignition coil primary, we would have a complete 1900 spark gap transmitter along with 21st century “comfort features” like a leather operating seat and a sun roof. What could we do with this lash-up? We could transmit Morse code to a range of about 10 miles, with the emission occupying a huge amount of spectrum, centered on a frequency determined by the L-C resonance of the antenna’s series inductance and parallel capacitance to ground.

With the modern resistive plugs and COP, I doubt it.  Even in the case where I used an ignition coil to drive mine with an antenna attached, I made it to the neighbors house.  A long way off from 10 miles.   My dipole is cut for the 40M band but I never measured it.   Consider I was transmitting closer to 25MHz, that certainly wasn't helping.   The FCC would have to be parked in front of my house to pick it up.       I wonder back in the early days how far the typical hams were actually able to communicate.  Consider the receiver I was using is a lot more sensitive than anything they had at the time.     

This last attempt will operate down to  6MHzish and should handle higher voltages once I replace that tiny tuning cap.     

[joeqsmith]

Shown with new tuning cap.  Running 5kV to the primary will cause it to breakdown but it easily handles 2kV. 

[joeqsmith]

Assembled and running with flyswatter.