Loop modulation

[kb3ouk]

You don't need the battery. The resistance of the carbon mic provides a constant absorptive load on the oscillator, which is the carrier's output level (the oscillator is running at full power into the mic, the mic absorbs part of the power). The changing resistance causes the power output to vary, which makes the modulation. A very crude way of making AM, but rather effective.

[VE3LYX]

"I hate to show my ignorance but just what is the purpose of the battery in previous examples of loop modulation?"
I am sorry I don't understand. I don't use a battery. Where did that come from?" I couldn't find it.
You assumption is correct. The mic functions as a variable resistor (absorbing a portion of the RF into the loop  that would normally have gone to the antenna or at least the next stage.)
Re the TPTG comment above,  No, you didn't try it on your TPTG  because if you did you would already know it works.
It is not an iffy thing.
Don

[VE3LYX]

I wouldn't have choosen crude , interesting perhaps or unusual but your assessment is correct.
It also works on other stages besides the oscillator. I have it on the buffer stage of a crystal controlled solid state QRP rig for 80M , on the PA stage of a two tube mopa, and on both my ARC5 transmitters. Plus my TNT which I have been using on air the last week or so. The TNT was probably a bad choice as they are not the most stable rig however I spent a day reworking its vari cap positioning and installing a vernier drive to get the capacitor free of hand capacity effects and make tuning easier so I'm not swishing around. Seems like a new radio now. Did an recorded on air test today. 
Don.  

[PA0NVD]

Hi everybody.
Interesting subject, and indeed the first AM modulation method, using carbon mics. But it started putting the mic in the ground of the antenna system, not in the coil of the transmitter. That came a little later. The first AM transmitter was a spark transmitter with a very high quenching resulting in a hundred thousand sparks a second, giving CW. The best there was was the Moretti spark gap, a spark gap with water jet quenching. This amazing gap could produce 1 kW RF with an efficiency of approx 65% max!! A figure not too bad many transistor amplifiers. All CW transmitters, Spark, arc, small machine transmitters etc. were modulated with a carbon mic in the antenna system. The antenna current however was half to several amps, so the carbon mics ran hot. There were carbon mics with oil cooling and systems with very many microphones in series and parallel. But Marzi made one of the best one's, a vessel with carbon powder constantly flowing out of a small hole wich was partly blocked by a tonge connected to the membrane of a microphone, but more often to a relay system driven by a normal carbon microphone, a mechanical amplifier.
In the years 1918, the system of connecting the carbon mic to the coil of the transmitter was used in the Netherlands by the first broadcast transmitter in Holland. Indeed was the modulation mixed AM and FM.
Please look at the attaceched photos, a diagram of a 1912 transmitter with AM modulation, a Lorenz microphone and the Marzi microphone.
I have more info about this old technology, so when you like to see e few pic's, mail me. All my books are already stored because I like to move out of Spain, but I have nice pic's in my computer.

[VE3LYX]

It really isnt a lot of FM. That is a kinda unfair assumption. Certainly little if any more then would be found in normal 1940s transmitters but I am tired of arguing about it , suffice to say I have used it and even recently. Some have talked with me probably not knowing that is what I am using. The secret to the deal is to make the mic in a fixed position and speak at it through a tube rather then holding it your hand. The Fm, if it occurs, doesnt come from the method but from hand capacity problems associated with most single tube Power oscillators. Once you mount the mic solidly and dont touch anything it become very stable You are NOT modulating the oscillator. You are absorbing output that otherwise would have gone to the antenna. The RF is already established. It either goes up the wire to the antenna or into the carbon mic. Anyway I have several and have had some decent longer distance QSOs with them. It has been a very interesting journey and often , almost always misunderstood. Actually doing it is a real education. It as also been used in the microwave industry BTW. Thanks for the post and the links. The modulation loop or absorption loop is typically one turn or less. Only enough to absorb . Too much like any type of modulator is too much. It doesnt take much to work well. Same as you would use for an RF sniffer lamp loop for those who have ever made and used one. I have spent a lot of time experimenting with this method. Very interesting to me anyway. 
donVe3LYX