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Old March 17th 14, 08:15 PM posted to rec.radio.shortwave
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Posts: 16
Default FM radio reception at ~24MHz?

Hi all, I picked up an SDR radio dongle, and have been playing around
with receiving. As I am playing around with it today, I am noticing
something odd (to me).

We have a college station locally that broadcasts at 88.1MHz FM. As
with many college radio stations, it has a very weak signal, but I can
just pull it in with rabbit ear antennas.

As I was poking around at ~24MHz, and I was able to pull this same
station in at 23.645MHz. Even more odd, I was getting a much better
signal at 23.645MHz than at the 88.1MHz "official" frequency.

Additionally, as I scanned around, I also found all of the other
stations doing this as well; 91.9MHz, which is too weak to tune in, I
can hear perfectly at 28.035MHz. I can also tune in to 96.1MHz at
30.240MHz, 105.5MHz at 30.325, and so forth.

So here's my question, is this something that is "normal" in radio, or
does this instead point to some peculiarity with my radio/software setup.

Thanks for any suggestions,

Jon

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Old March 17th 14, 09:29 PM posted to rec.radio.shortwave
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First recorded activity by RadioBanter: Mar 2008
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Default FM radio reception at ~24MHz?

On Mon, 17 Mar 2014, Jon Danniken wrote:

Hi all, I picked up an SDR radio dongle, and have been playing around
with receiving. As I am playing around with it today, I am noticing
something odd (to me).

We have a college station locally that broadcasts at 88.1MHz FM. As
with many college radio stations, it has a very weak signal, but I can
just pull it in with rabbit ear antennas.

As I was poking around at ~24MHz, and I was able to pull this same
station in at 23.645MHz. Even more odd, I was getting a much better
signal at 23.645MHz than at the 88.1MHz "official" frequency.

Additionally, as I scanned around, I also found all of the other
stations doing this as well; 91.9MHz, which is too weak to tune in, I
can hear perfectly at 28.035MHz. I can also tune in to 96.1MHz at
30.240MHz, 105.5MHz at 30.325, and so forth.

So here's my question, is this something that is "normal" in radio, or
does this instead point to some peculiarity with my radio/software setup.

Thanks for any suggestions,

Jon

FM stations apparently can use something around 26 MHz for remote feeds,
but I don't know the details other than it has come up in one of the
newsgroups before, someonehearing FM broadcasting in the wrong place.

Do those things do any conversion, or is it straight to baseband? If it
converts to some other frequency first, then maybe you are seeing an
image. And since those things aren't so well tuned, so image rejection
might not be so great. That doesn't explain why the wrong frequency would
have a stronger signal.

Find out if it converts to an IF first, then do some math.


MIchael

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Old March 17th 14, 10:10 PM posted to rec.radio.shortwave
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First recorded activity by RadioBanter: Aug 2013
Posts: 3
Default FM radio reception at ~24MHz?

On Mon, 17 Mar 2014 13:15:16 -0700, Jon Danniken
wrote:

Hi all, I picked up an SDR radio dongle, and have been playing around
with receiving. As I am playing around with it today, I am noticing
something odd (to me).

We have a college station locally that broadcasts at 88.1MHz FM. As
with many college radio stations, it has a very weak signal, but I can
just pull it in with rabbit ear antennas.

As I was poking around at ~24MHz, and I was able to pull this same
station in at 23.645MHz. Even more odd, I was getting a much better
signal at 23.645MHz than at the 88.1MHz "official" frequency.

Additionally, as I scanned around, I also found all of the other
stations doing this as well; 91.9MHz, which is too weak to tune in, I
can hear perfectly at 28.035MHz. I can also tune in to 96.1MHz at
30.240MHz, 105.5MHz at 30.325, and so forth.

So here's my question, is this something that is "normal" in radio, or
does this instead point to some peculiarity with my radio/software setup.

Thanks for any suggestions,

Jon


The 'front' end of many relatively low cost, wide range receivers has
very poor selectivity. The output of the Mixer stage is the SUM and
the DIFFERENCE between the local oscillator frequency and the incoming
signal. The result is when you tune a frequency, you are really tuning
to two different frequencies. The difference between one signal and
the IF and the Sum of another signal and the IF, Usually there is some
sort of pre-selector, bandpass filter or semi-tuned RF amplier that
effectively blocks the unwanted frequency, so there is no energy in
one of the two possibly tuned frequencies

For example if you have a conventional FM receiver, traditionally the
Intermediate frequency is 10.7 Mhz. So if you tuned the receiver

Another strategy is to 'up convert' instead of down convert. This puts
much more spectrum between the sum and difference frequency.

