Narrowband AM needed on future scanners

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bryan_herbert

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Looks like 8.33 kHz channel spacing is starting to catch on here in the United States. Ive been having trouble hearing pilots, their audio is low and Im having to crank the volume. While scouring the FCC database I noticed Scaled Composites now has 8.33 kHz frequencies on their license and my local airports licenses have been modified for the emission 6K00A3E. When I use my SDR, Im having to set the bandwidth to 6 kHz instead of 15 kHz.

So whatever you folks at Uniden are cooking up, make sure narrowband AM is available for all aviation bands
108-136.975, 138-144, 148-150.7, and 225-400 MHz.
 

ko6jw_2

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First of all the 8.33Mhz spacing is going to be standard in Europe on January 1, 2018. It has not been adopted in the US as of now.

Second, even my old BC396T can do 8.33Mhz steps at least in the search mode.
 

jonwienke

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Looks like 8.33 kHz channel spacing is starting to catch on here in the United States. Ive been having trouble hearing pilots, their audio is low and Im having to crank the volume.
Your problem has nothing to do with narrowbanding, but rather the fundamental difference between AM and FM. AM is NOT like FM; the demodulated volume level is directly proportional to received signal strength, not the deviation of the modulation. With AM, the farther the transmitter is from you, the weaker the signal, the lower the carrier amplitude, and the lower the demodulated audio level. With FM, as long as the signal level is far enough above the noise so that the receiver can see the frequency deviation accurately, the demodulated audio level will be fairly constant.

You're totally barking up the wrong tree.
 

bryan_herbert

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A mandatory cutover date in Europe does not mean it can't be used now here in the US, which it is.

The aircraft in question are within 20-miles of me talking to the approach controller, they're not hundreds of miles away dropping below the horizon, the signal meter on my receiver is slammed. I confirmed they're using narrowband with my SDR receiver.

If you're trying to listen to 121.7083 and the AM filter in your radio is 15 kHz, you are going to pick up any aircraft or airports operating on 121.7000 on top of the traffic you're trying to hear on 121.7083. In order to eliminate adjacent channel interference, the filter in your radio has to be set below 8.33 kHz, in this case 6 kHz is being used.

Railfans learned this the hard way after the FRA mandated narrowbanding. They would try to monitor the newer channels with the wider filters on their older radios but the traffic was low in audio and they were being slammed by adjacent channel interference.

Experiment and see for yourself, set the step in your scanner to 8.33 kHz and tune to a busy aviation freq. While someone is talking, go up one channel, I guarantee you will hear that same station. You hear it because the AM filter is set to 15 kHz and not 6 kHz as mandated for 8.33 kHz spacing
 

bryan_herbert

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Im not saying 8.33 kHz channels are being used for ATC, I never said that, Im saying narrowband radios are being used for voice on ATC freqs. Buy an SDR receiver, search through all of the aviation bands I listed above, and you'll see many aircraft radios are transmitting voice using only 6 kHz of bandwidth.

Even the FCC rules you posted state 8.33 kHz (narrowband) can be used for flight testing between 123.125 and 123.575 and Im seeing it every day here in Las Vegas.
 

nd5y

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You are confusing occupied bandwidth with channel spacing.
If you limit the maximum audio frequency in an AM transmitter to 3 kHz you get 6 kHz bandwidth. As far as I know aircraft radios have always been like that.
If the audio or user's voice doesn't have a lot of high frequency near 3 kHz then the bandwidth you see on a SDR will be less than 6 kHz.
 

UPMan

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If I recall my radio theory correctly, the bandwidth of an AM signal twice that of the highest audio transmitted.

EDIT: I see Tom said just that while I was typing (well, I started typing a reply, got called away, came back, finished...).
 

UPMan

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And just found a reference for air band modulation that says that the audio modulation shall not fall outside of the range from 300 to 2,700 Hz (this is inclusive of 25, 50, 100, and 8.33 kHz channel spacing). So, the maximum occupied bandwidth of any AM airband transmission should be no more than 5.4 kHz, regardless of the frequency spacing.
 

