A highly capable HF Data Mode for Android Phones

By Evgeny Slodkevich, UA3AHM/OH5HM, and Dieter Kuckelkorn, DL1DBY

When going to an outdoor camping trip, we will find that in many parts of the world there is no cell phone service avail-
able in the back country. To make matters worse, in these areas there is almost never a VHF/UHF ham radio repeater
in range when we need wide-area coverage. Apart from strictly local communications using VHF/UHF simplex radio,
how do we send messages to friends and family over great distances? How do we call for help? A similar problem can
even arise in an urban environment if a major disaster strikes like the break-down of the power grid.
In activities like back country trips in areas without cell phone coverage or in a widespread emergency with the loss of
our normal means of communication we can use satellite phones, but this technology is very expensive, requires sub-
scriptions and there is no guarantee that the complex infrastructure of satellite communications will work under all cir-
cumstances. The obvious solution for Ham Radio operators will be to switch to shortwave communication using battery
operated radios and often NVIS modes of operation. NVIS stands for Near Vertikal Incidence Skywave, which means
transmitting with special antennas straight up to communicate with other stations 30 km to 300 km (20 to 200 miles)
away with low power - which would be the most useful communications distance if help is needed. We could use SSB
voice communications, but this requires that the person we want to reach is sitting constantly at his or her radio to be
able to receive the message. This can be a problem: In a real emergency we probably won't have time for this. We could
instead use capable digital modes with automatic message handling capabilities like JS8Call, but these require notebook
computers or other complicated setups in the field which consume a lot of energy and can be difficult to recharge off-
grid on a reliable basis.
Evgeny UA3AHM/OH5HM and Sergej UA9OV have developed another mode of digital shortwave communications,
which aims to be easy to use, capable and - most importantly - friendly to the operator's resources. Apart from a low
power battery operated transceiver and a small digital interface, only an Android smartphone is needed, which can be
recharged with cheap and readily available consumer-grade solar chargers. Evgeny and Sergej have created an app
called "HFpager" which allows to use the smartphone's sound chip to encode and decode audio signals in the SSB audio
passband of the transceiver - similar to PC based modes like FT8 and JS8Call. It uses rates of transmission of 1.46,
5.86, 23.44 and 46.88 Baud. Modulation is 18-tone Incremental Frequency Shift Keying (IFSK) with forward error cor-
recting Reed-Solomon code RS(15,7) and a superblock by 4 RS blocks with interleaving.
It is possible to send text messages and GPS position reports with the position being instantly visible in Google Maps or
Maps.me, a service with allows the map to be stored on the smartphone and to be used off-line. All message are auto-
matically stored on the phone to be retrieved later, if desired. The sender can request an automatic confirmation by the
receiving station. There is the possibility to send and receive automatic beacon transmissions including the GPS posi-
tion in regular intervals, to let the outside world know that our station is still "alive and kicking" or to share the progress
we make on our way with our friends.
The app allows text messaging between two stations operating on the same frequency, like in a SSB or CW QSO. There
are no group calls like in JS8Call, nor is there a kind of rudimentary networking possible like in JS8Call, to keep things
as simple as possible. In contrast to JS8Call not only latin letters are allowed, but also cyrillic letters. The app includes
an audio frequency waterfall display well known from FT8 or JS8Call software.
