[N2HTT via rec.radio.amateur.moderated Admin]

73, de N2HTT

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Digital on Board

Posted: 21 Jul 2019 12:45 PM PDT
https://n2htt.radio/2019/07/21/digital-on-board/


Summer is in full swing here in the Hudson Valley, alternating between
humid rainy days, and blindingly hot humid sunny days. Did I mention humid?
So once again, on the drier days Im out whenever possible with one of the
portable setups doing a little operating. In these times of rock bottom
propagation, QRP does present a challenge. Between that and the humidity, a
guy can get restless.

I have not been interested at all up to now in the recent ham phenomenon of
FT8. I have done digital modes in the past, most notably PSK-31, but in
recent years my interest dropped off. The current controversies over FT8
being real ham radio were enough to fend me off. But, to be fair I should
not make up my mind about it until I tried it. So I started to think, not
seriously of course, about what it would take to try out FT8. Just to be
sure I have no interest in it whatsoever.

Well the first thing I would need was a computer to run the software. All
of the machines in our household are spoken for, and it would inconvenient
to move my office laptop for digital shack duties, so I started the usual
eBay search for an old, used, cheap, Linux worthy laptop. Pretty depressing.

Im not sure when the idea occurred to me, but for less than half the money
and one quarter the footprint, a Raspberry Pi might do the trick. The new
models of this $35 single board computer run quad core processors at 1.2
GHz, a far cry from the original. It was an intriguing idea, and I took a
look. I found a lot of material, much of it on Youtube. There are dozens of
videos covering all facets of Raspberry Pi mischief: I used several of them
as sources for my project and I will share the links as I go along.

One of the Youtube channels I found, SurvivalTechNord by Julian, OH8STN,
suddenly changed the whole idea and crystallized the project for me. I
highly recommend his work, the videos are packed with great information.

Running digital modes would be interesting, but running them portable, off
grid, battery operated QRP would be awesome! I had to do this, especially
since I instantly knew that I had the perfect substrate for a portable
digital platform: a bamboo cutting board.

Ive put lots of projects on these boards theyre cheap, readily available
in the produce section of your local grocery, and made from a renewable
resource. Every project is better on a bamboo cutting board.
Everythings better on a bamboo cutting board

I made a list of the hardware I would need:

bamboo cutting board (had one in stock)
Raspberry Pi (I chose to use a 3B+ rather than the newly released 4)
case for the Pi
power distribution for 12v to the radio and the Pi
DC-DC converter to supply 5v to the Pi
sound card interface
rig control interface
GPS (for off grid time synchronization)
FT-817
android tablet for UI (Raspberry Pi runs headless)
one of those cute bluetooth keyboard/mouse combos (optional, but handy)
12v battery to run everything (I have a Bioenno 4.5Ah lipo)
several class 10 micro SD cards and USB reader (really important, make
backups!)


Theres a lot to describe about this project, so Im going to focus on the
hardware in this post, and add follow on posts to discuss some other key
issues:

setting the Raspberry Pi up to run headless
installing the latest versions of ham radio digital software
setting the Pi up as an access point when there isnt Wifi available


Anyway, on to the hardware. By watching Julians videos, I selected the
sound card interface and the GPS unit for the project.

The sound card interface, a ZLP MiniProSC, is sold by a firm in the UK, but
ordering it for delivery to the States was not a problem, and it was about
a week from order to delivery. The MiniProSc is tiny, has very good
specifications, and no knobs. Audio levels are controlled from the
computer, very convenient. The other choice was a Signal Link, which is
more expensive, physically larger, and after reading reviews and comparing
specifications, less attractive to me.

The MiniProSC is shipped with connecting cables for the radio of your
choice. I ordered the data input cable for the FT-817, which will also work
with my FT-847. Oddly enough, the MiniProSC uses the old style larger type
B USB cable. There is nothing special about the bright blue one you see in
the photo, it was just the shortest cable of this variety I had on hand.
The MiniProSC

The GPS unit is a inexpensive USB dongle readily available on Amazon
(here). Julian provides a video showing the software installation and setup
necessary to allow Raspbian Linux to synchonize its system time to the NMEA
sentences coming from the GPS. You will only need this when not connected
to the internet; when you have an internet connection the time servers in
the cloud provide more accurate time fixes and the GPS is ignored.
GPS and rig control dongles

Julian also mentions an excellent Anderson Powerpole distribution block
sold as a kit by K9JEB, John. These are a superb alternative to
commercially made distribution blocks, and John offers an optional DC-DC
converter the size of a Powerpole, which can be substituted in up to three
locations on the board. I ordered my kit with one DC-DC converter, and the
filter capacitors. This block allows me to connect the battery to the
Raspberry Pi setup and the radio, and provides fused input. It works well,
but I found that the Pi with all the USB devices connected wanted more
current than the tiny converter could supply, and so consistently displayed
the dreaded lightning bolt icon on the desktop indicating the supply
voltage was sagging.
K9JEB power distribution block