Ubbe

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the maximum occupied bandwidth of any AM airband transmission should be no more than 5.4 kHz, regardless of the frequency spacing.
And Uniden scanners have a suitable filter for that bandwidth and not only for NFM12,5 and FM25KHz bandwidths?

/Ubbe
 

ko6jw_2

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I think there is some confusion here between bandwidth and channel spacing. I looked up the specifications of ICOM avionics radios that are capable of both 25Khz and 8.33Khz channel spacing. In either mode the bandwidth is virtually the same. That is 2.775 to 3.000Khz. These represent the -6db points of the bandpass filter. The difference is in the -60db bandpass which is much narrower in the 8.33Khz mode because of the closer channel spacing. However, in both cases the transmitted bandwidth is identical for all intents and purposes. My guess is that most scanners do not have this narrow a filter. More like 5Khz would be my guess. This is not something that needs to be corrected for casual listening. You won't hear the difference. I have optional Collins filters is my ham radios for SSB and CW. These filters are somewhat expensive and are not needed in a scanner.

For those who insist on narrow bandwidth the option to purchase dedicated air band radios is always available.

The 8.33Khz standard was first introduced in Europe around 1999. It will be mandatory in 2018. It effectively triples the number of channels available. While this is a good idea, a better idea would be to use SSB or digital modes. The use of AM is inefficient and antiquated. However, when you consider the sheer number of radios is use, it is unlikely that there will be a change in the near future.
 

Ubbe

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Using AM is an easy way of handling two transmissions on the same frequency and still hear both. So I guess its mostly of safety reasons they have opted to stay with AM.

The BC780 says:

FM/AM Mode IF:450KHz (BW:+/-10KHz)
NFM Mode IF:450KHz (BW:+/-6KHz)

That's 20KHz bandwidth for FM/AM mode and 12KHz for NFM which seems alright for FM modulation but not so much for AM.

No data presented for IF filters in BCDx36HP manual.

/Ubbe
 
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jonwienke

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I saw at least 3 IF filters when I opened my 436 up to do the internal GPS mod.

Upman has stated on multiple occasions that there are separate IF filters for FM and NFM. I don't know if AM has its own filter, but from Upman's comments, I suspect so.
 

Ubbe

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The mode selection in BCDx36HP includes WFM and FMB and there are more than 5 IF filters in a BC780, one for 45MHz, two for 10.7MHz broadcast FM and 2 for 450KHz. There is also 2 filters in the first high frequency IF.

UPman only state the fact how it should be, not how it actually are done in Uniden scanners.

/Ubbe
 

jonwienke

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No, he specifically stated that the x36 scanners have separate IF filters for FM and FMN, and that is why the x36 scanners perform better receiving FMN.
 

ShyFlyer

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So I guess its mostly of safety reasons they have opted to stay with AM.
Aviation has stayed with AM for a number of reasons. The primary reason is that it works. Another reason is that moving to a new/different form of communication involves a massive undertaking that would take decades to fully realize. Even with the incredible benefits of DataLink transmissions, DataLink won't be in widespread use for at least another 10 years.
 

gary123

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Technically speaking the actual AM siganal does not use any bandwidth even when modulated. In theory just the AM carrier freq is all that is required. However when the carrier freq is modulated you do get the sidebands that are created and transmitted. These could be filtered out but it is not necessary or cost effective to do so. The sidebands will extend from the carrier +/- by the maximum modulated freq. This is why the channels are spaced far enough apart so that one AM siganal does not cross into the bandwidth of another.
 

Ubbe

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No, he specifically stated that the x36 scanners have separate IF filters for FM and FMN, and that is why the x36 scanners perform better receiving FMN.
All scanners since 20 years back have seperate FM and NFM filters.
I where refering to the current discussion about AM bandwidth and the lack of proper filters for that mode.

/Ubbe
 
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