The app was tested by the two authors of this article on 20m over a distance of 2100 km (1300 miles) using only 1 watts
of transmitting power and a ground plane antenna hanging from a tree in a public park in Frankfurt (Germany). On the
other side near Moscow a 3-element beam and 20 watts were employed. This represents a typical configuration with
one station somewhere in the "great outdoors" and the other representing a home base. On the German side an Elecraft
K2 transceiver and a DigiLink Nano interface by HB9ZHK were used, on the Russian side an Yaseu FT-450 and a
RAIS-1 interface. The DigiLink Nano interface has its own sound card build-in which is reliably recognized by An-
droid. It was chosen because it draws very little current. The RAIS-1 interface and a special version RAYS-4 for Yaesu
transceivers use the smartphone's sound chip, but have their own vox circuitry integrated. The RAIS-1 does not need an
external voltage nor draws any current, the DigiLink Nano and the RAYS-4 receive the required voltage from the smart-
It was easily possible to stay in contact using this setup for the whole test period of three hours. The 1 watt signal was
received with S3 near Moscow. It would have been possible to significantly reduce transmitting power or use a less effi-
cient and smaller portable antenna, as HF Pager should be theoretically able to decode signals up to 27 dB below noise
In further testing it was possible to employ only 0.5 watts to keep the connection stable in a slightly different configura-
tion as described above. This time a dipole antenna in sloper configuration and 5 watts of transmitting power were used
on the other side near Moscow. As the 20m band was heavily congested due to a major HF contest and as the iono-
spheric conditions were pretty bad, so sometimes a lower transmission rate of 1.46 Baud had to be used. This proved to
be reliable over a period of several hours. In these difficult band conditions HFpager appeared to be as capable as JS8-
The current draw of the app was moderate. Over a time frame of three hours with intensive use of the app and full
brightness of the display the battery indicator off their smartphone phone went down from 100% to 63%. The app never
crashed even when using a cheap Chinese smartphone that has problems in this regard. The user interface of the app in-
cludes a waterfall display known from FT8 and JS8Call applications and indicators for receiving and transmitting. Text
sent by other stations other than the operator's correspondent will be decoded and and displayed as well, if the station is
on the same frequency, as all forms of encryption would be illegal in Ham Radio.
In Android, switching on OTG capabilities can be required as well as allowing for external sound chip support (look in
developer settings). It is essential that audio notifications are temporarily switched off in Android when using HFpager
for obvious reasons. It can be necessary to disable battery-saving mode for the app in the Android battery settings. As
with all HF digital modes it is important to switch off transceiver TX voice compression and to be careful not to over-
drive the transmitter as HFpager is a 100% duty cycle mode like FT8 and JS8Call. To be on the safe side 25 watts when
using a standard ham radio 100-watts transceiver and around 5 watts with a 10-watts QRP transceiver are recommended
TX power levels.
The user experience is meant to be as close as possible to popular messag-
ing apps like Whatsapp and Telegram, because HFpager is not only tar-
geted at the Ham Radio community. In the Russian Federation, only 16
percent of the territory have cell phone coverage due to the enormous size
of the country. Huge areas in Siberia and the Russian Far East can proba-
bly never get sufficient coverage. A solution like this is therefore very use-
ful for outdoor enthusiasts, geologists, hunters etc. Having this in mind the
app developers chose numerical station identifiers, like in DMR. To oper-
ate legally, the ham radio enthusiast has to transmit his or her call sign as
message text every 10 minutes. The automatic beacon feature has the ca-
pability to include the call sign automatically in CW at the end of every
As non-ham operator usually don't own radio gear and as not all ham gear
is up to the task, Evgeny has developed a very small and light battery-op-
erated low-power HF transceiver called "Uleyma-80" (named after a river
in Russia) with three fixed-frequency channels for net or emergency fre-
quencies that must be specified while ordering. Included is a 12 Volts Li-
Ion battery that allows operation of the transceiver for very long periods of
time, a carrying case and shortened dipole antenna tuned to the desired fre-
quency. The antenna can simply be thrown into the next tree, as NVIS
communication does not require great antenna heights. The transceiver
uses only 30 mA current on receive mode and is extremely simple to oper-
ate and specially designed for this purpose as it allows to connect a smart-
phone with a regular and widely available audio cable. It also allows to
use a typical smartphone headset with microphone for SSB voice trans-
missions, should the user prefer to communicate this way. Non-ham users
have to obtain a commercial radio license to use the transceiver, according
to the resident country's laws.
Apart from the Android app available in Google Play there is now a Windows base station software available and, as the
newest development, an Android-based gateway that allows messages to be relayed to and from the regular SMS mobile
phone service, to communicate with GSM phones around the world in both directions. Check your local laws whether
this service may be used by Amateur Radio operators in your country. Under development is also a HF/VHF crossband
repeater. This repeater can be placed at the outskirts of a settlement or city outside the urban high RF noise environment
and relay the HFpager messages on VHF to a home base in the center of the village or town. Please note that setting up
an unmanned ham radio station may require a special licence in many countries.