Original Part 15 PHILCO Record Player with Transmitter in action!

They had this then!

Wonder why this didn't take off....quite some engineering in 1939

Mark

That looks like a top of the line model, too bad he didn't lift the hood for a peek inside.

My interest in Part15 broadcasting began when someone gave me an old unit similar to this but it was not in a nice wood cabinet.  It was in a stamped bread pan chassis but it worked.  It had a single tube and the filament was wired in series with the motor to drop the voltage to the right level.  It did work and I visited a neighbor and heard it clearly on their AM set.  It was replaced with a KnightKit broadcaster and it was stowed somewhere and is now gone.

Neil

Maybe if the range is good one could copy the AM Transmitter section and make a few mods to it to allow you to conect it to a computer/whatever.  I still think it sounds better than the Talking House or iAM Transmitter and that is for sure.

Was the transmitter crystal controlled?  I remember at the School For The Blind a tume AM transmitter with a 6-10 foot copper pipe antenna did easily 2-3 miles.  We called our station WMSB and we got listeners.  We didn't even have a compressor/limiter at the time (we didn't even know what that was).  Someting about the tube transmitter caused us to get out.  What could that have been?  I don't remember anything about an ATU.  It was connect, let it warm up, plut it into the AUX out of a Stereo or Radio Shack mixer and with a few tape recorder mics we were a campus Radio station heard off campus 2-3 miles.

Only if I could figure out how to do it now.

... I remember at the School For The Blind a tume (tube?) AM transmitter with a 6-10 foot copper pipe antenna did easily 2-3 miles.  ...

Just to note that based on physics, an AM transmit system fully compliant with FCC Part 15 could not produce a groundwave field intensity generally useful to typical AM broadcast receivers at distances of 2-3 miles from the transmit antenna.

Below is a link to a thread with background information on this subject:

  http://www.radiodiscussions.com/showthread.php?664421-Part-15-AM-quot-Ground-Leads-quot

Now I haven't used a phono oscillator with tubes like a Knight or Lafayette, or one of the older ones, but looking at the circuit for one that used 2, 50C5 tubes, the power output might be more than 100 mw.

I think it was a tuneup for the Knight, with 42 volts across the final, and current was 10-20 milliamps. Now that I think about it, I might have all this wrong, it seems like the Knight is the one with the modulation transformer, not the series tube modulation, and the Knight is the one with the hefty power output. That's because the voltage isn't divided in half by the series modulation.

If a Knight pulled over 10 milliamps with 100 volts on the plate, that could be over a watt of input power. In our more primitive, earlier times with these units, it didn't seem to matter as much, since a short wire antenna inside someone's bedroom likely wouldn't have gotten out well anyway. You may or not have had more power, but putting the antenna outside is what probably made all the difference, whatever the power happened to be.

When they went to transistors later, like the Radio Shack versions, they took a drastic drop in power, just because of the technology at the time, low power germanium transistors, and 9 volt battery supply, you couldn't get much power output with that!

It may have been an Knight.  I remember plenty of those Knight Shortwave Radio's at the School For The Blind.  A lot of the students bought stuff from Popular Electronics magazine so that makes sense why it got out so far.

Being on the 3rd floor of what they called the Main Building also helped I'm sure with that copper pipe and the bed springs at the bottom of the antenna or some sort of spring as it was Thick that I remember.

It put out a nice clean signal when tuned up right and like I said we had listeners.  And now it comes to me we played Album Rock on AM.  We did so because we found it was easier to get a transmitter that got out on AM and at the time they were not as expensive as they seem to be now.

We were kids we didn't know how many watts these transmitters put out and still I am not for sure that is why I brought it up because I've yet to put out that good old signal I remembered back then.

I'd also like to have the same antenna design that they used back then too for I am sure that made the difference too.  I do remember it sounded very good however if you had a nice Radio.

These old phono oscillators did not need nor use a crystal.  Tuning was via a variable cap or inductor.

My KnightKit input power measured 183 mW but I didn't worry because it was certified by Allied Radio Corp. to conform with the FCC rules provided the antenna length was less than 10 feet.  Things were different in 1959, no one was uptight about a few tens of milliwatts over power.

Neil

Using NEC4.2

A 1650 kHz Part 15 AM system fully compliant with FCC §15.219 can produce a groundwave field intensity of 100 µV/m at a radius of 1 mile, for earth conductivity of 6 mS/m (about average for the US).