All that your experience says is that SDR probably has poor front end
selectivity.
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Old March 17th 14, 11:48 PM posted to rec.radio.shortwave
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First recorded activity by RadioBanter: Aug 2013
Posts: 35
Default FM radio reception at ~24MHz?

On 2014-03-17 21:29:35 +0000, Michael Black said:

On Mon, 17 Mar 2014, Jon Danniken wrote:

Hi all, I picked up an SDR radio dongle, and have been playing around
with receiving. As I am playing around with it today, I am noticing
something odd (to me).

We have a college station locally that broadcasts at 88.1MHz FM. As
with many college radio stations, it has a very weak signal, but I can
just pull it in with rabbit ear antennas.

As I was poking around at ~24MHz, and I was able to pull this same
station in at 23.645MHz. Even more odd, I was getting a much better
signal at 23.645MHz than at the 88.1MHz "official" frequency.

Additionally, as I scanned around, I also found all of the other
stations doing this as well; 91.9MHz, which is too weak to tune in, I
can hear perfectly at 28.035MHz. I can also tune in to 96.1MHz at
30.240MHz, 105.5MHz at 30.325, and so forth.

So here's my question, is this something that is "normal" in radio, or
does this instead point to some peculiarity with my radio/software setup.

Thanks for any suggestions,

Jon

FM stations apparently can use something around 26 MHz for remote
feeds, but I don't know the details other than it has come up in one of
the newsgroups before, someonehearing FM broadcasting in the wrong
place.

Do those things do any conversion, or is it straight to baseband? If
it converts to some other frequency first, then maybe you are seeing an
image. And since those things aren't so well tuned, so image rejection
might not be so great. That doesn't explain why the wrong frequency
would have a stronger signal.

Find out if it converts to an IF first, then do some math.


MIchael


It is also common to find FM broadcast feeds up around 460 MHz.

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Old March 18th 14, 12:21 AM posted to rec.radio.shortwave
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First recorded activity by RadioBanter: Mar 2014
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Default FM radio reception at ~24MHz?

On 03/17/2014 02:29 PM, Michael Black wrote:

FM stations apparently can use something around 26 MHz for remote feeds,
but I don't know the details other than it has come up in one of the
newsgroups before, someonehearing FM broadcasting in the wrong place.

Do those things do any conversion, or is it straight to baseband?


From my understanding, the tuner chip sends out an 8MHz (I think)
"window" of information at zero-IF to the modulator/interface chip
(RTL2832U). I did see a 28.800 crystal on mine when I opened it up to
put in some shielding, though.

Unfortunately I am rather new at all of this, so I'm not really up to
speed on radio stuff yet.

If it
converts to some other frequency first, then maybe you are seeing an
image. And since those things aren't so well tuned, so image rejection
might not be so great. That doesn't explain why the wrong frequency
would have a stronger signal.

Find out if it converts to an IF first, then do some math.


Thanks,

Jon



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Old March 18th 14, 12:22 AM posted to rec.radio.shortwave
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First recorded activity by RadioBanter: Mar 2014
Posts: 16
Default FM radio reception at ~24MHz?

On 03/17/2014 03:10 PM, matt weber wrote:

The 'front' end of many relatively low cost, wide range receivers has
very poor selectivity. The output of the Mixer stage is the SUM and
the DIFFERENCE between the local oscillator frequency and the incoming
signal. The result is when you tune a frequency, you are really tuning
to two different frequencies. The difference between one signal and
the IF and the Sum of another signal and the IF, Usually there is some
sort of pre-selector, bandpass filter or semi-tuned RF amplier that
effectively blocks the unwanted frequency, so there is no energy in
one of the two possibly tuned frequencies

For example if you have a conventional FM receiver, traditionally the
Intermediate frequency is 10.7 Mhz. So if you tuned the receiver

Another strategy is to 'up convert' instead of down convert. This puts
much more spectrum between the sum and difference frequency.

All that your experience says is that SDR probably has poor front end
selectivity.


