Dave Richards AA7EE

October 8, 2010

The SSTRAN AMT3000 – A Part 15 AM Transmitter

In the last post you saw my first steps towards putting a low power unlicensed (yet legal) AM broadcast station on the air from my house in Oakland, California.  I spent several months putting the programming together using a piece of free radio automation software called Zara Radio. Adding songs is not quite as easy a process as ripping songs into your iTunes.  As well as ripping the songs from CD, I have to trim any excess silence from the beginning of the song and then mark the exact segue point, so that the software knows at which point to start the next song.  It takes a little while to do this, and I have now done it for 1650 individual songs, as well as producing sweepers, promos and other little doo-das that all go to make a radio station sound like, well, a radio station. The software also automatically plays a newscast (grabbed from an internet feed) at the top of each hour after playing a news intro. It all sounds pretty nifty and I’m quite happy with the way it has worked out. So with the programming worked out, the next step was to build and install a transmitter. I had originally decided to use the Hamilton Rangemaster, but that was going to set me back the best part of a grand for the transmitter and cabling alone, not to mention the cost of outboard audio processing.  The AMT-3000 Part 15 AM transmitter is made by SSTRAN, has onboard limiting and compression and comes as a kit for around $100.  On checking reviews and write-ups, it seemed to be a high quality kit with a stable well-modulated signal and decent onbaord audio processing. The relatively high cost of the Rangemaster had dissuaded me from continuing with my low power broadcasting aspirations,  but the thought of getting on the air for $100 meant that the financial barrier to going through with this had just been removed. $200 is a little closer to the mark actually, as I would also have to fabricate a vertical antenna with loading coil and buy the cable to connect the outdoor transmitter to the studio, but this was within the realm of justifiability for me.

So this is what the SSTRAN AMT-3000 looked like when it arrived at my house. Exciting eh?

 

This kit arrives in a box. Who would have thought it?

 

On opening the box the first thing you see after the packing slip, is a clear and detailed instruction manual sitting on top of a well-packed kit. First impressions are very good.  At this point I definitely want a second date:

 

The instruction manual is detailed and clear.

 

When you open up the box, you see all the parts, including a wall-wart transformer, all knobs and connectors,  a high quality silk screened PC board and a plastic case with printed front and rear panels. You even get antenna and ground wires that are sufficient if you’re just intending to broadcast to the radios in your house (50-200 foot range):

 

This is what you get inside the box.

 

If you look closely at the above picture, you’ll see that instead of a front and rear panel, I have 2 rear panels.  An e-mail to SSTRAN solved the problem and a front panel arrived in my mailbox a few days later. It didn’t delay my building the kit as I could still complete the board, place it in the case if needed, and use the transmitter.

Although a pretty straightforward kit, this is not a project for the absolute beginner. The parts density is fairly low, but some of the individual parts have leads that are quite close together. If you have some experience soldering parts onto circuit boards you should be OK with this kit though. There is one SMT device which came pre-soldered to the board in the version that I bought.  There is also a version of the kit for $3 less that doesn’t have the SMT chip pre-soldered, but this wasn’t available at the time of ordering so I took a deep breath and decided to pony up the extra $3 (I’m a cheapskate and happen to like soldering SMT devices).

Another thing about this kit is that there are no coils or toroids to wind. I joked in the forums over at Hobby Broadcaster that I almost felt as if I hadn’t really built a transmitter because I hadn’t wound any inductors! The kit uses pre-wound inductors, which does make the whole assembly process faster and more straight forward. This is what the board looks like when fully assembled:

 

The finished board. At this point, you can plug it in and transmit!

