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?
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:
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):
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:
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:
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:
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:
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:
Here’s a closer view of my Home Depot/Orchard Supply Hardware/Safeway special:
Some gratuitous beefcake; the loading coil gets it’s close-up:
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…………