Air Distress - Now 121.505 MHz ?

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Re: Air Distress - Now 121.505 MHz ?

Postby mickydt » Wed Oct 17, 2018 2:26 pm

both aor and alinco added the 8.33 step
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Re: Air Distress - Now 121.505 MHz ?

Postby AndrewIrvine » Wed Oct 17, 2018 10:08 pm

The Uniden 3500 does 8.33, has done for over 10 years. I use Freescan, it doesn't do 8.33, but it accepts what you put in without rounding it off.
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Re: Air Distress - Now 121.505 MHz ?

Postby m0lsx » Wed Oct 17, 2018 11:23 pm

The problem is. And it's one we may know, but it's one we need to make new comers aware of.
An ability to accept 8.33. Does not mean it has a 8.33 khz bandwidth ability.
So 8.33khz is all but meaningless.
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Re: Air Distress - Now 121.505 MHz ?

Postby Minus1 » Thu Oct 18, 2018 8:29 am

The Uniden UBC125XLT sort of gets right,


In what way?
It turns 121.705 into 121.7083, which is the wrong frequency, and it doesn't even have narrow bandwidth anyway.
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Re: Air Distress - Now 121.505 MHz ?

Postby Scan125 » Thu Oct 18, 2018 12:14 pm

Minus1 wrote:
The Uniden UBC125XLT sort of gets right,


In what way?
It turns 121.705 into 121.7083, which is the wrong frequency, and it doesn't even have narrow bandwidth anyway.


Correct! Read my post link. The UBC125XLT gets most of them right, 16 out of 20. This is why Scan125 has an Air Band Correction function to solve this problem.

As for narrow band FM for air band then the UBC125XLT IS NARROW BAND FM. Sadly Uniden chose to label NFM as FM on this particular model. On the US BC125AT they have both FM and NFM.

To be quite honest I get the impression that just about every scanner manufacturer can not write decent, clear and accurate User Manuals and this just makes matters far worse. Couple this with other errors and it just becomes totally messed up out there.
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Re: Air Distress - Now 121.505 MHz ?

Postby Mark5R » Thu Oct 18, 2018 12:15 pm

Minus1 wrote:The numbers given for 8.33 kHz channels are CHANNELS they are NOT frequencies. They are NOT 5 kHz steps.

"128.855" is an 8.33 kHz bandwidth channel using a frequency of 128.85 MHz.
"128.85" is a 25 kHz bandwidth channel also using 128.85 MHz


I do hear what you are saying but at the risk of sounding dumb I still don't get it. How can 128.855 be a channel name? It makes no sense.

There are one or two pages NATS have updated to the new steps. Taking Leeds Bradford as an example they list the following

LEEDS APPROACH 134.580 MHz

Surely they are referring to the frequency in MHz as 134.580 MHz and not a channel name?
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Re: Air Distress - Now 121.505 MHz ?

Postby Scan125 » Thu Oct 18, 2018 6:46 pm

Mark5R wrote:
Minus1 wrote:The numbers given for 8.33 kHz channels are CHANNELS they are NOT frequencies. They are NOT 5 kHz steps.

"128.855" is an 8.33 kHz bandwidth channel using a frequency of 128.85 MHz.
"128.85" is a 25 kHz bandwidth channel also using 128.85 MHz


I do hear what you are saying but at the risk of sounding dumb I still don't get it. How can 128.855 be a channel name? It makes no sense.

There are one or two pages NATS have updated to the new steps. Taking Leeds Bradford as an example they list the following

LEEDS APPROACH 134.580 MHz

Surely they are referring to the frequency in MHz as 134.580 MHz and not a channel name?


We are locked into a bit of history wrt to radios. If we ignore what we think and go back then in many dedicated "radio applications" (military/air/other) sets were often produced to be switched to say Band/Channel A or B or I or II etc. Within reason the operator in the field needs to be able to tune to a given frequency by "switching" something as opposed to some variable know twiddling. Old radios just had some for of crude pointer with dubious calibration.

In reality an pilot does not care if Channel 123 is on 123.000 or 321.000 MHz. They just need a quick and accurate method of getting on the right frequency. With the advent of digital thumb wheel tuning then the old 25MHz separation between channels was a straight forward 6 thumb wheel control, nnn.nnn Example 123.025, 123.050, 123.075 and 123,100. Notice the clean alignment / separation of channel/frequency. For pilots/ATC the two terminologies are interchangeable. You will also note nobody talks about frequency or channel. Just, eg. contact Brest on nnn.yyy

To this day modern aircraft transceivers still only have the nnn.nnn. So how do you set a frequency of/aligned to 8.33kHz. This would require EIGHT thumb wheels, eg. 118,008,33 (0)

The solution is to keep just the 6 thumb wheels and map the unused wheel settings/gaps (used = n00, n25, n50 and n75, unused = n05, n10, n15, n20, n30, n35, n40, n45, n55, n60, n65, n70, n80, n85, n90 and n95) to the nearest 8.33kHz step.

