As I thought about having grown up around vacuum tubes I realized how much I miss them...the glow, the smell, the distorted audio, burns, and so on. Therefore I decided to scrounge around my junk box and build a tube something. The something is a Part 15 AM transmitter.

I wanted the transmitter to be transformer isolated from the power line, be frequency agile, be crystal controlled, and have some of the controls of the "big" transmitters. The junk box yielded all parts except for two tubes and a panel meter for a total cost of $17. The third tube (small one in back) came from the junk box and it is marked "Amperex" with a date code of 1951 yet it tests good and works fine.

The design is a plate modulated class C unit with the only non-tube concession being using a PLL to set the frequency by means of DIP switches. Otherwise, all is old school tube assembly techniques.

The finished (except for labels) unit:

The two front tubes were selected for the relatively highly visible filaments. The meter indicates the final plate current which is tuned for a dip with the knob next to the meter. The two switches control the filament power and the B+ power just like with the big units. The hole next to the switches will someday be used for a meter selector switch to select current or modulation but this hasn't been designed yet.

Here's a low light view showing the glow from the tubes:

The top of the cabinet is smooth with nothing but the tubes protruding. This was done by using a sub-panel as the chassis for the tube circuitry show here with the cover removed:

The power transformers can be seen in the lower left. One powers the filaments at 12.6 VAC and the other steps the 12.6 VAC from the first up to 120 VAC for the B+.

To make servicing and changing things easier the sub-panel rotates up and can be secured in place exposing the circuitry. In this view the circuitry can be seen as well as the tuning capacitor and the tank coil and the PLL is in the lower middle of the pic:

One off label use for tube equipment such as my first good stereo amplifier has been to warm cashews. So, to commission this unit here goes:

YUM!

Now for some boring details. The final input DC power is 74 mW when tuned and connected to a 3 meter wire antenna. The maximum modulation is 84%. The audio is not as clean as the solid state transmitters I use but it is acceptable. I found that the tubes are spec'd at 7% THD typical but using two negative feedback paths seems to have improved this.

Range tests haven't been done yet but a quick check with a cheap portable the signal from the antenna in my basement could be heard at the curb 100 feet away. More serious checks to come.

There is no matching to the antenna at all. The signal comes directly off the plate through a capacitor (for safety...stops the B+ from getting on the antenna). With this simple arrangement the harmonics are all down by 34 dBc or more. There is no hum seen on the scoped RF and audio and none is heard on the signal. The tank tunes from 1100 kHz to 1700 kHz.

During prototyping I tried coupling through a link coil to a 33 ohm load resistor and achieved good results. The transmitter loaded nicely up to 100 mW input and gave an efficiency of 68% which is pretty good for such a simple circuit. This has no meaning though when used with the simple wire antenna.

This was a lot of work doing the prototyping, the chassis and panel work, wiring, testing, and adjusting but the final product was worth it.

Neil

Seeing Neil's newly built tube transmitter kicked my brain into inventive mode. Let's talk outdoor transmitter installations.

Over at the hardware store they have a whole row of small outdoor tool sheds lined up along the parking lot on display. Pay for one and set it up in the back yard.

Below it will be a self-made concrete slab with built in ground rod and radials.

Inside the convenient walk-in shack will be your transmitter, maybe a tube model.

On top of the shack will be a ten-foot copper stick.

More deluxe than a simple weather box? You will think so this winter when you're in your shack doing some fine-tuning.

All this talk about tube transmitters sent me into the attic last night to see if I could dig up this little bugger.  Not only did I find it, but it still works!  Built this from a kit when I was in Jr. High so that would be about about 45 years ago. Darn I feel old now.

Had a bit of hum in the signal but could only hear the hum it on the stereo receiver in the living room, sounded fine into the portable radios.  Don't know what I'll do with it now that I've dug it out. Probably display it in the studio, I suppose. 