 

Looking at the board above, at the back from left to right are the 2 audio input jacks.  It is a mono transmitter, but if you have a stereo feed you can plug both channels in here and they will be summed to a mono signal.  If you have a mono feed, it can be plugged into either connector. Hiding behind the 15V regulator heatsink is the power connector.  The kit comes with a wall-wart that outputs 16V AC, but you can also run this from a DC input; I used 24V DC from 2 gel cells and it worked fine. Finally at the far right is the antenna and ground connector. You can see the 3 RF chokes that can be placed in or out of circuit with jumpers in order to combat hum due to stray RF. 2 of the chokes serve to isolate the power input and one isolates the audio input ground from the board ground.  Because these are RF chokes, they allow audio to pass, so if you have an audio ground loop you will still need to fit an audio isolation transformer. In front of the regulator heatsink is the blue 8 position DIP switch that is used to set the transmit frequency. The transmit frequency is derived from a PLL synthesizer which gets it’s reference frequency from a 4MHz crystal (for the US version with 10KHz channel spacing), making this transmitter stable enough for most Part 15 purposes. The 4 position DIP switch to the right controls the switching in and out of circuit of several inductors for use when tuning the supplied indoor antenna. When using a base loaded outdoor vertical, the on-board inductors are not used.

The whole thing looks pretty nifty when you put it in the case:

 

Your own mini AM broadcasting station (substitute the phrase "medium wave" for AM if you're in the UK.)

 

On the front from left to right is the audio gain control, the pot that sets the modulation level (it sets the point beyond which limiting occurs), and the compression level control.

Oh, here’s the bottom of the board too, just to prove that I can solder:

 

Look Ma, I can solder!

 

Many folk buy these in order to broadcast programming to their vintage radios. I don’t know what the AM band sounds like in other countries (or medium wave band as it is more correctly referred to in the UK), but in the US it is mainly conservative talk radio produced with ratings (and not quality of content) in mind.  The conservative part doesn’t concern me, but the fact that much of it is highly opinionated banter designed solely to push the emotional buttons of listeners does bother me no end. John N8ZYA refers to it in his blog as drivel and to my mind he’s right on the mark.

Anyway, if you just want to broadcast around your house, you’re pretty much done at this point.  You can connect the supplied pieces of ground and antenna wire, tune them up per the instructions, and you’re ready to blanket your homestead with good sounding AM broadcasts. If, like me, you’re hoping to cover a slightly larger area, then the next step is to build an outdoor vertical antenna with a loading coil.

The instructions for a base loaded vertical made from readily obtainable parts are on the SSTRAN website, and there is a drawing and parts list also. All the info is contained on the site, so I’m not going to repeat it here, but this is the loading coil made from 16 AWG magnet wire wound on a former of 3″ white schedule 40 PVC:

 

\

This loading coil is BEEFY.

 

PVC has a tendency to absorb moisture over time, so the pipe was painted with 2 coats of varnish, the coil wound, and the finished coil coated with varnish.  The ends of the pipe were masked with tape so that the end caps could be cemented into place afterwards.

Here’s the finished antenna and transmitter installation.  The transmitter was housed in a Rubber-maid container.  If I decide to make this installation more permanent, I’ll search around for a white plastic box with weatherproofing seals. You can’t see the top of the antenna, but it’s just a length of copper plumbing pipe with a cap soldered on the end:

 

Schedule 40 pipe and Rubber-maid containers - the giveaway signs that this is not a high-tech installation on the roof of some government building, but just another radio experiment at my house.

 

Here’s a closer view of my Home Depot/Orchard Supply Hardware/Safeway special:

 

At this early stage, the antenna wire is still connected to the coil tap with an alligator clip; tuning has not yet been finalized. Audio and power cables enter the box through small drilled holes and will be sealed with silicone caulk.

 

Some gratuitous beefcake; the loading coil gets it’s close-up:

 

If this Part 15 thing doesn't work out, perhaps I could use it on Top Band......

 

So how does it work? Weeeelll……some people get great coverage and results right off the bat.  I am not one of those people, and I think I’m going to have to put quite a bit of effort into perfecting this system if I want it to work to my satisfaction. The transmitter itself seems to be doing everything it should.  The carrier is stable. The audio processing sounds good.  Considering all the processing is taken care of by one chip I’m quite happy with the way it sounds.

