Clean frequencies

[ajhenderson]

I was filling in my table of available channels in the Phoenix area this week and have come across something unusual. In the x-band, where you have advised Part 15s to go, there are a few stations that, by day only, seem to be repeating lower frequencies with great clarity. For example, on 1620 I heard 620. On another x-band freq, I heard 1010, and at times, another English langauge station. Seeing that none of the frequencies that were coming in on the upper band have a real mathematical formula to them, should I assume this is anything similar to a second harmonic? There are no stations that, multiplied by 2, would equal anything from 1610-1700. Just curious.

 

[carlvanorden]

> I was filling in my table of available channels in the> Phoenix area this week and have come across something> unusual. In the x-band, where you have advised Part 15s to> go, there are a few stations that, by day only, seem to be> repeating lower frequencies with great clarity. For example,> on 1620 I heard 620. On another x-band freq, I heard 1010,> and at times, another English langauge station. Seeing that> none of the frequencies that were coming in on the upper> band have a real mathematical formula to them, should I> assume this is anything similar to a second harmonic? There> are no stations that, multiplied by 2, would equal anything> from 1610-1700. Just curious.> It's an excellent question and I'm not offering a complete answer on it.....however I've noticed this as well......It doesn't happen a lot and it doesn't happen everyday, but I will say this:........do you live near any of these stations??....if so, the second harmonic you are hearing could be a a third......I live near 2 FM's and 1AM...sometimes they appear on other channels!...My part 15 AM when it is experiencing a very good day appears on several channels in my yard, but not outside of it....same thing with my part 15FM.....yesterday it was on 5 distinct channels, but of course strongest on my frequency....in the field it couldn't be heard but on my frequency.The only other possibility is that the stations are simulcasted.....................for part 15 AM, because the antenna is SO small, we do indeed recommend to be at the high end of the dial.....even the coil I build, at the very best, tunes as low as 1100AM....and it gets risky there because it is difficult to promise someone who lives 300 miles from you that it will work at their chosen or desired frequency......The high end of the dial works best for part 15am because the antenna comes closer to the wavelength....there is the ability to have a nightwave as well at the high end of the dial. Don't forget; 1,000 watts at 550am is nearly as good as 50,000 watts at 1600AM, therefore, the thinking is...."I have 1/10th of a watt; I'm best at the low end of the dial."...that is rational thinking but it doesn't work that way, but the other way. It is best to think that there are lower powered stations at the high end of the band and that you have a better shot at a clearer signal there than you do any higher up......All things being equal, when I test my antennas, I try a lot of things and I found I consistantly get a much better signal at 1600am then I get at 1400am, operating at a very legal standpoint......hope that answers some of your questions!!<P ID="signature">______________
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[KeithE]

> I was filling in my table of available channels in the
> Phoenix area this week and have come across something
> unusual. In the x-band, where you have advised Part 15s to
> go, there are a few stations that, by day only, seem to be
> repeating lower frequencies with great clarity. For example,
> on 1620 I heard 620. On another x-band freq, I heard 1010,
> and at times, another English langauge station. Seeing that
> none of the frequencies that were coming in on the upper
> band have a real mathematical formula to them, should I
> assume this is anything similar to a second harmonic? There
> are no stations that, multiplied by 2, would equal anything
> from 1610-1700. Just curious.

On some radios (including my car radio), 2nd-order intermod can happen between some stations, especially if your location is close enough to both transmitters to get extrememly strong signals. 2nd-order intermod frequencies are the sum and the difference of the signals in question (f1 + f2 or f2 - f1, where f2 > f1).

Using Phoenix AM stations as examples, I can hear a combination of KTAR 620 and KDUS 1060 on 1680. You could hear a combination of KTAR 620 and KXXT 1010 on 1630 but it isn't as likely given the distance between the transmitters. A combination of KFYI 550 and KMIK 1580 shows up on 1030. You can figure others from these examples.