WEAK-AM

Isn’t that one of the first questions we always ask when we go on the air? Well recently I purchased a Hamilton Rangemaster to add to my stable of transmitters (which includes the SSTran, the Ramsey AM25C, the Vectronics AM-1, and the venerable Knight Wireless Broadcaster). Of course, I was excited to find out how far it would go. And, I’ve developed an interesting technique for DXing that I want to share with you.

But first, the Rangemaster is awesome! I like it very much. I also like my SSTRan very much (but for somewhat different reasons). I bought the Rangemaster because I wanted to be able to operate it withough worrying about complaints about unlicensed operation.

So what did I find? Well, first of all, a little background. I am operating on a cluttered frequency (aren’t they all?) in a major metropolitan area, from out in the suburbs. I don’t have an ideal site. I have a small yard with a lot of trees. I wish I lived on a farm with hundreds of acres to put up experimental antennas… alas, that is not the case. Temporarily, I’m just using the CB whip antenna. The transmitter is grounded to a single 6′ ground rod. I’m watering the ground rod with a garden hose at a slow trickle. I plan to add additional ground rods over time.

When I first put the Rangemaster on the air, I didn’t pay close enough attention to the power adjustment instructions, and the range was very poor. Then I re-read the instructions and brought the power up to 100 mW, and the range improved markedly. I don’t own a field strength meter (I wish I had bought that Potomac FIM-41 I wanted when I was a kid)… So here is what I can tell you… my signal is really strong for about 200-300 feet, such that you can comfortably listen to it in a neighbor’s house on a reasonably good quality portable radio with little noise in the background. In my neighborhood, that’s about 2-3 houses away. The signal is intelligible for about .25 mile (2 blocks). At night, when the QRM is bad, you can just about listen to it a block away.

Now for the interesting part. I decided to see how far it was really going… despite the QRM. So I used the tone oscillator on my Shure M267 mixer, which is driving the transmitter, to generate a 1 kHz tone. Now, that tone is REALLY annoying to listen to at close range, but as the signal starts to get weak, it’s not as bad. You will find, if you try this, that the tone will stand out even if your signal is buried under 3 or 4 other stations in the muck, and you will be able to hear it for a long way. The ear is very sensitive in the 1 kHz range. So I put on the tone and drove off to work. In the morning, local sunrise is around 7 a.m. and it didn’t go far… about a mile was the farthest that I could hear it. But on the way home around 4:30, I was shocked to discover that as I was about 2 miles from my home, I started to hear it! And today, after I made some improvements to the ground system, I actually heard it several miles away. But the useful “broadcasting” range remains only a couple of blocks at most.

Now, what conclusions can be drawn about this? First, don’t try this experiment unless you are not afraid to be noticed. A 1 kHz tone on the AM dial left on all day stands out like a sore thumb. (So maybe don’t leave it on all day like I did). Second, the useful range of a Part 15 transmitter is a LOT less than the distance it actually can be be detected. On an area basis, if two blocks is equal to 1/4 mile, then the useful coverage area of my station is very roughly 0.2 square miles. But the absolute limit of reception is considerably farther. Now I will say, if you wanted to key the tone with Morse code, you could probably communicate at some distance… (yes, I am a ham radio operator). But that’s not why we get into LPAM, is it?

I noticed some interesting things while driving around. As you drive away, the signal rapidly drops off to a low level, such that you would think it would fade out entirely… but it “hangs in there” for a long, long distance (with occasional fade-outs). It’s amazing to me that after the signal gets weak, I can drive quite a bit farther before it actually disappears.

In any case, I would be interested to hear from people that try this experiment. How far can YOU go?!