[OZAPRS] Brisbane APRS

[Carlos Peco-Berrocal]

Hi Glen,

The transient at the beginning/end of the transmission is unique. I
even compared two portable radios and differences in battery level and
temperature made them drift differently during the transmission
period.

Those transients can be recorded and then new transmissions are
correlated with the entries in your database. There are standards for
that database so you can share fingerprints with "your mates".

I found a couple of photos, I don't know if the link to imgur will work:

https://imgur.com/a/kTJ4S

That repeater was off frequency few hertzs, you can see the vertical
pink lines (markers).

The last image was HF, there are lots of interesting details in the waterfalls.


If you monitor Canberra Approach you'll see the air traffic controller
in the center (GPS disciplined radio from some Germans you know) with
the aircraft appearing at the sides. The automatic simultaneous
transmission detection available in the current generation of ATC
radios from the same German brand exploits that: if two simultaneous
transmissions occur, it is highly likely that their instantaneous
frequencies will be few hertz apart so they will appear as two peaks
in the FFT of the complex envelope.

https://www.youtube.com/watch?v=Y983aZelEkM

Back in the days of CB27 in AM you could hear a beating tone when two
transmissions happened at the same time and had power levels in the
same range (if one transmission is MUCH stronger than the other,
you'll hear nothing but the strongest transmission).

Sorry for the long post, I hope you find it interesting. Happy to take
some toys to your place so you can tinker a bit with this.

cheers
Carlos VK1EA



On 10/29/17, Glen English VK1XX <glenlist at pacificmedia.com.au> wrote:
> hmm
>
> how much sag is that  in kHz, or even better Hz/sec ?
>
>
> On 29/10/2017 6:35 PM, vk4tec at tech-software.net wrote:
>> http://59.167.159.165/aprs.jpg
>>
>> -----Original Message-----
>> From: OZAPRS [mailto:ozaprs-bounces at aprs.net.au] On Behalf Of Glen English
>> VK1XX
>> Sent: Sunday, 29 October 2017 5:00 PM
>> To: ozaprs at aprs.net.au
>> Subject: Re: [OZAPRS] Brisbane APRS
>>
>> Andrew, there are a few checkpoints to this
>>
>> 1) I gather you know precisely what frequency error your spectrum analyser
>> has.
>>
>> 2) I gather your spectrum analyser , if FFT / digital  type, has
>> sufficient resolution bandwidth to tell a non aliased story.
>>
>> 3) It is not usual for station's TX  PLL /synthesisers to not be fully
>> settled during the keyup phase, and take 30- 100mS to establish fine
>> frequency lock .
>>
>> 4) I would expect +/- 5ppm for most radios, that being about +/- (5 x
>> 145.175 Hz)
>>
>> 5) It's probably not going to hurt much until +/- 1kHz, and not really
>> hurt until +/- 2kHz, depending on the TX deviation. (IE how far outside
>> the Receiver IF filter the modulation protrudes, as the received signal
>> will be asymmetrical in the receiver's IF passband, the higher the
>> deviation, the greater the extremes)
>>
>> How far out are they ?
>>
>> -glen
>>
>>
>> On 29/10/2017 5:51 PM, vk4tec at tech-software.net wrote:
>>> Hello
>>>
>>>
>>>
>>> I just had a quick look on 145.175 MHz on a spectrum analyser.
>>>
>>>
>>>
>>> I can see stations off frequency
>>>
>>>
>>>
>>> I can see transmitters shifting frequency on keyup
>>>
>>>
>>>
>>> Andrew
>>>
>>>
>>>
>>> _______________________________________________
>>> OZAPRS mailing list
>>> OZAPRS at aprs.net.au
>>> http://lists.aprs.net.au/mailman/listinfo/ozaprs
>>
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