Transmitter output power would need to be increased by about 21.2X in order for that system to produce a 100 µV/m field at a distance of 3 miles, and its field at 1 mile would increase then to about 460 µV/m (other things equal).

Depending on r-f noise, interference from other signals at the receive sites, and the weak signal performance of the AM receiver, that 100 µV/m field probably would provide only marginally useful reception for most listeners.

Refresh my memory,

what was the allowed power for a campus station at the time? Was there a provision in the fcc rules at the time these oscillators were big that might have allowed for a campus to transmit with more power?

I am not that old despite what my kids say lol
In high school I was working radio in many forms, the vocational school i was attending had a licensed 175 watt Fm on a shared frequency, my high school had a station which was on a shared frequency and i was running an AM station on 1610 for a 1/4 mile using a Heathkit 35 in 1 Electronics Lab and 20 feet of wire out the window.

The ground came from the roof flashing just under my window on the second story house we were renting.

Good Times.

... is permitted under FCC §15.221, using only the AM broadcast band.  It doesn't limit the field strength/radiated power of the transmit system on the campus, but does limit the field at the perimeter of the campus, per this clip:

15.221 (a)(2) At the perimeter of the campus, the field strength of any emissions, including those within the frequency band 525-1705 kHz, shall not exceed the general radiated emission in §15.209.

Unless the campus property is rather large and the intended reception area is rather small and near the center of the campus, Part 15 AM stations can have better results when operating under §15.219 than §15.221.

Fry is pretty spot on with AM usefulness, my range is only so big becuase of the sheer lack of RF interference in the rural hills of Texas. Any lightening, TV, power line, etc can wipe my signal out once you get a mile or more from the Transmitter. Stay in the clear and my signal can be detected an absurdly long distance away. (The Wife's Carolla can pull the station in noisily about 6 miles out, detectable but totally unlistenable by any stretch at 10 miles or more)

From post #4 : "Someting about the tube transmitter caused us to get out.  What could that have been?"

The Knight Kit did very well on range with an honest 10 foot antenna but there was more going on than I was aware of at the time.  Back then, all the equipment and the house used two wire power circuits so nothing was grounded  except through the neutral at the power panel.  This meant the whole house wiring was probably acting as a "ground lead".

Another factor is that the tube circuits operate at high voltage (150 to 300 volts) and the wire antenna is coupled to the oscillator through a capacitor connected to the tube plate.  This means there was a high RF voltage (120 V or thereabouts) at the feed end of the wire antenna.  This is about what one gets at the radiator feed point using a solid state transmitter with a loading coil so from the point of view of the antenna the simple tube transmitter output was essentially the same as the modern base coil loaded solid state transmitter but without the coil losses.

Again we were Kids we didn’t know anything about part 15. All we know is we’re having fun and this transmitter WAS on a LARGE campus of ½ mile. The MAIN building (North Wing 3) was at the top of the building. I’m trying to remember if we used the steam radiator as ground probably so. The antenna was near the window standing on the floor with a wood block at the bottom to hold the antenna straight from falling.

Come to think if I’m quite sure it was either a Knight or a heath. The Lofyette transmitters were a issue at the time as a student had one and there was issues with it. The electronics/machine shop teacher tried to fix the Lafyette one but it was too costly. He liked Heathkits and Knight kits and told us if we bought the kits he would help put it together. I remember wanting one as well.

We all decided to save and do it as a large one transmitter project and take turns playing Radio DJ’s. Even our gym coach played with it at times too. The lady who did announcements during school listened to the station all day as we would use an 8 track tape recorder and record some of our shows and play them on the transmitter. WE had lunch menu’s announced and if a student had a wonderful report card their name was mentioned. It raised the moral for the students and was used to encourage good grades and for good behavior on campus and of course the meaning of cooperation and teamwork.

I’m quite sure the FCC knew about our Radio station because the kids talked about it and so too did some of the teachers. We most likely didn’t break the rules because we had such a large campus and the transmitter was in dead center of it. You probably can google the School For the Blind in Lansing and see why.

Same thing up here. I'm in the boonies, in a town of 600 people.  It's pretty easy to get 1 1/2 miles during the day, but I get a block or two and night.  Very little man made noise here.  The band is dead silent between stations.  I'm sure I'd never get this coverage even 10 miles away in the "big" town of 16,000 people.  Lots of industry, power lines, etc over there!

There are advantages to not living in the city. Good  Part 15 AM is only one. 

TIB