I forget what it's called, but I recall some time back a reference talking about when broadcasting on one AM frequency (1700 in this case), that it also has an affect on another frequency somewhere else..
What is that called? And how much concern should applied to it?
I searched "harmonics", but didn't find what I was looking for, so I guess that's the wrong term.
Just not your find 🙂
Harmonics are what it sounds like to me you are looking for, though they are less of an issue at part 15 levels and with PLL circuits. My first AM unit was a hand-built-on-perf-board VCO nightmare based on a schematic in Ernie Wilson's book (now in our library). I tried to broadcast on 740 and my signal also appeared on 1480. When I tried 1420, I also showed up on 710 which was occupied - big no-no 🙂
Ernie's book said to try frequencies like 880 (which would hit 1760 and 440 - both out of band). Newer circuits, some built-in filtering and better shielding have really helped, although I have read that some of the imported high power (1 watt ++) FM units available on ebay are splatter city.
I gave up on the VCO and bought a kit 🙂
Testing at 1550kHz, a very clear channel at this location, for an unknown reason puts a whistle at 1570kHz, where there is a station about 30-miles away that just barely comes in under normal conditions. The station at 1570 has some interesting programs, and so I am actually interfering with my own ability to hear it, as the "whistle" is almost gone over near other homes. But the big question is, what is causing that whistle?
The only high power stations near 1550 are at 1600 and 1490, which should not be an issue.
Another search topic is "image frequency". Superheterodyne receivers (almost all receivers) receive on two frequencies: the signal frequency and the image frequency.
For part 15 operations on AM and FM this is rarely a problem due to the relative weak signals involved but you should know about images.
Neil
Sounds like your transmitter is being modulated by 20 kHz, putting a sideband at 1570 (1550 kHz + 20 kHz = 1570 kHz. I would think you might also be putting a whistle on 1530, if there is a station there.
What kind of transmitter are you using? Does this occur even when you are not modulating?
In my area there is a licensed station on 1530. I can hear a tone on stations at 1600 and also 1460. These are 70 kHz removed from 1530. I suspect their transmitter uses something called "Pulse Duration Modulation" which involves a 70 kHz squarewave which has the duty cycle varied with the modulation. The 70 kHz is supposed to be filtered out, but sometimes gets through.
If your console or processing is feeding 20 kHz noise into the transmitter, it will cause the situation you describe. Try disconnecting the input to the transmitter to see if the whistle on 1570 goes away.
RadioMonkey, you are very evolved as monkeys go. Your idea has proven to be the key to the whistle.
The modulation for 1550 AM is being picked up on an FM stereo radio tuned to a Wholehouse at 107.1. Indeed, when the modulation is cut, the whistle disappears. Bingo.
The Wholehouse is set for mono, but the FM radio must generate a 20kHz tone on its own. I'll study it some more now that I know what to study.
But the other idea from Radio8Z, about images, is also something I need to sharpen up about, so thank you for that.
The Wholehouse transmitter must still be generating the 19 kHz stereo pilot tone, even in mono mode. With 19 kHz modulating your transmitter, you have a sideband at 1550 + 19, or 1569 kHz...this will show up as a 1000 Hz tone when tuned to the 1570 station (difference between 1569 kHz and the 1570 kHz station's carrier).
You could try a parametric equalizer on your transmitter's input and tune the high adjustment until the tone or whistle goes away, or find a cheap NRSC 10 kHz filter. The NRSC filter basically brickwalls everything above 10 kHz...either of these solutions may be cost-prohibitive, however.
Thank you for the suggested solution to filter the (probable) pilot tone being fed to the AM input, but tell me what is a "NRSC 10 kHz filter." What I mean is, I know what a filter is... it's a low-pass filter... but what is NRSC?
And if it's "cheap," why would it be "cost prohibitive?"
There, I've put you on the spot. Try to get out of that!
Oh, I'm being so silly since the radiation.
That's all behind us now, but I am using this thread to avoid opening yet another new one.
My 1550 AM was taken off the air about a week ago when I noticed strange behavior where the modulation level seemed to jump around, sometimes being distorted.
This is an AMT3000 transmitter modified for use with a loading coil which I built and tuned to resonance, but in all the experimentation some of the connections were done in haste and might have been failing, not to mention a lot of dust down there under the corner of the desk.
Removed and cleaned everything, carefully re-installed, and now 1550 is back in action with a stable and beautifully modulated signal.
The Part 15 life is the way to absolute contentment and joy.