I’m experiencing two problems. One is that there is an AC hum on the carrier that I’m pretty sure is happening because RF from the antenna is being re-radiated by the house AC wiring. This will be a tough and maybe impossible problem to solve, as I am one tenant out of 10 in this house and don’t have access to the other tenant’s rooms.  There are a couple of solutions in my head, but they may not be possible given my current living situation.

The external antenna certainly does increase the coverage area over the supplied wire antenna. With the supplied wire antenna I could receive my transmissions all over the house, but not too far outside. With the external antenna coverage  seems to go as far as 1000 feet in some directions, but only a block or two in other directions. I don’t believe that this is in any way a fault of the transmitter but of my imperfect installation. I’m using a cold water pipe as a ground connection and don’t know how good a ground it is.  Although the antenna is about 15 feet off the ground, it is shielded on one side by the house and on another side by apartment buildings next door.  If I could mount the antenna on my roof I think I’d get better coverage (as long as I were able to ensure that the ground connection is not radiating in order to keep the FCC happy), but roof access isn’t too easy here.

I’d prefer to ground mount the transmitter so that I can ensure that the ground lead is very short and connects directly into the ground to avoid any possible misinterpretation of the ground lead rule if the FCC were ever to inspect, but ground space is limited on my plot and I have to be careful not to overstep any boundaries with my landlord and fellow tenants.

So…….I’m going to sit back and not do too much with this project for the time being. I may decide to use this transmitter with a short wire antenna just to broadcast around the house, or I may get another burst of enthusiasm and decide to try a different installation in the hope of increasing the coverage (and eliminating the hum on the carrier.)  It sounds great with a short wire antenna and I know that with the right installation it will sound great with an external antenna also. I’m hoping to cover an area of radius 3/4 mile around my house and I do think it can be achieved.

I’m also realizing how even my QRP ham radio activities are easier than Part 15 operation. With QRP, I run 5 watts and have no restrictions on antennas at all.  In fact, if I want, I can run up to 1500 watts on most ham bands into any antenna I want. Engineering a Part 15 system is truly a challenge.

Now if the FCC could just relax the Part 15 antenna restrictions and allow me to hook this thing up to an ATU and long wire antenna…………

 

 

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14 Comments »

  1. Dave – good to see you back! You don’t post often, but when you do, it’s a doozie!

    I’m curious about your antenna. How tall is the vertical part and what are you using as an RF ground? And do you plan to keep operating the transmitter from right at the base of the antenna? As I’m sure you’re aware, even the cheapest brand of RG50 you can find would have minimal losses at the freq your operating. Locating the transmitter away from the antenna might solve your hum problem…

    Comment by John AE5X — October 8, 2010 @ 8:48 pm | Reply

  2. John – I read your posts often. As I don’t read many radio magazines these days, your blog keeps me up to date with interesting new ham radio kits and ideas. Many thanks!

    The FCC ruling which applies to my transmitter is Part 15.219 –

    “15.219 Operation in the band 510-1705 kHz.

    (a) The total input power to the final radio frequency stage (exclusive of
    filament or heater power) shall not exceed 100 milliwatts.

    (b) The total length of the transmission line, antenna and ground lead
    (if used) shall not exceed 3 meters.

    (c) All emissions below 510 kHz or above 1705 kHz shall be attenuated at
    least 20 dB below the level of the unmodulated carrier. Determination of
    compliance with the 20 dB attenuation specification may be based on measure-
    ments at the intentional radiator’s antenna output terminal unless the in-
    tentional radiator uses a permanently attached antenna, in which case com-
    pliance shall be demonstrated by measuring the radiated emissions.”

    As you can see from (b), using coax to feed the antenna would put me in violation of the rules.

