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…………



May 27, 2010

A Different Type Of QRP

My radio interests have taken a different course in the last month or so and I’m not sure whether it’s appropriate to include them in this blog or not, so let me tell what I’ve been up to and you can decide;  I’d welcome your comments.

I had been earning my living as a DJ/announcer/voiceover guy since 1987 until last year when the paid work all came to an end for me. I really thought I’d gotten over the DJ bug and in some ways, I think I have.  As a youngster, I very much wanted to prove myself – to both myself and my peers.  Being perfectly honest, there was a need for a certain amount of ego gratification too. Well, I don’t feel the need to prove myself to anyone anymore and as for the ego gratification part – well, I think that age has cured me of the need for that.

But interests picked up at an early age never go away completely and so it was that a month or two ago I became interested in setting up a Part 15 AM broadcast station.

Most people reading this blog will know about the FCC regulations that cover Part 15 devices, which can be broadly divided into 2 types – non-intentional radiators and intentional radiators.  Non-intentional radiators are devices that emit RF as a by-product of what they do and then happen to leak some of it. This category includes the local oscillators of radio receivers and the crystal controlled clocks in computers, for example.  Intentional radiaters are devices for which the RF radiation is the main point of the device.  Baby monitors,  cordless phones and garage remote controls are examples of intentional radiators.

Part 15 regulations also cover something which is quite attractive to me,  and that is the ability to operate a low power unlicensed yet completely legal broadcast station. On the FM broadcast band, there is a strict field strength requirement at a set distance from the antenna that makes it unlikely you’ll be able to achieve a range much greater than 200 feet.  This is fine if you want to play music from your iPod on your radio, but you’re not going to build much of an audience with a Part 15 FM station. The criteria for transmitters on the AM broadcast band are much more lenient; instead of field strength measurements at a specified distance from the antenna, the main rules are that the input power to the final amplifier cannot exceed 100mW and the total length of the antenna and ground lead cannot exceed 3 meters.

100mW to a 10 foot antenna on the AM broadcast band isn’t much, but if you engineer the whole system for maximum efficiency, it can be possible (from what I’ve read) to achieve a range of up to a mile (or even more) from the transmitter. What makes this even more appealing is that the FCC allows the use of multiple transmitters to increase coverage. Place a few transmitters around the edge of your primary coverage area, and now you’re starting to cover a significant part of a city.  Some small town residents can cover a significant part of their entire community with one well-placed transmitter. The same discipline of maximizing the efficiency of the whole system that applies to QRP ham operating also applies to Part 15 AM operating. After all – it is also QRP. In fact quite a few operators of Part 15 AM broadcasting stations are licensed hams.

Many Part 15 AM broadcasters are either people who want to broadcast old-time programming to their restored antique radios, or Realtors with their “Talking House” transmitters broadcasting details of houses for sale to prospective buyers parked outside.  A smaller, but very enthusiastic subset of  Part 15 AM operators are the folk who run their own radio broadcasting stations.  The FCC don’t recognize these outfits as radio stations; they are simply classified as intentional radiators.

At this point, you’ve either completely tuned out or have at least some level of marginal interest. Here’s a picture to break the monotony:


Ant Radio broadcasting from the Pill Hill district of Oakland, California


Not a great photo I’m afraid.  At some point I’ll take a better lit and processed picture, but at least you can see what it looks like. This is where I do my ham operating too.  In the shelf unit to the left is the FT-817,  KK-1 straight key and Bencher paddle along with the Fort Tuthill 80 and 2N2/40 as well as my soldering station, 13.8V regulated power supply – oh, and my DVD player to boot! There is also my Signalink USB sitting on top of the Tut 80.

On the right is AM broadcast central. At the bottom is a Mackie mixer on a pullout shelf. Above it the Denon dual CD player (2 separate units – the control unit at the bottom and the CD trays in the unit above it.) The microphone is an EV RE27N/D. There is a Shure SM7 (also on a boom arm) out of sight of the camera for guests.  At the top of the rack are the 2 audio processing units.  The lower one is the first in the audio chain after the mixer.  It is an Aphex compellor which provides compression, leveling and peak limiting of the signal. Above it is an Inovonics 222 which provides pre-emphasis, a lowpass filter (to limit the bandwidth of the transmitted signal) and more peak limiting. It supports asymmetrical carrier modulation to modulate the transmitter to as much as 130% – another way to maximize the range of this QRP signal. The Inovonics 222 is quite popular with AM amateur radio operators to help them squeeze maximum efficiency from their signals.

If you have any interest in this, I thoroughly recommend Hobby Broadcaster (link opens in a new window) – the site for Part 15 AM and FM broadcasters.  It’s run by broadcast engineer Bill DeFelice who also actively participates in and moderates a great set of forums. There are other sites that deal with Part 15 broadcasting,  but Bill really sets the tone in his forums with helpful friendly comments and advice as well as equipment reviews.  He also posts his online finds for those who are looking for deals on good affordable gear for their stations.

When I’m not DJ’ing live (which will be most of the time) the computer runs the whole station in automation and so far, it’s not sounding too bad at all. I’m currently spending a lot of time recording and producing all the station ID’s as well as adding to the song library. When Ant Radio hits the airwaves, I’ll probably post an air-check so you can hear what my little broadcast station sounds like.

Maybe I’ll even get the occasional DX report 🙂

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