Thanks Matt.

Jon

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Old March 18th 14, 04:59 AM posted to rec.radio.shortwave
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First recorded activity by RadioBanter: Jun 2006
Posts: 487
Default FM radio reception at ~24MHz?

matt weber wrote:
All that your experience says is that SDR probably has poor front end
selectivity.


Almost none. The dongle was designed to pick up megawatt digital TV stations
with short antennas (about 4-6 inches) antennas. Using it as an SDR is
a happy accident.

The ones sold as DVB-T dongles come with software for decoding digital
TV broadcasts, some come with digital audio broadcast decoding software, most
don't. The most popular E4000 receiver dongle did not come with FM broadcast
reception software.

Geoff.

--
Geoffrey S. Mendelson, N3OWJ/4X1GM/KBUH7245/KBUW5379

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Old March 18th 14, 05:16 AM posted to rec.radio.shortwave
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First recorded activity by RadioBanter: Mar 2008
Posts: 618
Default FM radio reception at ~24MHz?

On Tue, 18 Mar 2014, Geoffrey S. Mendelson wrote:

matt weber wrote:
All that your experience says is that SDR probably has poor front end
selectivity.


Almost none. The dongle was designed to pick up megawatt digital TV stations
with short antennas (about 4-6 inches) antennas. Using it as an SDR is
a happy accident.

To be fair, it's not related to an SDR, but to a specific and cheap
implemtnation of the idea. One can argue that a lot of low end wide band
receivers don't have good front end selectivity, either.

If there's a conversion to a high IF, image rejection can be done with a
low pass filter. That's true with any receiver that converts to a high
enough first IF. I seem to recall from a description that the Racal
receiver that used the Wadley Loop had both a good low pass filter and a
traditional LC front end, and one could switch between the two. The low
pass was good enough for the image rejection, but of course meant all the
signals from DC to 30MHz or wherever the LPF cutoff was seen by the first
tube, so it had to handle all of that. Not necessarily a good thing.

Michael

The ones sold as DVB-T dongles come with software for decoding digital
TV broadcasts, some come with digital audio broadcast decoding software,
most don't. The most popular E4000 receiver dongle did not come with FM
broadcast reception software.

Geoff.

--
Geoffrey S. Mendelson, N3OWJ/4X1GM/KBUH7245/KBUW5379


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Old March 18th 14, 12:39 PM posted to rec.radio.shortwave
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First recorded activity by RadioBanter: May 2009
Posts: 952
Default FM radio reception at ~24MHz?


matt weber wrote:
All that your experience says is that SDR probably has poor front end
selectivity.


On 3/18/2014 12:59 AM, Geoffrey S. Mendelson wrote:

Almost none. The dongle was designed to pick up megawatt digital TV stations
with short antennas (about 4-6 inches) antennas. Using it as an SDR is
a happy accident.


A great tutorial / construction article on how to use the dongle as a DC
to daylight software defined radio (SDR):

QST magazine, January 2013, pp 30-35.

If you know a member of the ARRL, they can print a copy of this article.


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Old March 18th 14, 01:56 PM posted to rec.radio.shortwave
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First recorded activity by RadioBanter: Sep 2012
Posts: 327
Default FM radio reception at ~24MHz?

On 03/17/2014 01:15 PM, Jon Danniken wrote:
Hi all, I picked up an SDR radio dongle, and have been playing around
with receiving. As I am playing around with it today, I am noticing
something odd (to me).

We have a college station locally that broadcasts at 88.1MHz FM. As
with many college radio stations, it has a very weak signal, but I can
just pull it in with rabbit ear antennas.

As I was poking around at ~24MHz, and I was able to pull this same
station in at 23.645MHz. Even more odd, I was getting a much better
signal at 23.645MHz than at the 88.1MHz "official" frequency.

Additionally, as I scanned around, I also found all of the other
stations doing this as well; 91.9MHz, which is too weak to tune in, I
can hear perfectly at 28.035MHz. I can also tune in to 96.1MHz at
30.240MHz, 105.5MHz at 30.325, and so forth.

So here's my question, is this something that is "normal" in radio, or
does this instead point to some peculiarity with my radio/software setup.

Thanks for any suggestions,

Jon


Now you get to experience the joy of building a crude preselector. A 20
dB pad might work, try that first.


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