[OZAPRS] Brisbane APRS

[Andrew Rich]

I used to check RADAR encoders with a logic analyser 

And change valves 

I used to work MIR on C64 and a Baycom modem 

Those were the days before SMART phones

Andrew 


> On 31 Oct 2017, at 2:09 pm, Glen English VK1XX <glenlist at pacificmedia.com.au> wrote:
> 
> yeah, I  used to do TX key-up freq chirp characteristic /amplitude
> envelope fingerprinting in the mid 1980s with a microbee and parallel
> port ADC...
> 
> cheers
> 
> 
> On 31/10/2017 2:55 PM, Carlos Peco-Berrocal wrote:
>> 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
>>>>> 
>>>>> 
>>>>> 
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