Here are a few pictures of a small spark transmitter I've just about finished. I still have a few small things left to do, but for the most part it is done. The condenser is a glass plate type which is under the helix, and the coil is a 2 in. MESCO. According to my calculations it should roughly tune 200 to 450 meters plus all other bands

Great work Chris, I am too young to remember the old "spark gap" days, but find them very interesting.

Did you make the spark coil? I built a similar unit many years ago (as a snot nosed kid) using a Model T spark coil with an untuned output. Earned the coveted WATVs award (Worked All TVs) and made myself a general PITA in the neighborhood. I was still getting blamed for interfering with radios and TVs after I moved out. Go figure!

I have an older friend here in town who remembers when the ore boats used to anchor in the harbor and screw up everybodies radios with their spark rigs. This was at the end of the spark era.

Not sure what you intend to do with the rig, but it sure is cool!

73

Dave

Thanks Dave. The spark coil is an original I found a few years ago. I had thought it was an engine coil until I looked up the info on the tag and found it was originally sold for radio.

I'm not sure what I'm going to do with it yet. I built it mainly for the coolness factor, plus that coil was laying around.

Nicely done Chris!

Tho for authenticity you might want to find a spark key with big contacts or silver solder a couple old mercury dimes to the contacts.

Photo courtesy of Henry Rogers - WA7YBS and Sharon Rogers - KK7EI, owners of Western Historic Radio Museum.

John

Chris:

Very nice!

Here is my homebrew Spark Transmitter and it's companion homebrew Receiver:





BR,

Terry

Thanks John. Yep its just a telegraph key, but the contacts seem to handle the current just fine. With the coil adjusted for a good spark, there is no spark visible at the key. I guess I should remove the shorting bar, though.

I made a spark key with big contacts similar to the one in the picture, but that will be for my next TX. I'm thinking a 2KW rotary gap.


Very nice Terry! What material is that under the helix and spark gap? I need to get to work on a receiver, but I think it will just be a simpler double slider coil. I haven't decided on a detector yet.

Chris:

It is marble.

BR,

Terry

I have a little electric buzzer here that I was playing with some time ago. Noticed any nearby AM radio produced a nice buzz when the thing was on. Thinking then, hey, add a tuned circuit and some caps and antenna, and I could maybe have a QRP spark transmitter?

A buzzer can indeed be used to make a transmitter. The circuit was invented by Nikola Tesla in the 1890s, though his actual machines were of higher power.

Select a coil and condenser to resonate at the desired frequency. Connect them in series, and connect the two free ends in shunt to the buzzer contacts. When the battery current is turned on, the electromagnet will energise, drawing the contacts open. The magnetic field will rapidly collapse, inducing a high-voltage pulse in the magnet coil, which will charge the condenser through the coil and the battery. The contacts spring closed, re-energising the electromagnet, and simultaneously discharging the condenser into the radio frequency coil—which are now connected as a parallel tuned circuit—and producing a damped wave train at the resonant frequency.

To maximise the power output, choose a relatively large capacity and relatively low inductance for the tuned circuit. The signal can be coupled out by a second coil in inductive relationship to the first.

Hey All,

I heard a story way back about ships in port using the cat's whisker buzzer for low power communications between the docks. Kind of neat if true.

I rebuilt a VERY old buzzer (100 y.o.?) and it has a nice very nigh pitched clean buzz on the broadcast band.

K

To maximise the power output, choose a relatively large capacity and relatively low inductance for the tuned circuit. The signal can be coupled out by a second coil in inductive relationship to the first.


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I think a c0-worker of mine was building a replica of this Tesla machine a few years back. I should look him up and see how it turned out.

Buzzers were used with old-time cat whisker crystal detectors to find the "sweet spot" on the crystal.... not to transmit a signal.

http://en.wikipedia.org/wiki/File:Cryst ... buzzer.svg

Think of it as a primitive signal generator!

Rich

Probably very true.

The idea of using the buzzer to transmit a signal over short distances while in port, so as to prevent drastic interference, seems sensible and quite likely. Making the required change in connections on a temporary basis would not have presented a problem. Ideal for testing and for practice or social sessions between operators while in port.

Trevor.

In fact, portable transmitters were made on the plan I described above:

http://books.google.com/books?id=KpZEAAAAIAAJ&pg=PA264&dq=%22signal+corps%22+%22power+buzzer%22&hl=en&ei=XtcwTomxCc2RgQeapPXoDA&sa=X&oi=book_result&ct=result&resnum=8&ved=0CEwQ6AEwBw#v=onepage&q&f=false

Tesla described this circuit, and the details of some of his oscillators utilising it, in a 1919 article intended for amateur experimenters:

http://rpmgt.org/ELECTRICAL_OSCILLATORS.htm

Tesla oscillators of this kind are still manufactured to-day:

http://www.sciencefirst.com/Tesla-Coil.html