    The problem a lot of Part 15 operators have is when they wish to mount the transmitter and antenna high above ground level.
    Although the Part 15 rules aren’t completely clear on this, from the results of FCC inspections of Part 15 stations that have taken place, some Field Inspectors are satisfied with elevated installations if it can be demonstrated that the long ground lead is not radiating. This is done by taking field strength measurements with the ground lead connected and then disconnected. Some Part 15 operators install RF chokes or other types of filters to prevent their ground leads from radiating.

    Of course, the easy way to ensure compliance is to mount the transmitter and antenna at ground level so that the ground lead is very short. That way you can get a good RF ground with an installation that will almost definitely pass inspection.

    I’m using a piece of solid wire that connects to a nearby cold water pipe for a ground John. I don’t know if it’s a good RF ground or not, but it’s kind of a moot point because I am supposed to insulate the ground lead from radiation, as my installation is about 15 feet above ground. My ground lead does pass through a ferrite core at the transmitter end (you can see it in the close-up picture of the transmitter box) but I don’t know how effective it is. Obviously this arrangement will have to change in some way if I am to continue with this external antenna.

    This has made me realize how much easier the constraints are to us QRP operators. 5 watts into any antenna system seems like paradise compared to these Part 15 restrictions. Nevertheless, I’m glad that we are allowed to emit any kind of RF at all on this band.

    Comment by aa7ee — October 8, 2010 @ 10:36 pm | Reply

  3. In answer to your question about the vertical tube John, it telescopes for exact tuning, but the approximate length for my vertical part when tuned is 101″. That leaves approximately 19″ more for the lead to the loading coil, the length of the loading coil and the ground lead. It’s not much!

    Comment by aa7ee — October 8, 2010 @ 10:43 pm | Reply

  4. Thanks Dave – I knew that was a restriction on the 160-190 kHz Lowfer band but didn’t realize it applied here as well. With ~60 mW (based on 100 mW ‘input’) from the transmitter coupled with the imposed inefficiency of the antenna requirements, your ERP is probably in the single digit microwatts.

    Comment by John AE5X — October 8, 2010 @ 11:58 pm | Reply

  5. Dave –

    Below is a clip from a post I made on the web on the subject of long, radiating conductors between an elevated Pt 15 AM tx + 3-m whip and a functional r-f ground. Maybe you will find it interesting.

    Rgds,

    Rich
    ___________

    Re: Setting new precedent for outdoor Part 15, From Keith Hamilton
    « Reply #18 on: July 10, 2010, 10:21:58 AM »

    Quote from: (omitted) on July 10, 2010, 09:02:42 AM
    \\ It is my opinion, that no matter what ‘corrective’ action was taken at KENC, it would never be good enough. Seems to me that this particular agent and his office have it out for Ken Cartright.//

    The FCC would have no legal basis per 47 CFR Section 15.219 to cite any unlicensed, intentionally radiating transmit system in the AM broadcast band that actually met 47 CFR Part 15.219.

    However it appears from the FCC NOUO that KENC did not do so before the filter was installed, and that installing the filter “made no appreciable difference” to that determination.

    This whole issues revolves around the total length of the conducting path between the transmitter and the functional r-f ground used by the Part 15 AM system (something buried in the earth, typically).

    Some Part 15 AM microbroadcasting proponents define the “ground lead” as stated in 15.219(b) to be limited to a short conductor attached from the transmitter to the top of second conductor, and that a Part 15 AM system meets Part 15.219(b) as long as the sum of the length of that short conductor and the length of whatever conductor is attached to the antenna connector of the transmitter does not exceed 3 meters.

    That second conductor is defined by these proponents to be “ground.” In the KENC case, the length of that second conductor (the tower) appears to have a length of something like 40 feet, where it connects at its base to one or more buried ground rods — which ground rods are the functional r-f ground for the system.

    What if a single 40-foot conductor was used between the transmitter at the top of the tower and the ground rods? Probably those same proponents of Part 15 AM microcasting then would recognize that such a configuration would not be compliant with 15.219(b).