TIB

From another thread:  Here's something else to speculate on, I wonder if anyone's tried 100 milliwatts into a full length broadcast antenna tower as a test, and how much range did it get?

A typical 1/4-wave vertical tower system used by AM broadcast stations produces an inverse distance groundwave field of about 305 mV/m at 1 km, for 1000 watts of applied power.  This has been proven by thousands of real-world measurements using accurately-calibrated FI meters, and as predicted by mathematics.

If the power applied to that system was reduced to 100 mW, the inverse distance field at 1 km would drop by 40 dB -- to about 3 mV/m.

A MW field of about 150 µV/m can produce a fairly useful signal to receivers in locations with low radio noise levels.  If earth conductivity is average (5 mS/m) then the 150 µV/m groundwave from this 100 mW system using 1650 kHz would be located about 4 miles away.

This is more than speculation, but hopefully more useful, also.

I owe Keith Hamilton a Christmas card!  Just did a five-hour gig at the local Chamber of Commerce car show.  Set up the Rangemaster, the processing, and my laptop, and played 50's and 60's vintage music (to match the cars) along with recorded thanks to the event sponsors and coordinators.

It was held in our local Home Depot parking lot, so there was little chance of getting a decent ground for the xmtr.  But even without grounding of any sort, the signal was to-the-wall over the whole parking lot, and clearly receivable 1/4 mile away on an "average" car radio.  I was impressed!  (Thanks, Keith!)

Oh, and for those who must know (radio geeks like me), the setup was a simple HP laptop with its internal equalizer rolling off the 32HZ and 16kHz bands, all others flat.  Into an Aphex 320A as an AGC, then into a Texar (pre-NRSC) Eagle with fairly robust compression but moderate clipping and 125% positive peaks, then right into the Rangemaster, which was tuned to 100% output.  I used ZaraRadio as programming software, which is an amazing program for the price (FREE!), but I think I'll look for something that may cost money but is more malleable.  Any suggestions are most welcome.

It's pretty rewarding when others can see and hear what we do with this hobby of ours.  And then have good things to say about it, too!

Mark

The AMT5000 transmitter is such a wonderful experience.

After hours when rolled up for the day I set the power level down to 34 mW so there is no liklihood anyone will complain. Indoor reception is perfect.

In the daytime when out on the campus trimming shrubbery and mowing grass the AMT5000 set at 100 mW provides a solid signal to the Sangeon U1 yard radio.

But every once in awhile when the daykight is dim the corner where the AMT5000 is located gets dark and the power control is hard to see. I fear it might accidentally get set high, as high as 360 mW.

When that happens I usually don't discover the problem for an entire week, and when I do, of course, I am appropriately filled with guilt and plan to turn that power down as soon as possible.

With my schedule it's never easy to know when I'll have the free time and appropriate lighting to re-set the RF power.

Of course there is an ultimate moment when the power gets lowered to where it belongs.

May the AMT5000 be declared consecrated as holy.

http://www.iamradio.net/

For those who are short on cash. Reg: $249.  Get one today for $195.

Does anyone have experience with the North County Radio AM88, particularly in the LW part 15 region, 160-190kHz? http://www.northcountryradio.com/Kitpages/am88.htm

Does anyone know of other transmitter kits or even off the shelf transmitters that work at 160-190kHz? (I wonder if it is possible to mod a talking house to work that low)

The part 15 dial maps got me thinking about LW again (Thank you Carl!). I have no idea what the range would be for part 15 LW, and range would not be my goal. My thought is LW is a neat little niche that is commercially unserved in the US. At one point there was a push for designing a part 15 LW transmitter. Every now and then it comes up then fizzles.

Amazon has the TECSUN PL-380 for $42. It can tune LW, and it has variable IF bandwidth. That would make listening to part 15 LW transmissions possibly without a $100+ radio.

http://www.progressive-concepts.com/products/transmission-equipment/spec...

Interesting Topic huh?

