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"Low" FM channels.


TN.Frank
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So I'm surfing around and I find some music stations that come in under Wide FM(can't get em' with AM, go figure) down at 27.340.000, 28.150.000 and 49.835.000. FM, as far as I know starts at 88MHz and goes through 108MHz or there abouts. So, what are these stations doing down so low? Also, not to digress but I did catch some talk at 128.175.000-870 but only for a second. Need to keep checking in that range to see what's there.

Also, after doing the rtfsdr scan I found a fairly strong and constant tone at 432.000.400, no idea what that is.

Edited by TN.Frank
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So I'm surfing around and I find some music stations that come in under Wide FM(can't get em' with AM, go figure) down at 27.340.000, 28.150.000 and 49.835.000. FM, as far as I know starts at 88MHz and goes through 108MHz or there abouts. So, what are these stations doing down so low? Also, not to digress but I did catch some talk at 128.175.000-870 but only for a second. Need to keep checking in that range to see what's there.Also, after doing the rtfsdr scan I found a fairly strong and constant tone at 432.000.400, no idea what that is.

http://www.hamradio.lt/bp-432.htm

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If you want to reach AM, you'll probably need one of these http://www.nooelec.com/store/ham-it-up-v1-0-rf-upconverter-for-software-defined-radio.html

edit:

and if you want to reach up into 2.4+GHz something like http://www.aliexpress.com/item/2012-best-selling-L-O1998MHZ-MMDS-down-converter/670265064.html

Edited by midnitesnake
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I've been poking around 49,800,000 with NFM on. I came by a baby monitor. That's with the stock antenna, so it must be somewhere close by. By the looks of it, RTL SDRs might pick them up on 49 MHz, 902 MHz. It gave me the creeps, to be honest. Just thought it might be of interest to you.

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I did hear the call letters of the station broadcasting in the 49MHz range, it's actually a local FM radio station that lives at 102.5FM, Wow Country. Why it's also down at 49MHz is a really puzzle. I've also found out NOAA weather station, it's over at 162.393.000 even though they say they're broadcasting at 162.400.000.

I was also listening to a couple guys talk smack to each other yesterday in the 27MHz CB range, it was a hoot, LOL

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  • 3 weeks later...

I did hear the call letters of the station broadcasting in the 49MHz range, it's actually a local FM radio station that lives at 102.5FM, Wow Country. Why it's also down at 49MHz is a really puzzle. I've also found out NOAA weather station, it's over at 162.393.000 even though they say they're broadcasting at 162.400.000.

I was also listening to a couple guys talk smack to each other yesterday in the 27MHz CB range, it was a hoot, LOL

I got my dongle today, after reading around I decided to look for the local weather station it broadcasts around 162.418 here even though it claims to broadcast on 162.425. This makes me wonder if all weather stations broadcast 7kHz below what they claim.

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The RTL-SDR is crazy good for the price but there are some things about it that just aren't that good. I would suggest searching for calibrating a rtl-sdr. The weather channels are typically pretty good at being on frequency and if you know what it is transmitting at you offset your sdr to match that frequency. The other problem is that these things have poor filtering and shielding. You may be seeing two signals (internal of the sdr mixed with external signal maybe) and end up with a signal showing up on your screen that is out of place and in reality not at that frequency. This could explain a FM station down that low. Also if you are near the Broadcast transmitter it could also just be spurious transmissions from the radio station that you are picking up because you are so close.

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IIRC way back from Class at DeVry a station that broadcasts as a certain freq. will also have ghost freqs. at intervals of it's original broadcast freq. So 102.5 FM will also come in at half of that freq roughly and twice that freq. Like I said, this was back in '83 and I'm a bit fuzz on this but I do remember something about antennas and picking up signals at different sizes of antenna so 1/4 meter, 1/2 meter, 1 meter, ect can pick up the same freq. IIRC.

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What you are talking about is the First, second, third, etc.... harmonic frequencies. This is exactly what I was referring to. Normally you don't worry about it because these harmonics are being broadcast at such a low power compared to the primary that you wont see them. If you are near a high power broadcast transmitter though you may end up picking up on these. There can also be things such as your VFO mixing with A RF signal and producing a image. RF can be funny like that and these cheap SDR's don't have much hardware to combat that sort of thing.

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The RTL-SDR is crazy good for the price but there are some things about it that just aren't that good. I would suggest searching for calibrating a rtl-sdr. The weather channels are typically pretty good at being on frequency and if you know what it is transmitting at you offset your sdr to match that frequency. The other problem is that these things have poor filtering and shielding. You may be seeing two signals (internal of the sdr mixed with external signal maybe) and end up with a signal showing up on your screen that is out of place and in reality not at that frequency. This could explain a FM station down that low. Also if you are near the Broadcast transmitter it could also just be spurious transmissions from the radio station that you are picking up because you are so close.

It would make sense to calibrate, however as I pointed out, all other channels are exactly where they should claim to be broadcasting. If I were to calibrate based on the weather channel assuming it was in fact broadcasting at the frequency it should, would then suggest that all other stations were in fact broadcasting off their frequency by that much. It seems more reasonable that the weather stations might be off, than that every station except the weather station is on a wrong frequency.

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It could be they are way off freq. The only other thing I would suggest would be to check with other narrow band stations (you may have done this already) rather than broadcast wide band FM. The reason for this is that it is easier to find the center frequency on a narrow band channel giving better resolution. Again you may have already done this and if so you found a station way out of band, a good find if so!

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  • 1 month later...

If you hear radio / TV but not other noises at 49MHz, I'd bet on something like wireless headphones or wireless speakers. A harmonic of the original wide FM signal would be wider bandwidth than regular FM and would sound distorted (over-modulated).

http://www.alibaba.com/showroom/wireless-headphone-%252849-mhz%2529.html

https://www.yumpu.com/ro/document/view/18167308/cew112-49mhz-ms-customer-care

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It's not wifi, it's wireless (in the same way not all animals are cows). These devices typically use 900mhz, the frequency of your cordless home phone.

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  • 2 weeks later...

Interesting. Maybe they changed frequencies or their equipment is off :P.

It's not uncommon for small transmitters (and SDR's) to drift around. You can be sure the commercial power stations are locked to one or more frequency standards like a GPS disciplined ovenized crystal. The little guys aren't tuned as often or as critical and 7 kHz is slightly out of spec but not a huge deal in the grand scheme of things. Accuracy depends on how you measure the center frequency. If they ever put up a 1 kHz tone, measure the deviation of the carrier 3dB down on each side of the carrier lobe and divide in half. If the center is off their assigned frequency, you might have a future career at the FCC, they could use a few people like you!

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