Thursday, March 17, 2022

FEB 2017 REBOOT: 2022 APRS on the cheap!


Time to finally start going back in the time machine to see how things have changed since the early HVDN days.

Feb 7th 2017:  The first HVDN article

Fellow Hudson Valley amateur radio related organization known as the Overlook Mountain Amateur Radio Club (OMARC) used to have a message board that I used to post content to.

Some of those articles were then mirrored to start HVDN, such as the "Mobilinkd TNC2: Bluetooth APRS on the cheap!"  one but the photo links are now broken since they closed down that message board last year and is where those original images were hosted.  Is that you McFly?

Since the time of that article, Rob WX9O has released his TNC3, which basically does the same thing as the earlier TNC2, but the main improvements for $124.95 include:

    • Bluetooth 4.2 support (dual-mode EDR/LE) iOS Compatible
    • 1200 and 9600 baud capability!!
    • USB serial port
    • DCD and TX indicator LEDs
    • More powerful DSP and modem
    • Higher dynamic range ADCs and DACs
    • Buffered inputs and outputs
    • Input amplifier for improved input level matching across a wider range of radios
    • Optimized for packet radio
    • 900mAh rechargeable battery lasts for 2 full days

Now supposing you already have a smartphone,  VHF/UHF radio and the TNC3 plus cable to hook this all together, you have a pretty portable and capable APRS system.  But is there a cheaper way in 2022?  Lets find out!

Its not about being cheap: 2022 APRS on the cheap!

The TNC3 is actually really great, but without a smartphone and its built in GPS and whatever APRS smartphone/tablet application you want to use, you have a few limitations.

The Appalachian Trail Golden Packet (ATGP) is fast approaching on July 16th 2022 and a specialized APRS Appliance is being developed by an amazing team using off the shelf parts and open software to fill a gap left by manufacturers of ready to use APRS equipment.

It would be easy to just use the TNC3 with an appropriate VHF/UHF radio, but that may not be best for some events such as the historic Appalachian Trail Golden Packet event.

Here is a general block diagram of what the APRS Appliance contains:

Direwolf software is run on a Raspberry Pi computing device which provides a software emulated TNC. 

The DINAH interface provides a safe and stable way to connect the Raspberry Pi to a VHF/UHF capable radio which can handle FSK 9600 baud or AFSK 1200 baud data. 

For a fixed location station, this is all that is needed in addition to the appropriate power sources for the Raspberry Pi and VHF/UHF radio plus an antenna for the radio.

If there was a need for this to be taken mobile, including a USB GPS with external antenna would be needed for continual location updates.  

The additional USB Wi-Fi device shown provides a secondary connection to either debug or provide connection to a smart phone or tablet to view or control the device.

A bill of materials for the basics as above, excluding the VHF/UHF radio, power supply system and antenna would cost about $200 or less.

You said cheap APRS!!

Portable APRS that is full featured is actually a hard thing to do still, even almost after 20+ years and this should not be the case.

With the mobile Kenwood TM-D710 radio being hard to purchase new and the hand held radios they also made being discontinued like the TH-D72 and TH-D74, the only off the shelf options for a full self contained APRS device are the less feature rich Anytone AT-D878UVII+Lanch HG-UV98 and a few Yaesu radios such as the FT-5DR, FTM-300DR  and FTM-400.

Trusted on the International Space Station, the Kenwood TM-D710
is considered by many to be the best mobile APRS radio ever made

Needless to say, to do what the APRS Appliance offers is actually not a bad deal, plus the parts may be on hand or you can repurpose them for other projects.

Surplus commercial radios like the Motorola CDM750 or CDM1250 are great for higher power mobile APRS installations when used in conjunction with the APRS appliance.

Stop being cheap!!!

An option to use a $20 interface cable, old smartphone and almost any inexpensive $50 VHF/UHF handheld radio will not be close to the same data capability as what the Mobilinkd TNC3 or an APRS Appliance can offer for true versatility with an appropriate radio, like an Alinco DR-135 or surplus commercial gear by Motorola, Vertex, Kenwood, etc.

The primary reason is that a lower cost radio does not have the same quality of RF switching and front end needed for a reliable APRS system in mission critical or dense signal environments.

A second reason is that using "voice activation" or "VOX" functions found in some radios would not offer the faster switching and turn around time needed to process higher volume packets.  Dropping packets or partial decoding means failure.

Summer 2022:  More planning than just buying sun screen

For the ATGP event on July 16th 2022 and maybe even the Hudson River Radio Relay event on June 11th 2022, it is critical for all participants to use the same or nearly the same equipment for reliable communications.

Now is a great time to start thinking about your capability and to test it in advance with local friends and not during the actual event.

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