Before I explain the next bit let us look at why 8.33kHz. Quite simple because 25 divided by 8.333333333 is 3. They could have chosen as 12.5kHz separation but that would have reduced the number of channels available. Or 25/4 = 6.25kHz. However the smaller the separation the close the channels and one gets in set to set calibration and alignment issues and discrimination ability between channels.

Now back to 8.33 then we only need to have two additional settings on the thumb wheels between each of the n00, n25, n50 and n75 settings. So taking the n00 to n25 gap one has the option of n05, n10, n15 and n20 for just only two required frequencies of 8.33 and 16.66. So they chose n10 and n15. But what about n05. To avoid a dead channel/setting n05 maps back to n00.

So the terminology of channel as opposed to frequency makes absolute sense based on long history of the past and that one can not dial 8.33 let alone 8.333333333333333333333333333.

For ATC "Contact Brest on nnn.xyz" is clear and precise. For the pilot dialling in nnn.xyz is also dead simple. What frequency is actually used is basically irrelevant to both parties and they are on ones who count.

So when LEEDS APPROACH 134.580 MHz is listed the actual frequency is 134.583333333333 MHz.

The pilot will just set 134.580 on his dials and let the radio kit do whatever it does.

Hope this sort of clarifies what is going on.
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Re: Air Distress - Now 121.505 MHz ?

Postby radiostationx » Thu Oct 18, 2018 9:40 pm

Excellent explanation Nigel, 25mhz into 3 equally spaced "channels".
Reminds me of trying to express π 22/7 as a decimal (an after school detention favourite from our maths teacher) he would let you go on for the full hour , no calculator but you could use log tables /slide rule. :wink:
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Re: Air Distress - Now 121.505 MHz ?

Postby m0lsx » Fri Oct 19, 2018 8:38 am

Scan125 wrote:
Before I explain the next bit let us look at why 8.33kHz. Quite simple because 25 divided by 8.333333333 is 3. They could have chosen as 12.5kHz separation but that would have reduced the number of channels available. Or 25/4 = 6.25kHz. However the smaller the separation the close the channels and one gets in set to set calibration and alignment issues and discrimination ability between channels.

Now back to 8.33 then we only need to have two additional settings on the thumb wheels between each of the n00, n25, n50 and n75 settings. So taking the n00 to n25 gap one has the option of n05, n10, n15 and n20 for just only two required frequencies of 8.33 and 16.66. So they chose n10 and n15. But what about n05. To avoid a dead channel/setting n05 maps back to n00.

So the terminology of channel as opposed to frequency makes absolute sense based on long history of the past and that one can not dial 8.33 let alone 8.333333333333333333333333333.



That is a silly thing to say, of course you can dial in an 8.33khz channel. Even a very basic radio will be capable of that.
What ATC has done is said we are going to have an 8.33khz step & then only done so in the most confusing manner possible & thus made their radio system, potentially VERY unsafe.
We now have a frequency assigned to the channel, which is different to the one being used. But only on some channels.
Imagine the chaos if we said we are going to rename all of our Cities & from now on Manchester will be known as Liverpool, but letters need to say Grimsby. But what was Grimsby will now be Nottingham & Newcastle will be known as Grimsby but the mail for Grimsby will need to be addressed to Leicester.
Calling one frequency, an incorrect frequency is stupid in the extreme. Unless what you want to do is cause confusion, as it will very obviously ultimately lead to confusion, especially during an emergency situation. That is why we ALL use UTC as a time standard. As like that, we are all using the same standard & understand each other perfectly during an emergency situation, when communication details matter.
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Re: Air Distress - Now 121.505 MHz ?

Postby Minus1 » Fri Oct 19, 2018 9:55 am

They could have chosen 10 kHz. Which would save a digit spoken with every frequency change requested by ATC.
Yes, it would need .#25 and .#75 to first move to .#2 and .#7 respectively, but just as practical as changing to 8.33 kHz, and far more sensible in the long term.
Sadly that would have required some common sense and initiative, which is sadly lacking in this world.
It's like this absurd dividing things by two endlessly, leading to a never ending sequence of unnecessary digits.
25 - 12.5 - 6.25 - 3.125 - FFS :rolleyes:
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