If you're in a crisis and need a transmitter in a hurry, enter POCKET FM!

http://www.pocket-fm.com/

What's in your pocket?

check this little guy out, I might have to grab one when it gets relisted.

http://www.ebay.com/ulk/itm/131602741412 

So I want to start my own radiostation at home. Only issue is I would like to take the legal way of a unlisenced radio station. From what I understand with part 15 you can only got 300ft with a fm transmitter so I wont be going FM. With AM I see you can go about a mile wich is good enough for me. I would like to know whats a good part 15 transmitter that can give me a deicent range. I have herd about the talking house transmitter but im not sure how far of a signial I can get with it. I do live in a desert so the wether is pritty clear most of the time. 

Thanks

Knightkit Wireless Broadcaster on eBay...

http://www.ebay.com/itm/KNIGHTKIT-WIRELESS-BROADCASTER-AM-TRANSMITTER-WO...

Before I decided to take out the batteries and pack up the FIM-71 until the next project comes along, and list the CM-10 on eBay I thought I'd pop out to the front yard and shoot a quick demonstration video of this transmitter in action, and also show the extreme variabilities in Part 15 transmitters.  The variations you will see were similar with the WH 3.0.

Note this was NOT done in an "open field" but how many Part 15 broadcasters do you know that put their transmitters in open fields?  Further, how many NOUO's have you see where the operator got to tell the FCC agent "But wait, lets take my transmitter over to the fieid over there..."

No, this is in the front yard. Next to a big ol' pine tree (that actually absorbs signal).

Note also that the readings you see on the meter are NOT the actual field strengh readings.  Per the graphs in the FIM-71 manual, at the test frequency, to determine actual field strength the reading must be multiplied by the antenna factor for the frequency.  In this case that is 2.25.  So that reading of 5 mV (yes, that's mV not uV) is actually more like 11 mV, or 11,000 uV/m.   You can do your own math for the other readings demonstrated in the video. A LEGAL reading at this frequency would be roughly 111 uV/m.

Now I realize there are those who will say "that can't possibly be right" even when seeing the meter for themselves.  Personally, I don't much care. As far as I'm concerned, for someone who has no intention of ever using a Part 15 FM transmitter, my work here is done.  And I challange others who may feel so inclined to do such testing to not only report actual numbers, but demonstrate them. I trust big city engineers also have access to video recorders.

Even accounting for various variables we're WAY past the limits here.

See:

https://youtu.be/wEnkSU0NCN0

TIB

Today I ordered the $249 "Enhanced" version of this transmitter. I already have a V5 that is not "enhanced." I will do a side-by-side comparison for those who are interested for both audio quality as well as range.  Not scientific by NYC standards, but a better than guestimate for those who might be toying with the idea of buying one. Based on UPS shipping from New Jersey, I should see it in Florida by Thursday.

Out of the box the unmodified V5 had the old Illinois company's label on the bottom. The audio was/is not great for music. However, running the ABN Old Time Radio was adequate.

The older V5 with the antenna fashoned into an inverted L around a window at ground level, could be heard on the fringe at 1600 feet. So I am guessing that with the antenna vertical and outside the performance would be better.

Will keep you guys posted.

There was a topic started 'over there' by wdcx about the Talking House power supply, but I thought it was apropos enough to continue over here (considering that Mighty1650 asked a question which has yet to be answered).

The Talking House power supply is prone to hum, as it is grounded to the electrical outlet to which it is plugged in.  Computers and other electrically noisy devices, if they are on the same circuit, are usually the cause.  John asked if it could be because the device (and power supply) is rated at 18 volts.  I have tried running the TH on 12 volts, and it does work (how well in terms of range, I never measured); I also noted no increase or decrease in hum.

To eliminate the hum generated by noise, you can usually experiment and try plugging in the wall wart to other outlets on different circuits, but sometimes an entire room (i.e., your studio, with all its devices) is covered by a single circuit.

