Re: No 2m but still an APRS igate?

"John D. Hays" <john@...>

Hi Sander,

Maybe I can clarify this a little bit.   If you look at the wiki information in the link you included, 144.39 nor 1200 baud is universally used for APRS or igates.  You will find there are UHF networks and 9600 baud networks, including 9600 baud UHF networks.

Obviously, anyone using a UDR56K for an APRS tracker is only going to be able to report to 70cm igates.   

The UDR56K was designed to be both a user radio and an infrastructure (igate, D-STAR gateway, AMPRNET, RMS, etc.) radio.  If you are using it in infrastructure, say as an igate, it can do that.  A frequency, modulation (FSK, GMSK, 4FSK) and baud rate (4800-56000) would be selected and the proper application loaded and run.  Any trackers would need to match the same combination.

A lot of thought went into the selection of the band and the modes under which the radio would operate.  

The concept of "Universal" Digital Radio means it can be used for more than one application.  So while one user might want to use it for 9600 baud APRS, another might want to run it at 56Kbps for file transfers in an RMS function, or to pass AMPRNET or D-STAR DATA traffic.   Yet another user might want to run D-STAR Voice or Codec-2.   

2 meters, suffers from being "too popular" -- in many countries it is only 2 mHz wide, with FM repeaters, weak signal, APRS, satellites, etc. all trying to squeeze into the band.  A 56Kbps signal is not permitted in the US FCC regulations on 2 meters and would not be friendly to other spectrum users if it were.   

The 219 and 222 band is interesting and may lend itself to a UDR type radio (the band is only available in a limited area such as US/Canada), so the thought is that it is better to use 70cm as a good place to start:
  • 10-30 mHz. of spectrum in most areas
  • reasonable feedline/connectors (compared to microwave)
  • descent propagation (not as good as 2m, obviously but better than 33/23 cm)
  • allows a data signal of up to 100 kHz bandwidth and 56 baud signal rate, with spectrum to support it
I believe that both tactical users (e.g. Emcomm) and experimenters will have a new world opened up to them with a less crowded spectrum.  In the US, it seems a lot of people think the lower end of 70cm is at 440 mHz., when there is an additional 10-20 mHz below that. (To me it's silly to try to squeeze APRS into 440-450, one proposal puts it on a frequency that is a repeater output in some band plans.)

Here's the good news though.  If you have a current igate on 2m, you can replace the computer with a UDR56K, attach a USB-to-Serial cable  to the TNC and radio to continue to service the 144.39 net, while offering 9600 baud or better APRS on the UDR56K's 70cm radio.  Then you would have a dual band igate with less power requirement (by loosing the computer) and a much smaller package.  Attach a diplexer and dual band antenna and you are ready to go.

John D. Hays
PO Box 1223, Edmonds, WA 98020-1223 

On Tue, May 22, 2012 at 10:27 AM, Sander Pool <sander_pool@...> wrote:

Right, if you have a local userbase that runs APRS on 70cm then this would be a great radio. It could even be an advantage that no one 'in the know' will be pinging your igate with 144.390 packets. But as a general purpose APRS igate it would not be very useful.

To be clear, I have nothing against UHF or any other frequency, of course. Clearly there are advantages to running 9600 baud packet over UHF. I was questioning the use of APRS in the flyer. I suppose anyone with the knowledge to set up a linux based APRS igate would not be confused and quickly see that it is not usable on the most often used frequencies.


    Sander W1SOP

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