    But physics and field experience show that there is no practical electrical difference in the performance of a Part 15 AM system using a single 40-foot “ground” conductor, or one using two conductors in series whose total length is 40 feet — where the shorter length is called a “ground lead” and the longer length is called “ground.” Calling the longer lead ground doesn’t make it function as a ground, as far as r-f energy is concerned.

    The accurate definition of a ground lead used at radio frequencies is the conducting path leading from the r-f ground terminal/chassis of an electrical device, and connecting it to a functional r-f ground (ground rods, or similar).

    Such long conducting paths (whether comprised of one conductor or more) between the transmitter and a functional r-f ground carry all of the r-f current that the transmitter can produce in that installation.

    An r-f current flowing along a conducting path between the ground rods and the transmitter produces radiation. That is the reason that 15.219(b) includes the length of that conducting path in the 3-meter limit. Effectively it is a radiating part of the antenna system, along with the “3-meter whip.”

    Comment by R. Fry — October 10, 2010 @ 7:42 pm | Reply

  6. Your comments make perfect sense Rich and it was a very similar line of reasoning that made me realize that if I am to operate a Part 15 station with an elevated installation, the only antenna configuration whose compliance I would feel confident in would be one in which there actually is no ground lead and the 2 antenna wires connected to the transmitter comprise a much-shortened dipole antenna.

    The transmitter I am using has chokes on the circuit board that prevent RF currents from flowing in the power leads and the ground side of the audio input cable, so such a set-up should be compliant with 15.219

    Comment by aa7ee — October 11, 2010 @ 1:44 pm | Reply

  7. While Mr. Fry brings up some *interesting* observations he is not the FCC. The 15.219b rule simply states “and ground *lead* (if used)” which anybody proficient in the English language will interpret as the wire between the device and the ground. The FCC rule is vague but Mr. Fry believes he is the sole authority for on both the rules and the FCC interpretation. Mr. Fry believes his personal interpretation of what ground *lead* is the Gospel truth and anybody who disagrees with his point of view is not only wrong but is of lesser intelligence. There are many and multiple instances of this never ending rant spewed across many radio and hobby oriented online forums.

    The only thing Mr. Fry fails to acknowledge time and again is that various installations have been inspected by FCC inspecters and given a passing grade. His must believe his one year of broadcast engineer allows him to play the expert.

    Comment by Rob Martin — October 11, 2010 @ 3:07 pm | Reply

  8. Rob Martin wrote, “While Mr. Fry brings up some *interesting* observations he is not the FCC. The 15.219b rule simply states “and ground *lead* (if used)” which anybody proficient in the English language will interpret as the wire between the device and the ground.”

    And I do, however no interpretation is necessary. A functioning r-f ground does not exist at the top of vertical “massive wire” or any other conducting path such as a tower, flagpole etc. Physics, and the experience of the “Part 15 AM” operators who were cited for using long, conducting paths to a functional r-f ground (something buried in the earth) both show this to be true.

    “The FCC rule is vague…”

    No, I submit that an accurate understanding of the rule is what may be vague.

    “His (sic) must believe his one year of broadcast engineer allows him to play the expert.”

    My broadcast engineering experience includes the list below, which starts in 1959 and was continuous for over forty years through my retirement from Harris.

    * RF Applications Engineer, Harris Corporation Broadcast Division
    * Field Supervisor, AM/FM/TV Broadcast Service at Radio Corporation of America (later: RCA Corp)
    * Staff Engineer at WJR, The Goodwill Station (Detroit)
    * Chief Engineer at WSAM and WSAM-FM, Saginaw, MI
    * Staff Engineer at WKNX-TV, Saginaw, MI
    * Chief Engineer at KPLY (AM), Crescent City, CA

    Comment by R. Fry — October 11, 2010 @ 8:12 pm | Reply

  9. Hello Dave,

    Nice to see another post on your station again. I continue to keep reading and learning about micro broadcasting. As luck would have it, I ran across a micro broadcaster at a local “car show” this weekend. They were using it to keep event participants informed about the activities at the show and playing 60’s music. They also had it “hard wired” with speakers placed along the course which covered about a mile. It worked well but were only getting about 1/4 mile range (over the air) due to the tall buildings and the transmission site along the river. They were using a steel whip for an antenna. The audio was excellent for an AM station.