You CAN ground the Talking House by other means.  Someone 'over there' states, it would then not be 'complaint', which is NOT necessarily true.  You would be potentially voiding the certification, but really, all the certification means is that the transmitter can be sold in the U.S.  You as the operator are still responsible for ensuring that it is 'complaint' (compliant) with Part 15.

Probably the best way to do that would be to run a short ground wire from the chassis to a good ground - the antenna + feedline (none if you are using the wire antenna) + ground wire has to be at most 3 meters in length (regardless of what that length was when the Talking House was certified - the antenna + the length of the ground wire in the power lead is probably greater than 3 meters even in its certified state, and heaven forbid if you plug in an extension cord or power bar).

Hum can also be generated in the audio feed, so I always use a ground isolator.  And if you do ground the transmitter by the chassis, you can use a better quality power supply that is better filtered.  You can also thread torroids over the power lead (no matter what you use), and it does make a difference.

Well, it was bugging me last night.  I just knew the skeptics and naysayers would look at my previously posted video and scream that it wasn't valid because I wasn't in a field, away from trees, the house, etc.  Now, even though I knew it would make a marginal difference, I decided today to head back to the field (although I didn't hike out as far as usual) and make a similar video to the previous one, only this time, out in the open.  So I did.   It didn't make the trasnmitter any more legal.  Taking into account variables like positioning of the power and audio cables (you noticed in the previous video they vary as you move the cables) the difference between open field testing and front yard next to a pine tree and an enclosed porch (no metal in the porch however, it's all wood and glass) is really quite negligible.

Then, I figured as long as I went to all this trouble, and people had asked, I moved the transmitter and meter out to 30 meters apart (98.4 feet) to get a field strength reading at the Canadian distance. Now, you'll note in the video the reading at this distance is 260 uV/m (.26 mV). Which, once the antenna factor is applied is a field strength of 585 uV/m.  I believe the Canadian limit at 30 meters is 100 uV/m, correct?  Uh oh!

Couldn't decide if I should add this video to the other topic, or start new. I guess I decided on starting a new one.

Video is at:

https://youtu.be/HyHrlWbdK4M

TIB

This device is not certified.  The FCC ID on it is bogus. See the attached letter from the FCC: 

I have the orginal V1 and a V5.  Both circuit boards are marked  V 1.1 So what is the difference?  The V5 BTW is not enhanced.

I was thinking, if the weather holds, I could maybe test a couple more FM transmitters.

I'm somewhat intrigued by the "C. Crane Digital FM Transmitter 2" What intrigues me, I guess, is that they advertise it as broadcasting up to 45 feet in line of sight use. But there are about a million references to a "mod" to crank up the power (it's mentioned in a ton of the Amazon reviews, there are videos on Youtube, and of course Radio Brandy has it too). Yet the HB review (granted from 2011 if I remember) stated it was right at the legal limit, but I have a list of issues with the HB test/review of this transmitter (but again, it is a 4 year old review).   But it's cheap, and it would be interesting to test.  And I truly hope no US Part 15 broadcasters are relying on Radio Brandy for any sort of information that is legal. That is a scary site indeed.  What amuses me most is the three mile coverage promised with their Ultimate FM Broadcast Antenna.  The use of which, with a certified FM transmitter makes it un-certified and puts the burden of proof for it's legal use on the user.  It's clearly a gain antenna. Do these users get out their field strength meters and adjust it to the legal level?  But in any event 250 uV/m at 3 meters is just that and no special antenna is going to make that amount of current go any further.  That site suggests so many violations of FCC Rules and the laws of physics I'm sure it's leading many into illegal broadcasting and they don't even realize it. 

Oops, little tangent there.  Other than the C. Crane, I was considering one of the Ramsey units. Any votes on which Ramsey transmitter might be good to test?  Don't know if there's time to build a kit before the snow files but I could give it a shot. 

TIB