Consequently I found a very nice, inexpensive adjustable DC-DC converter on
Amazon (here). This works great, it can provide several amps if necessary
with very good voltage regulation and includes a nice display and switch
controlled start up. I havent noticed any switching noise on the
perceptible on the ham bands. And, I can still charge my tablet from the DC
converted on the power distribution block if I need to.
DC-DC converter

Connecting the Pi to the DC converter required making a power cable. I
cannibalized an old wall wart for a two-wire cable that appeared robust
enough to carry the amp or so we might draw on startup. The micro usb
connectors are available on Amazon, about $3 for ten. You will need 10,
because these things are really hard to solder to the plastic melts easily
if overheated. Use the finest soldering tip you have, crank the temp down,
and use the minimum dwell time necessary to flow the solder. I added some
heat shrink tubing for a strain relief, and a 90 degree usb adapter
(Amazon, here) so I could route the cable with the least amount of stress.
Some cable ties finish the job, once installed dont mess with it. And make
a few spares.
Homebrew micro USB cable and right angle adapter

Rig control is provided by an FTDI usb dongle (pictured above with the GPS
dongle), again available from Amazon here. No problem if you stick with
FTDI devices, no drivers needed for Raspian linux, it was truly plug and
play. Not much to say, it just works.

The Raspberry Pi is the 3B+ version, available just about everywhere. I
decided to avoid the newer version 4, as there is a reported problem with
powering it from the USB C cables available, requiring a converter from
micro usb.

I found a nice case for it as well, that includes a heat sink set and a
tiny fan that runs from the 5v GPIO pins. Looks classy. I was going to
provide an Amazon link for it, but apparently the 3B+ version has been
replaced the the version 4 case. Alas

I chose to use 64 Gb micro SD cards for the persistent store on the Pi, but
in retrospect that might be overkill; 32 Gb is plenty, you could probably
get away with less. The issue is that you will need several of these cards
so you can back up the software as you make progress with the various
installation and configuration tasks. Trying to set up the access point I
hosed up my setup several times, having those backups avoided the loss of
hours of work.
Raspberry Pi 3B+ in classy case

All this stuff needs to be stuck down to the cutting board, and my favorite
medium for this is 3M Scotch brand Extremely Strong hook and loop
fasteners. They dont appear to have any other name, heres the package:
Scotch Extermely Stong fastener

There not actually hooks and loops, more like little mushrooms, and the
tape is genderless. Clean both surfaces with a little isopropyl alcohol,
let them dry. Stick to device, place carefully and stick to the board.
Allow to set for a few hours undisturbed and your good to go. In the
unlikely event that you ever want to remove something from the board, it
can with effort be peeled off.

The idea of using the tablet, connected over wifi to the Pi as a user
interface is brilliant. You can operate without direct connection to the
rig giving you a lot of flexibility for your portable setup. The type of
tablet does not matter; excellent VNC client apps are available free for
Android and IOS devices which bring the Pis desktop to your device and
allow you to interact with the ham radio software GUI. This photo is of my
nVidia tablet running VNC client
Rasberry Pi desktop running WSJTX on an Android tablet via VNC

While it is not necessary, these cute little bluetooth keyboard/mouse
combinations (Amazon, here) eliminate the need to use the touch screen and
virtual keyboar on your tablet, yielding finer and more positive control of
the sofware in my opinion.
Cute Bluetooth keyboard

The rig is of course my venerable Yaesu FT-817 (original model) which has
come out of mothballs and is ideal for this application. I have also used
the board with my FT-847, the big brother of the 817, and this also works
quite well.
FT-817 listening to FT8 on 40m

I havent done tests but by rough calculation I should get 4 hours of
operation from the 4.5 Ah battery. Once started, the Pi draws a steady 0.5
amp. The FT-817 draws about 2 amps at 5 watts on transmit. The receive
current is about 350 ma. So in receive mode FT8 the setup is drawing about
850 ma, on transmit 2.35 amps. During QSO you have a 50% transmit/receive
duty cycle, so the average QSO draw is 1175 ma. If you assume your time is
spent two thirds listening and one third in QSO, overall average
consumption would be 625 ma, and allowing for not depleting the battery
much beyond 50% you could run for the setup for about 4 hours. More than
enough for me.
Bioenno 4.5 AH LiFePo Battery

In the next post I will go over the software used in the project. Lots of
interesting stuff to talk about. Until then,

73,

de N2HTT