    I’m not sure I could solder as well as you…so have been looking at the Ramsey FM 25B which is a little more “basic”. With a little luck, I might be able to put this kit together and will probably use a “dipole” antenna. (perhaps a set of rabbit ears extended the proper length) for the first test. I’m faced with antenna restrictions here at home and would not be able to place a steel whip in the attic. You’re right about the difference between Ham radio and broadcasting…antennas are NOT a problem with ham radio.

    Sorry to hear about the “hum”. Hope you will be able to work out this “bug”. I can’t help but be concerned about the transmitter being so close to the antenna on your set up. Not much of an electronics background here so may be worried about something that makes no difference? I assume you hope to minimize power loss to the antenna.

    Thanks again for posting. I’m making broadcasting friends here in the valley but the going is slow. My time line is still around the first of the new year. I’ll be looking forward to reading more about your station there. 72’s

    Comment by John N8ZYA — October 14, 2010 @ 1:03 am | Reply

    • Hi John –

      Glad you got to see a Part 15 AM station in action. I wish I had seen that one.

      Have fun with the FM transmitter. If you’re thinking of eventually covering a greater area than your house, it is much harder to do on FM while still remaining within the constraints of the FCC Part 15 rules. The restrictions for Part 15 FM broadcast band devices pretty much ensure that you won’t cover greater than a 200 foot radius from the transmitter. Have fun with it though – it sounds like a worthwhile project.

      Good to hear that you continue to enjoy listening to “Auntie Beeb” as she is affectionately known in the UK.

      Comment by aa7ee — October 15, 2010 @ 12:06 am | Reply

  10. Nice Blog Dave. You are indeed a minimalist with ur QRP AM and amateur station. See you on Hobby Broadcaster

    73, me

    Comment by John — October 14, 2010 @ 2:21 pm | Reply

  11. Hello Dave,

    Actually they have a beginner AM kit which costs at about $35. I think I’ll try that one just as a test unit. It’s manually tuned (with a pot) but only 100 solder points and estimated completion to assemble is 3 hours. Even I should be able to put that together. hihi. I’ve been looking at some audio mixers and mikes. I’ll keep it simple at first to see how it goes. It should be fun. Still thinking of AM for the distance.

    Comment by John N8ZYA — October 18, 2010 @ 2:16 am | Reply

  12. Hi Dave,

    I have a transmitter setup similar to yours except I also added 30 20-foot ground radials.
    Beside the radials, I also have 4 ground rods about 6 feet apart driven at a 45 degree angle.
    Not much choice since my yard is 5 feet of soil over bedrock.

    Are you sure the ‘hum’ is on the carrier, not the audio ? I had hum on the audio that went away when I unplugged the audio feed.
    My transmitter is about 200 feet from the house and I had to isolate the audio ground with a RadioShack ground look isolator.

    Tuning also took a while using a mix of a Digital meter, a scope and an old AM receiver with a real signal strength meter.

    I get about 1.5 miles all around except to the east, there I get almost 4 miles.
    It must be some kind of underground iron ore vein or something :)

    Andre

    Comment by Andre — October 19, 2010 @ 7:35 pm | Reply

  13. Andre – your experience, along with the comments I’ve received from others, makes me realize how very important a good RF ground is for these small Part 15 AM transmitters. Most of my experience is at higher frequencies where groundwave propagation doesn’t happen as much.

    Comment by aa7ee — October 21, 2010 @ 3:45 am | Reply


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